LTC2348-18
1
234818fa
For more information www.linear.com/LTC2348-18
Typical applicaTion
FeaTures DescripTion
Octal, 18-Bit, 200ksps
Differential ±10.24V Input SoftSpan ADC
with Wide Input Common Mode Range
The LTC
®
2348-18 is an 18-bit, low noise 8-channel si-
multaneous sampling successive approximation register
(SAR) ADC with differential, wide common mode range
inputs. Operating from a 5V low voltage supply, flexible
high voltage supplies, and using the internal reference
and buffer, each channel of this SoftSpan
TM
ADC can be
independently configured on a conversion-by-conversion
basis to accept ±10.24V, 0V to 10.24V, ±5.12V, or 0V to
5.12V signals. Individual channels may also be disabled
to increase throughput on the remaining channels.
The wide input common mode range and 118dB CMRR of
the LTC2348-18 analog inputs allow the ADC to directly
digitize a variety of signals, simplifying signal chain de-
sign. This input signal flexibility, combined with ±3LSB
INL, no missing codes at 18 bits, and 96.7dB SNR, makes
the LTC2348-18 an ideal choice for many high voltage
applications requiring wide dynamic range.
The LTC2348-18 supports pin-selectable SPI CMOS (1.8V
to 5V) and LVDS serial interfaces. Between one and eight
lanes of data output may be employed in CMOS mode,
allowing the user to optimize bus width and throughput.
L, LT, LT C , LT M, Linear Technology and the Linear logo are registered trademarks and
SoftSpan is a trademark of Linear Technology Corporation. All other trademarks are the property
of their respective owners. Protected by U.S. Patents, including 7705765, 7961132, 8319673.
Other Patents pending.
applicaTions
n 200ksps per Channel Throughput
n Eight Simultaneous Sampling Channels
n ±3LSB INL (Maximum, ±10.24V Range)
n Guaranteed 18-Bit, No Missing Codes
n Differential, Wide Common Mode Range Inputs
n Per-Channel SoftSpan Input Ranges:
±10.24V, 0V to 10.24V, ±5.12V, 0V to 5.12V
±12.5V, 0V to 12.5V, ±6.25V, 0V to 6.25V
n 96.7dB Single-Conversion SNR (Typical)
n −109dB THD (Typical) at fIN = 2kHz
n 118dB CMRR (Typical) at fIN = 200Hz
n Rail-to-Rail Input Overdrive Tolerance
n Guaranteed Operation to 125°C
n Integrated Reference and Buffer (4.096V)
n SPI CMOS (1.8V to 5V) and LVDS Serial I/O
n Internal Conversion Clock, No Cycle Latency
n 140mW Power Dissipation (Typical)
n 48-Lead (7mm x 7mm) LQFP Package
n Programmable Logic Controllers
n Industrial Process Control
n Power Line Monitoring
n Test and Measurement
Integral Nonlinearity vs
Output Code and Channel
0.1µF2.2µF0.1µF0.1µF
1.8V TO 5V5V15V
–15V
SAMPLE
CLOCK
234818 TA01a
S/H
S/H
S/H
S/H
S/H
S/H
S/H
S/H
MUX
VCC VDD VDDLBYP OVDD
EIGHT SIMULTANEOUS
SAMPLING CHANNELS
DIFFERENTIAL INPUTS IN+/IN– WITH
WIDE INPUT COMMON MODE RANGE
+5V
0V
–5V
FULLY
DIFFERENTIAL
+10V
0V
–10V
TRUE BIPOLAR
+10V
0V
–10V
ARBITRARY
+10V
0V
–10V
UNIPOLAR
SDO0
SDO7
SCKO
SCKI
SDI
CS
BUSY
CNV
• • •
• • •
LVDS/CMOS
PD
IN0+
IN0–
IN7+
IN7–
18-BIT
SAR ADC
CMOS OR LVDS
I/O INTERFACE
0.1µF
REFIN
47µF
0.1µF
GNDREFBUFVEE
LTC2348-18
TRUE BIPOLAR DRIVE (IN– = 0V)
ALL CHANNELS
±10.24V RANGE
OUTPUT CODE
–131072
–65536
0
65536
131072
–2.0
–1.5
–1.0
–0.5
0
0.5
1.0
1.5
2.0
INL ERROR (LSB)
234818 TA01b
LTC2348-18
2
234818fa
For more information www.linear.com/LTC2348-18
pin conFiguraTionabsoluTe MaxiMuM raTings
Supply Voltage (VCC) .....................–0.3V to (VEE + 40V)
Supply Voltage (VEE) ................................ –17.4V to 0.3V
Supply Voltage Difference (VCC – VEE) ......................40V
Supply Voltage (VDD) ..................................................6V
Supply Voltage (OVDD) ................................................6V
Internal Regulated Supply Bypass (VDDLBYP) ... (Note 3)
Analog Input Voltage
IN0+ to IN7+,
IN0– to IN7– (Note 4) .........(VEE – 0.3V) to (VCC + 0.3V)
REFIN .................................................... –0.3V to 2.8V
REFBUF, CNV (Note 5) ............. –0.3V to (VDD + 0.3V)
Digital Input Voltage (Note 5) .....–0.3V to (OVDD + 0.3V)
Digital Output Voltage (Note 5) .. –0.3V to (OVDD + 0.3V)
Power Dissipation .............................................. 500mW
Operating Temperature Range
LTC2348C ................................................ 0°C to 70°C
LTC2348I .............................................–40°C to 85°C
LTC2348H .......................................... –40°C to 125°C
Storage Temperature Range .................. –65°C to 150°C
(Notes 1, 2)
1
2
3
4
5
6
7
8
9
10
11
12
36
35
34
33
32
31
30
29
28
27
26
25
IN6–
IN6+
IN5–
IN5+
IN4–
IN4+
IN3–
IN3+
IN2–
IN2+
IN1–
IN1+
13
14
15
16
17
18
19
20
21
22
23
24
IN0–
IN0+
GND
VCC
VEE
GND
REFIN
GND
REFBUF
PD
LVDS/CMOS
CNV
48
47
46
45
44
43
42
41
40
39
38
37
IN7+
IN7–
GND
VEE
GND
VDD
VDD
GND
VDDLBYP
CS
BUSY
SDI
SDO7
SDO–/SDO6
SDO+/SDO5
SCKO–/SDO4
SCKO+/SCKO
OVDD
GND
SCKI–/SCKI
SCKI+/SDO3
SDI–/SDO2
SDI+/SDO1
SDO0
TOP VIEW
LX PACKAGE
48-LEAD (7mm × 7mm) PLASTIC LQFP
TJMAX = 150°C, θJA = 53°C/W
orDer inForMaTion
LEAD FREE FINISH TRAY PART MARKING* PACKAGE DESCRIPTION TEMPERATURE RANGE
LTC2348CLX-18#PBF LTC2348CLX-18#PBF LTC2348LX-18 48-Lead (7mm × 7mm) Plastic LQFP 0°C to 70°C
LTC2348ILX-18#PBF LTC2348ILX-18#PBF LTC2348LX-18 48-Lead (7mm × 7mm) Plastic LQFP –40°C to 85°C
LTC2348HLX-18#PBF LTC2348HLX-18#PBF LTC2348LX-18 48-Lead (7mm × 7mm) Plastic LQFP –40°C to 125°C
Consult LTC Marketing for parts specified with wider operating temperature ranges. *The temperature grade is identified by a label on the shipping container.
For more information on lead free part marking, go to: http://www.linear.com/leadfree/
LTC2348-18
3
234818fa
For more information www.linear.com/LTC2348-18
elecTrical characTerisTics
converTer characTerisTics
SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS
VIN+ Absolute Input Range
(IN0+ to IN7+)
(Note 7) lVEE VCC – 4 V
VIN– Absolute Input Range
(IN0– to IN7–)
(Note 7) lVEE VCC – 4 V
VIN+ – VIN– Input Differential Voltage
Range
SoftSpan 7: ±2.5 • VREFBUF Range (Note 7)
SoftSpan 6: ±2.5 • VREFBUF/1.024 Range (Note 7)
SoftSpan 5: 0V to 2.5 • VREFBUF Range (Note 7)
SoftSpan 4: 0V to 2.5 • VREFBUF/1.024 Range (Note 7)
SoftSpan 3: ±1.25 • VREFBUF Range (Note 7)
SoftSpan 2: ±1.25 • VREFBUF/1.024 Range (Note 7)
SoftSpan 1: 0V to 1.25 • VREFBUF Range (Note 7)
l
l
l
l
l
l
l
–2.5 • VREFBUF
–2.5 • VREFBUF/1.024
0
0
–1.25 • VREFBUF
–1.25 • VREFBUF/1.024
0
2.5 • VREFBUF
2.5 • VREFBUF/1.024
2.5 • VREFBUF
2.5 • VREFBUF/1.024
1.25 • VREFBUF
1.25 • VREFBUF/1.024
1.25 • VREFBUF
V
V
V
V
V
V
V
VCM Input Common Mode Voltage
Range
(Note 7) lVEE VCC – 4 V
VIN+ – VIN– Input Differential Overdrive
Tolerance
(Note 8) l−(VCC − VEE) (VCC − VEE) V
IIN Analog Input Leakage Current l–1 1 µA
CIN Analog Input Capacitance Sample Mode
Hold Mode
50
10
pF
pF
CMRR Input Common Mode
Rejection Ratio
VIN+ = VIN− = 18VP-P 200Hz Sine l100 118 dB
VIHCNV CNV High Level Input Voltage l1.3 V
VILCNV CNV Low Level Input Voltage l0.5 V
IINCNV CNV Input Current VIN = 0V to VDD l–10 10 μA
SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS
Resolution l18 Bits
No Missing Codes l18 Bits
Transition Noise SoftSpans 7 and 6: ±10.24V and ±10V Ranges
SoftSpans 5 and 4: 0V to 10.24V and 0V to 10V Ranges
SoftSpans 3 and 2: ±5.12V and ±5V Ranges
SoftSpan 1: 0V to 5.12V Range
1.3
2.6
2.0
4.0
LSBRMS
LSBRMS
LSBRMS
LSBRMS
INL Integral Linearity Error SoftSpans 7 and 6: ±10.24V and ±10V Ranges (Note 10)
SoftSpans 5 and 4: 0V to 10.24V and 0V to 10V Ranges (Note 10)
SoftSpans 3 and 2: ±5.12V and ±5V Ranges (Note 10)
SoftSpan 1: 0V to 5.12V Range (Note 10)
l
l
l
l
–3
–4
–2.5
–2.5
±1
±1.5
±0.75
±0.75
3
4
2.5
2.5
LSB
LSB
LSB
LSB
DNL Differential Linearity Error (Note 11) l−0.9 ±0.2 0.9 LSB
ZSE Zero-Scale Error (Note 12) l−550 ±80 550 μV
Zero-Scale Error Drift ±2 μV/°C
FSE Full-Scale Error (Note 12) l−0.1 ±0.025 0.1 %FS
Full-Scale Error Drift ±2.5 ppm/°C
The l denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C. (Note 6)
The l denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C. (Note 9)
LTC2348-18
4
234818fa
For more information www.linear.com/LTC2348-18
DynaMic accuracy
inTernal reFerence characTerisTics
reFerence buFFer characTerisTics
SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS
SINAD Signal-to-(Noise +
Distortion) Ratio
SoftSpans 7 and 6: ±10.24V and ±10V Ranges, fIN = 2kHz
SoftSpans 5 and 4: 0V to 10.24V and 0V to 10V Ranges, fIN = 2kHz
SoftSpans 3 and 2: ±5.12V and ±5V Ranges, fIN = 2kHz
SoftSpan 1: 0V to 5.12V Range, fIN = 2kHz
l
l
l
l
93.0
87.6
90.0
84.2
96.5
90.6
93.2
87.3
dB
dB
dB
dB
SNR Signal-to-Noise Ratio SoftSpans 7 and 6: ±10.24V and ±10V Ranges, fIN = 2kHz
SoftSpans 5 and 4: 0V to 10.24V and 0V to 10V Ranges, fIN = 2kHz
SoftSpans 3 and 2: ±5.12V and ±5V Ranges, fIN = 2kHz
SoftSpan 1: 0V to 5.12V Range, fIN = 2kHz
l
l
l
l
93.7
87.7
90.2
84.3
96.7
90.7
93.2
87.3
dB
dB
dB
dB
THD Total Harmonic Distortion SoftSpans 7 and 6: ±10.24V and ±10V Ranges, fIN = 2kHz
SoftSpans 5 and 4: 0V to 10.24V and 0V to 10V Ranges, fIN = 2kHz
SoftSpans 3 and 2: ±5.12V and ±5V Ranges, fIN = 2kHz
SoftSpan 1: 0V to 5.12V Range, fIN = 2kHz
l
l
l
l
–109
–111
–113
–114
–101
–104
–104
–103
dB
dB
dB
dB
SFDR Spurious Free Dynamic
Range
SoftSpans 7 and 6: ±10.24V and ±10V Ranges, fIN = 2kHz
SoftSpans 5 and 4: 0V to 10.24V and 0V to 10V Ranges, fIN = 2kHz
SoftSpans 3 and 2: ±5.12V and ±5V Ranges, fIN = 2kHz
SoftSpan 1: 0V to 5.12V Range, fIN = 2kHz
l
l
l
l
101
105
105
105
110
112
114
115
dB
dB
dB
dB
Channel-to-Channel
Crosstalk
One Channel Converting 18VP-P 200Hz Sine in ±10.24V Range,
Crosstalk to All Other Channels
−109 dB
–3dB Input Bandwidth 7 MHz
Aperture Delay 1 ns
Aperture Delay Matching 150 ps
Aperture Jitter 3 psRMS
Transient Response Full-Scale Step, 0.005% Settling 360 ns
SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS
VREFIN Internal Reference Output Voltage 2.043 2.048 2.053 V
Internal Reference Temperature Coefficient (Note 14) l5 20 ppm/°C
Internal Reference Line Regulation VDD = 4.75V to 5.25V 0.1 mV/V
Internal Reference Output Impedance 20 kΩ
VREFIN REFIN Voltage Range REFIN Overdriven (Note 7) 1.25 2.2 V
SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS
VREFBUF Reference Buffer Output Voltage REFIN Overdriven, VREFIN = 2.048V l4.091 4.096 4.101 V
REFBUF Voltage Range REFBUF Overdriven (Notes 7, 15) l2.5 5 V
REFBUF Input Impedance VREFIN = 0V, Buffer Disabled 13 kΩ
IREFBUF REFBUF Load Current VREFBUF = 5V, 8 Channels Enabled (Notes 15, 16)
VREFBUF = 5V, Acquisition or Nap Mode (Note 15)
l1.5
0.39
1.9 mA
mA
The l denotes the specifications which apply over the full operating temperature range,
otherwise specifications are at TA = 25°C. AIN = –1dBFS. (Notes 9, 13)
The l denotes the specifications which apply over the full
operating temperature range, otherwise specifications are at TA = 25°C. (Note 9)
The l denotes the specifications which apply over the full
operating temperature range, otherwise specifications are at TA = 25°C. (Note 9)
LTC2348-18
5
234818fa
For more information www.linear.com/LTC2348-18
DigiTal inpuTs anD DigiTal ouTpuTs
SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS
CMOS Digital Inputs and Outputs
VIH High Level Input Voltage l0.8•OVDD V
VIL Low Level Input Voltage l0.2•OVDD V
IIN Digital Input Current VIN = 0V to OVDD l–10 10 μA
CIN Digital Input Capacitance 5 pF
VOH High Level Output Voltage IOUT = –500μA lOVDD–0.2 V
VOL Low Level Output Voltage IOUT = 500μA l0.2 V
IOZ Hi-Z Output Leakage Current VOUT = 0V to OVDD l–10 10 μA
ISOURCE Output Source Current VOUT = 0V –50 mA
ISINK Output Sink Current VOUT = OVDD 50 mA
LVDS Digital Inputs and Outputs
VID Differential Input Voltage l200 350 600 mV
RID On-Chip Input Termination
Resistance
CS = 0V, VICM = 1.2V
CS = OVDD
l90 106
10
125 Ω
MΩ
VICM Common-Mode Input Voltage l0.3 1.2 2.2 V
IICM Common-Mode Input Current VIN+ = VIN– = 0V to OVDD l–10 10 μA
VOD Differential Output Voltage RL = 100Ω Differential Termination l275 350 425 mV
VOCM Common-Mode Output Voltage RL = 100Ω Differential Termination l1.1 1.2 1.3 V
IOZ Hi-Z Output Leakage Current VOUT = 0V to OVDD l–10 10 μA
The l denotes the specifications which apply over the
full operating temperature range, otherwise specifications are at TA = 25°C. (Note 9)
power requireMenTs
SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS
VCC Supply Voltage l0 38 V
VEE Supply Voltage l–16.5 0 V
VCC − VEE Supply Voltage Difference l10 38 V
VDD Supply Voltage l4.75 5.00 5.25 V
IVCC Supply Current 200ksps Sample Rate, 8 Channels Enabled
Acquisition Mode
Nap Mode
Power Down Mode
l
l
l
l
1.8
3.8
0.7
1
2.2
4.5
0.9
15
mA
mA
mA
μA
IVEE Supply Current 200ksps Sample Rate, 8 Channels Enabled
Acquisition Mode
Nap Mode
Power Down Mode
l
l
l
l
–2.8
–4.9
–1.1
–15
–2.2
–4.0
–0.8
–1
mA
mA
mA
μA
CMOS I/O Mode
OVDD Supply Voltage l1.71 5.25 V
IVDD Supply Current 200ksps Sample Rate, 8 Channels Enabled
200ksps Sample Rate, 8 Channels Enabled, VREFBUF = 5V (Note 15)
Acquisition Mode
Nap Mode
Power Down Mode (C-Grade and I-Grade)
Power Down Mode (H-Grade)
l
l
l
l
l
l
15.2
13.4
1.6
1.4
65
65
17.5
15.4
2.1
1.9
175
450
mA
mA
mA
mA
μA
µA
The l denotes the specifications which apply over the full operating temperature
range, otherwise specifications are at TA = 25°C. (Note 9)
LTC2348-18
6
234818fa
For more information www.linear.com/LTC2348-18
aDc TiMing characTerisTics
SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS
fSMPL Maximum Sampling Frequency 8 Channels Enabled
7 Channels Enabled
6 Channels Enabled
5 Channels Enabled
4 Channels Enabled
3 Channels Enabled
2 Channels Enabled
1 Channel Enabled
l
l
l
l
l
l
l
l
200
225
266
300
375
450
625
1000
ksps
ksps
ksps
ksps
ksps
ksps
ksps
ksps
tCYC Time Between Conversions 8 Channels Enabled, fSMPL = 200ksps
7 Channels Enabled, fSMPL = 225ksps
6 Channels Enabled, fSMPL = 266ksps
5 Channels Enabled, fSMPL = 300ksps
4 Channels Enabled, fSMPL = 375ksps
3 Channels Enabled, fSMPL = 450ksps
2 Channels Enabled, fSMPL = 625ksps
1 Channel Enabled, fSMPL = 1000ksps
l
l
l
l
l
l
l
l
5000
4444
3750
3333
2666
2222
1600
1000
ns
ns
ns
ns
ns
ns
ns
ns
tCONV Conversion Time N Channels Enabled, 1 ≤ N ≤ 8 l450•N500•N550•N ns
tACQ Acquisition Time
(tACQ = tCYC – tCONV – tBUSYLH)8 Channels Enabled, fSMPL = 200ksps
7 Channels Enabled, fSMPL = 225ksps
6 Channels Enabled, fSMPL = 266ksps
5 Channels Enabled, fSMPL = 300ksps
4 Channels Enabled, fSMPL = 375ksps
3 Channels Enabled, fSMPL = 450ksps
2 Channels Enabled, fSMPL = 625ksps
1 Channel Enabled, fSMPL = 1000ksps
l
l
l
l
l
l
l
l
570
564
420
553
436
542
470
420
980
924
730
813
646
702
580
480
ns
ns
ns
ns
ns
ns
ns
ns
The l denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C. (Note 9)
SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS
IOVDD Supply Current 200ksps Sample Rate, 8 Channels Enabled (CL = 25pF)
Acquisition or Nap Mode
Power Down Mode
l
l
l
1.6
1
1
2.6
20
20
mA
μA
μA
PDPower Dissipation 200ksps Sample Rate, 8 Channels Enabled
Acquisition Mode
Nap Mode
Power Down Mode (C-Grade and I-Grade)
Power Down Mode (H-Grade)
l
l
l
l
l
140
125
30
0.36
0.36
169
152
40
1.4
2.8
mW
mW
mW
mW
mW
LVDS I/O Mode
OVDD Supply Voltage l2.375 5.25 V
IVDD Supply Current 200ksps Sample Rate, 8 Channels Enabled
200ksps Sample Rate, 8 Channels Enabled, VREFBUF = 5V (Note 15)
Acquisition Mode
Nap Mode
Power Down Mode (C-Grade and I-Grade)
Power Down Mode (H-Grade)
l
l
l
l
l
l
17.7
16.1
3.2
3.0
65
65
20.4
18.5
3.8
3.7
175
450
mA
mA
mA
mA
μA
µA
IOVDD Supply Current 200ksps Sample Rate, 8 Channels Enabled (RL = 100Ω)
Acquisition or Nap Mode (RL = 100Ω)
Power Down Mode
l
l
l
7
7
1
8.5
8.0
20
mA
mA
μA
PDPower Dissipation 200ksps Sample Rate, 8 Channels Enabled
Acquisition Mode
Nap Mode
Power Down Mode (C-Grade and I-Grade)
Power Down Mode (H-Grade)
l
l
l
l
l
166
151
55
0.36
0.36
199
180
69
1.4
2.8
mW
mW
mW
mW
mW
power requireMenTs
The l denotes the specifications which apply over the full operating temperature
range, otherwise specifications are at TA = 25°C. (Note 9)
LTC2348-18
7
234818fa
For more information www.linear.com/LTC2348-18
aDc TiMing characTerisTics
The l denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C. (Note 9)
SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS
tCNVH CNV High Time l40 ns
tCNVL CNV Low Time l420 ns
tBUSYLH CNV↑ to BUSY Delay CL = 25pF l30 ns
tQUIET Digital I/O Quiet Time from CNV↑l20 ns
tPDH PD High Time l40 ns
tPDL PD Low Time l40 ns
tWAKE REFBUF Wake-Up Time CREFBUF = 47μF, CREFIN = 0.1μF 200 ms
CMOS I/O Mode
tSCKI SCKI Period (Notes 17, 18) l10 ns
tSCKIH SCKI High Time l4 ns
tSCKIL SCKI Low Time l4 ns
tSSDISCKI SDI Setup Time from SCKI↑(Note 17) l2 ns
tHSDISCKI SDI Hold Time from SCKI↑(Note 17) l1 ns
tDSDOSCKI SDO Data Valid Delay from SCKI↑CL = 25pF (Note 17) l7.5 ns
tHSDOSCKI SDO Remains Valid Delay from SCKI↑CL = 25pF (Note 17) l1.5 ns
tSKEW SDO to SCKO Skew (Note 17) l–1 0 1 ns
tDSDOBUSYL SDO Data Valid Delay from BUSY↓CL = 25pF (Note 17) l0 ns
tEN Bus Enable Time After CS↓(Note 17) l15 ns
tDIS Bus Relinquish Time After CS↑(Note 17) l15 ns
LVDS I/O Mode
tSCKI SCKI Period (Note 19) l4 ns
tSCKIH SCKI High Time (Note 19) l1.5 ns
tSCKIL SCKI Low Time (Note 19) l1.5 ns
tSSDISCKI SDI Setup Time from SCKI (Notes 11, 19) l1.2 ns
tHSDISCKI SDI Hold Time from SCKI (Notes 11, 19) l–0.2 ns
tDSDOSCKI SDO Data Valid Delay from SCKI (Notes 11, 19) l6 ns
tHSDOSCKI SDO Remains Valid Delay from SCKI (Notes 11, 19) l1 ns
tSKEW SDO to SCKO Skew (Note 11) l–0.4 0 0.4 ns
tDSDOBUSYL SDO Data Valid Delay from BUSY↓(Note 11) l0 ns
tEN Bus Enable Time After CS↓l50 ns
tDIS Bus Relinquish Time After CS↑l15 ns
LTC2348-18
8
234818fa
For more information www.linear.com/LTC2348-18
CMOS Timings
0.8 • OVDD
0.2 • OVDD
50% 50%
234818 F01
0.2 • OVDD
0.8 • OVDD
0.2 • OVDD
0.8 • OVDD
tDELAY
tWIDTH
tDELAY
LVDS Timings (Differential)
+200mV
–200mV
0V 0V
234818 F01b
–200mV
+200mV
–200mV
+200mV
tDELAY
tWIDTH
tDELAY
Figure 1. Voltage Levels for Timing Specifications
Note 1: Stresses beyond those listed under Absolute Maximum Ratings
may cause permanent damage to the device. Exposure to any Absolute
Maximum Rating condition for extended periods may affect device
reliability and lifetime.
Note 2: All voltage values are with respect to ground.
Note 3: VDDLBYP is the output of an internal voltage regulator, and should
only be connected to a 2.2μF ceramic capacitor to bypass the pin to GND,
as described in the Pin Functions section. Do not connect this pin to any
external circuitry.
Note 4: When these pin voltages are taken below VEE or above VCC, they
will be clamped by internal diodes. This product can handle input currents
of up to 100mA below VEE or above VCC without latch-up.
Note 5: When these pin voltages are taken below ground or above VDD or
OVDD, they will be clamped by internal diodes. This product can handle
currents of up to 100mA below ground or above VDD or OVDD without
latch-up.
Note 6: –16.5V ≤ VEE ≤ 0V, 0V ≤ VCC ≤ 38V, 10V ≤ (VCC – VEE) ≤ 38V,
VDD=5V, unless otherwise specified.
Note 7: Recommended operating conditions.
Note 8: Exceeding these limits on any channel may corrupt conversion
results on other channels. Refer to Absolute Maximum Ratings section for
pin voltage limits related to device reliability.
Note 9: VCC = 15V, VEE = –15V, VDD = 5V, OVDD=2.5V, fSMPL = 200ksps,
internal reference and buffer, true bipolar input signal drive in bipolar
SoftSpan ranges, unipolar signal drive in unipolar SoftSpan ranges, unless
otherwise specified.
Note 10: Integral nonlinearity is defined as the deviation of a code from a
straight line passing through the actual endpoints of the transfer curve.
The deviation is measured from the center of the quantization band.
Note 11: Guaranteed by design, not subject to test.
Note 12: For bipolar SoftSpan ranges 7, 6, 3, and 2, zero-scale error
is the offset voltage measured from –0.5LSB when the output code
flickers between 00 0000 0000 0000 0000 and 11 1111 1111 1111 1111.
Full-scale error for these SoftSpan ranges is the worst-case deviation of
the first and last code transitions from ideal and includes the effect of
offset error. For unipolar SoftSpan ranges 5, 4, and 1, zero-scale error is
the offset voltage measured from 0.5LSB when the output code flickers
between 00 0000 0000 0000 0000 and 00 0000 0000 0000 0001. Full-
scale error for these SoftSpan ranges is the worst-case deviation of the
last code transition from ideal and includes the effect of offset error.
Note 13: All specifications in dB are referred to a full-scale input in the
relevant SoftSpan input range, except for crosstalk, which is referred to
the crosstalk injection signal amplitude.
Note 14: Temperature coefficient is calculated by dividing the maximum
change in output voltage by the specified temperature range.
Note 15: When REFBUF is overdriven, the internal reference buffer must
be disabled by setting REFIN = 0V.
Note 16: IREFBUF varies proportionally with sample rate and the number of
active channels.
Note 17: Parameter tested and guaranteed at OVDD = 1.71V, OVDD = 2.5V,
and OVDD = 5.25V.
Note 18: A tSCKI period of 10ns minimum allows a shift clock frequency of
up to 100MHz for rising edge capture.
Note 19: VICM = 1.2V, VID = 350mV for LVDS differential input pairs.
aDc TiMing characTerisTics
LTC2348-18
9
234818fa
For more information www.linear.com/LTC2348-18
Typical perForMance characTerisTics
Integral Nonlinearity
vs Output Code and Range
Integral Nonlinearity
vs Output Code and Range
Integral Nonlinearity
vs Output Code and Range
Integral Nonlinearity
vs Output Code DC Histogram (Zero-Scale) DC Histogram (Near Full-Scale)
Integral Nonlinearity
vs Output Code and Channel
Integral Nonlinearity
vs Output Code and Channel
Differential Nonlinearity
vs Output Code and Channel
±10.24V RANGE
TRUE BIPOLAR DRIVE (IN– = 0V)
ALL CHANNELS
OUTPUT CODE
–131072
–65536
0
65536
131072
–2.0
–1.5
–1.0
–0.5
0
0.5
1.0
1.5
2.0
INL ERROR (LSB)
vs Output Code
Integral Nonlinearity
234818 G01
±10.24V RANGE
FULLY DIFFERENTIAL DRIVE (IN– = –IN+)
ALL CHANNELS
OUTPUT CODE
–131072
–65536
0
65536
131072
–2.0
–1.5
–1.0
–0.5
0
0.5
1.0
1.5
2.0
INL ERROR (LSB)
vs Output Code
Integral Nonlinearity
234818 G02
ALL RANGES
ALL CHANNELS
OUTPUT CODE
0
65536
131072
196608
262144
–0.5
–0.4
–0.3
–0.2
–0.1
0.0
0.1
0.2
0.3
0.4
0.5
DNL ERROR (LSB)
vs Output Code
Differential Nonlinearity
234818 G03
±10.24V AND ±10V
RANGES
±5.12V AND ±5V
RANGES
TRUE BIPOLAR DRIVE (IN– = 0V)
ONE CHANNEL
OUTPUT CODE
–131072
–65536
0
65536
131072
–2.0
–1.5
–1.0
–0.5
0
0.5
1.0
1.5
2.0
INL ERROR (LSB)
vs Output Code
Integral Nonlinearity
234818 G04
±10.24V, ±10V,
±5.12V, AND ±5V
RANGES
FULLY DIFFERENTIAL DRIVE (IN– = –IN+)
ONE CHANNEL
OUTPUT CODE
–131072
–65536
0
65536
131072
–2.0
–1.5
–1.0
–0.5
0
0.5
1.0
1.5
2.0
INL ERROR (LSB)
vs Output Code
Integral Nonlinearity
234818 G05
UNIPOLAR DRIVE (IN– = 0V)
ONE CHANNEL
0V TO 5.12V
RANGE
0V TO 10.24V AND 0V TO 10V
RANGES
OUTPUT CODE
0
65536
131072
196608
262144
–2.0
–1.5
–1.0
–0.5
0
0.5
1.0
1.5
2.0
INL ERROR (LSB)
vs Output Code
Integral Nonlinearity
234818 G06
±10.24V RANGE
σ = 1.3
CODE
–6
–4
–2
0
2
4
6
0
10000
20000
30000
40000
50000
60000
70000
80000
90000
COUNTS
DC Histogram (Zero-Scale)
234818 G08
±10.24V RANGE
σ = 1.4
CODE
131056
131059
131062
131065
131068
0
10000
20000
30000
40000
50000
60000
70000
80000
90000
COUNTS
DC Histogram (Near Full–Scale)
234818 G09
TA = 25°C, VCC=+15V, VEE=–15V, VDD=5V,
OVDD=2.5V, Internal Reference and Buffer (VREFBUF =4.096V), fSMPL = 200ksps, unless otherwise noted.
TRUE BIPOLAR DRIVE (IN
–
= 0V)
±10.24V RANGE
ARBITRARY DRIVE
IN
+
/IN
–
COMMON MODE
SWEPT –10.24V to 10.24V
OUTPUT CODE
–131072
–65536
0
65536
131072
–2.0
–1.5
–1.0
–0.5
0
0.5
1.0
1.5
2.0
INL ERROR (LSB)
vs Output Code
Integral Nonlinearity
234818 G07
LTC2348-18
10
234818fa
For more information www.linear.com/LTC2348-18
Typical perForMance characTerisTics
32k Point FFT fSMPL = 200kHz,
fIN=2kHz
SNR, SINAD vs VREFBUF,
fIN=2kHz
THD, Harmonics vs VREFBUF,
fIN=2kHz
SNR, SINAD
vs Input Frequency
THD, Harmonics
vs Input Frequency
THD, Harmonics vs Input
Common Mode, fIN = 2kHz
32k Point FFT fSMPL=200kHz,
fIN=2kHz
32k Point FFT fSMPL=200kHz,
fIN=2kHz
32k Point Arbitrary Tw o -Tone FFT
fSMPL=200kHz, IN+=–7dBFS 2kHz
Sine, IN–=–7dBFS 3.1kHz Sine
SNR = 96.7dB
THD = –109dB
SINAD = 96.5dB
SFDR = 110dB
±10.24V RANGE
TRUE BIPOLAR DRIVE (IN– = 0V)
FREQUENCY (kHz)
0
20
40
60
80
100
–180
–160
–140
–120
–100
–80
–60
–40
–20
0
AMPLITUDE (dBFS)
f
IN
= 2kHz
32k Point FFT f
SMPL
= 200kHz,
234818 G10
SNR = 96.7dB
THD = –119dB
SINAD = 96.6dB
SFDR = 120dB
FULLY DIFFERENTIAL DRIVE (IN– = –IN+)
±10.24V RANGE
FREQUENCY (kHz)
0
20
40
60
80
100
–180
–160
–140
–120
–100
–80
–60
–40
–20
0
AMPLITUDE (dBFS)
f
IN
= 2kHz
32k Point FFT f
SMPL
= 200kHz,
234818 G11
SFDR = 119dB
±10.24V RANGE
ARBITRARY DRIVE
6.2kHz
SNR = 96.7dB
FREQUENCY (kHz)
0
20
40
60
80
100
–180
–160
–140
–120
–100
–80
–60
–40
–20
0
AMPLITUDE (dBFS)
IN
–
= –7dBFS 3.1kHz Sine
200kHz, IN
+
= –7dBFS 2kHz Sine,
32k Point Two-Tone FFT f
SMPL
=
234818 G12
SNR = 93.2dB
THD = –113dB
SINAD = 93.2dB
SFDR = 114dB
TRUE BIPOLAR DRIVE (IN
–
= 0V)
±5.12V RANGE
FREQUENCY (kHz)
0
20
40
60
80
100
–180
–160
–140
–120
–100
–80
–60
–40
–20
0
AMPLITUDE (dBFS)
f
IN
= 2kHz
32k Point FFT f
SMPL
= 200kHz,
234818 G13
SNR
SINAD
REFBUF VOLTAGE (V)
2.5
3
3.5
4
4.5
5
90.0
92.0
94.0
96.0
98.0
100.0
SNR, SINAD (dBFS)
SNR, SINAD vs V
REFBUF
, f
IN
= 2kHz
234818 G14
TRUE BIPOLAR DRIVE (IN– = 0V)
±2.5 • VREFBUF RANGE
TRUE BIPOLAR DRIVE (IN– = 0V)
±2.5 • VREFBUF RANGE
THD
2ND
3RD
REFBUF VOLTAGE (V)
2.5
3
3.5
4
4.5
5
–130.0
–125.0
–120.0
–115.0
–110.0
–105.0
–100.0
THD, HARMONICS (dBFS)
f
IN
= 2kHz
THD, Harmonics vs V
REFBUF
,
234818 G15
2ND
THD
3RD
2V
P-P
FULLY DIFFERENTIAL DRIVE
–14.5V ≤ V
CM
≤ 10.5V
±10.24V RANGE
INPUT COMMON MODE (V)
–15
–10
–5
0
5
10
15
–160
–140
–120
–100
–80
–60
–40
–20
0
THD, HARMONICS (dBFS)
Common Mode, f
IN
= 2kHz
THD, Harmonics vs Input
234818 G18
TA = 25°C, VCC=+15V, VEE=–15V, VDD=5V,
OVDD=2.5V, Internal Reference and Buffer (VREFBUF =4.096V), fSMPL = 200ksps, unless otherwise noted.
TRUE BIPOLAR DRIVE (IN
–
= 0V)
±10.24V RANGE
SNR
SINAD
FREQUENCY (Hz)
100
1k
10k
100k
78.0
82.0
86.0
90.0
94.0
98.0
102.0
SNR, SINAD (dBFS)
vs Input Frequency
SNR, SINAD
234818 G16
TRUE BIPOLAR DRIVE (IN
–
= 0V)
±10.24V RANGE
THD
2ND
3RD
FREQUENCY (Hz)
100
1k
10k
100k
–130.0
–120.0
–110.0
–100.0
–90.0
–80.0
–70.0
THD, HARMONICS (dBFS)
vs Input Frequency
THD, Harmonics
234818 G17
LTC2348-18
11
234818fa
For more information www.linear.com/LTC2348-18
Typical perForMance characTerisTics
SNR, SINAD vs Temperature,
fIN=2kHz
THD, Harmonics vs Temperature,
fIN = 2kHz INL, DNL vs Temperature
Positive Full-Scale Error vs
Temperature and Channel
Negative Full-Scale Error vs
Temperature and Channel
Zero-Scale Error vs
Temperature and Channel
SNR, SINAD vs Input Level,
fIN=2kHz
CMRR vs Input Frequency
and Channel
Crosstalk vs Input Frequency
and Channel
SNR
SINAD
INPUT LEVEL (dBFS)
–40
–30
–20
–10
0
96.0
96.5
97.0
97.5
98.0
SNR, SINAD (dBFS)
f
IN
= 2kHz
SNR, SINAD vs Input Level,
234818 G19
±10.24V RANGE
TRUE BIPOLAR DRIVE (IN– = 0V)
IN
+
= IN
–
= 18V
P-P
SINE
ALL CHANNELS
FREQUENCY (Hz)
10
100
1k
10k
100k
1M
65.0
75.0
85.0
95.0
105.0
115.0
125.0
135.0
CMRR (dB)
CMRR vs Input Frequency
234818 G20
±10.24V RANGE
CH2
CH1
IN0– = 18VP-P SINE
ALL CHANNELS CONVERTING
IN0
+
= 0V
±10.24V RANGE
CH7
FREQUENCY (Hz)
10
100
1k
10k
100k
1M
–135.0
–130.0
–125.0
–120.0
–115.0
–110.0
–105.0
–100.0
–95.0
–90.0
–85.0
–80.0
CROSSTALK (dB)
Crosstalk vs Input Frequency
234818 G21
SNR
SINAD
TEMPERATURE (°C)
–55
–35
–15
5
25
45
65
85
105
125
94.0
94.5
95.0
95.5
96.0
96.5
97.0
97.5
98.0
SNR, SINAD (dBFS)
f
IN
= 2kHz
SNR, SINAD vs Temperature,
234818 G22
±10.24V RANGE
TRUE BIPOLAR DRIVE (IN– = 0V)
THD
2ND
3RD
TEMPERATURE (°C)
–55
–35
–15
5
25
45
65
85
105
125
–125.0
–120.0
–115.0
–110.0
–105.0
–100.0
–95.0
THD, HARMONICS (dBFS)
f
IN
= 2kHz
THD, Harmonics vs Temperature,
234818 G23
±10.24V RANGE
TRUE BIPOLAR DRIVE (IN– = 0V)
MAX INL
MIN INL
MAX DNL
MIN DNL
TEMPERATURE (°C)
–55
–35
–15
5
25
45
65
85
105
125
–2.0
–1.5
–1.0
–0.5
0
0.5
1.0
1.5
2.0
INL, DNL ERROR (LSB)
INL/DNL vs Temperature
234818 G24
±10.24V RANGE
TRUE BIPOLAR DRIVE (IN– = 0V)
TEMPERATURE (°C)
–55
–35
–15
5
25
45
65
85
105
125
–0.100
–0.075
–0.050
–0.025
0.000
0.025
0.050
0.075
0.100
FULL–SCALE ERROR (%)
vs Temperature
Positive Full–Scale Error
234818 G25
±10.24V RANGE
ALL CHANNELS
TEMPERATURE (°C)
–55
–35
–15
5
25
45
65
85
105
125
–0.100
–0.075
–0.050
–0.025
0.000
0.025
0.050
0.075
0.100
FULL–SCALE ERROR (%)
vs Temperature
Negative Full–Scale Error
234818 G26
±10.24V RANGE
ALL CHANNELS
±10.24V RANGE
ALL CHANNELS
TEMPERATURE (°C)
–55
–35
–15
5
25
45
65
85
105
125
–5
–4
–3
–2
–1
0
1
2
3
4
5
ZERO-SCALE ERROR (LSB)
Zero-Scale Error vs Temperature
234818 G27
TA = 25°C, VCC=+15V, VEE=–15V, VDD=5V,
OVDD=2.5V, Internal Reference and Buffer (VREFBUF =4.096V), fSMPL = 200ksps, unless otherwise noted.
LTC2348-18
12
234818fa
For more information www.linear.com/LTC2348-18
Typical perForMance characTerisTics
Internal Reference Output
vs Temperature Supply Current vs Sampling Rate
Step Response
(Large-Signal Settling)
Power Dissipation vs Sampling
Rate, N-Channels Enabled
Step Response
(Fine Settling)
Supply Current vs Temperature
Power-Down Current
vs Temperature PSRR vs Frequency
Offset Error
vs Input Common Mode
I
OVDD
I
VDD
I
VEE
I
VCC
TEMPERATURE (°C)
–55
–35
–15
5
25
45
65
85
105
125
–4
–2
0
2
4
6
8
10
12
14
16
18
20
SUPPLY CURRENT (mA)
Supply Current vs Temperature
234818 G28
I
OVDD
I
VDD
–I
VEE
I
VCC
TEMPERATURE (°C)
–55
–35
–15
5
25
45
65
85
105
125
0.01
0.1
1
10
100
1000
POWER-DOWN CURRENT (µA)
vs Temperature
Power-Down Current
234818 G29
TA = 25°C, VCC=+15V, VEE=–15V, VDD=5V,
OVDD=2.5V, Internal Reference and Buffer (VREFBUF =4.096V), fSMPL = 200ksps, unless otherwise noted.
V
CC
OV
DD
V
EE
V
DD
IN
+
= IN
–
= 0V
FREQUENCY (Hz)
10
100
1k
10k
100k
1M
50
60
70
80
90
100
110
120
130
140
150
PSRR (dB)
234818 G30
WITH NAP MODE
t
CNVL
= 420ns
N = 8
N = 1
N = 2
N = 4
SAMPLING FREQUENCY (kHz)
0
200
400
600
800
1000
0
20
40
60
80
100
120
140
160
180
POWER DISSIPATION (mW)
234818 G34
V
CC
= 21.5V, V
EE
= –16.5V
V
CM
= –16.5V to 17.5V
V
CC
= 38V, V
EE
= 0V
V
CM
= 0V to 34V
±10.24V RANGE
INPUT COMMON MODE (V)
–16.5
0
17.5
34
–2.0
–1.5
–1.0
–0.5
0
0.5
1.0
1.5
2.0
OFFSET ERROR (LSB)
234818 G31
15 UNITS
TEMPERATURE (°C)
–55
–35
–15
5
25
45
65
85
105
125
2.045
2.046
2.047
2.048
2.049
2.050
2.051
INTERNAL REFERENCE OUTPUT (V)
234818 G32
I
OVDD
I
VDD
I
VEE
I
VCC
WITH NAP MODE
t
CNVL
= 1µs
SAMPLING FREQUENCY (kHz)
0
40
80
120
160
200
–4
–2
0
2
4
6
8
10
12
14
16
18
SUPPLY CURRENT (mA)
234818 G33
±10.24V RANGE
IN
+
= 200.0061kHz SQUARE WAVE
IN
–
= 0V
SETTLING TIME (ns)
–100
0
100
200
300
400
500
600
700
800
900
–131072
–98304
–65536
–32768
0
32768
65536
98304
131072
OUTPUT CODE (LSB)
234818 G35
±10.24V RANGE
IN
+
= 200.0061kHz
SQUARE WAVE
IN
–
= 0V
SETTLING TIME (ns)
–100
0
100
200
300
400
500
600
700
800
900
–250
–200
–150
–100
–50
0
50
100
150
200
250
DEVIATION FROM FINAL VALUE (LSB)
234818 G36
LTC2348-18
13
234818fa
For more information www.linear.com/LTC2348-18
pin FuncTions
Pins that are the Same for All Digital I/O Modes
IN0+ to IN7+, IN0− to IN7− (Pins 1, 2, 3, 4, 5, 6, 7, 8, 9,
10, 11, 12, 13, 14, 47, and 48): Positive and Negative
Analog Inputs, Channels 0 to 7. The converter simultane-
ously samples and digitizes (VIN+ – VIN–) for all channels.
Wide input common mode range (VEE ≤ VCM ≤ VCC – 4V)
and high common mode rejection allow the inputs to ac-
cept a wide variety of signal swings. Full-scale input range
is determined by the channel’s SoftSpan configuration.
GND (Pins 15, 18, 20, 30, 41, 44, 46): Ground. Solder
all GND pins to a solid ground plane.
VCC (Pin 16): Positive High Voltage Power Supply. The
range of VCC is 0V to 38V with respect to GND and 10V to
38V with respect to VEE. Bypass VCC to GND close to the
pin with a 0.1μF ceramic capacitor. In applications where
VCC is shorted to GND this capacitor may be omitted.
VEE (Pins 17, 45): Negative High Voltage Power Supply.
The range of VEE is 0V to –16.5V with respect to GND and
–10V to –38V with respect to VCC. Connect Pins 17 and 45
together and bypass the VEE network to GND close to Pin
17 with a 0.1μF ceramic capacitor. In applications where
VEE is shorted to GND this capacitor may be omitted.
REFIN (Pin 19): Bandgap Reference Output/Reference
Buffer Input. An internal bandgap reference nominally
outputs 2.048V on this pin. An internal reference buffer
amplifies VREFIN to create the converter master reference
voltage VREFBUF = 2•VREFIN on the REFBUF pin. When
using the internal reference, bypass REFIN to GND (Pin
20) close to the pin with a 0.1μF ceramic capacitor to filter
the bandgap output noise. If more accuracy is desired,
overdrive REFIN with an external reference in the range
of 1.25V to 2.2V.
REFBUF (Pin 21): Internal Reference Buffer Output. An
internal reference buffer amplifies VREFIN to create the
converter master reference voltage VREFBUF = 2•VREFIN on
this pin, nominally 4.096V when using the internal bandgap
reference. Bypass REFBUF to GND (Pin 20) close to the
pin with a 47μF ceramic capacitor. The internal reference
buffer may be disabled by grounding its input at REFIN.
With the buffer disabled, overdrive REFBUF with an ex-
ternal reference voltage in the range of 2.5V to 5V. When
using the internal reference buffer, limit the loading of any
external circuitry connected to REFBUF to less than 10µA.
Using a high input impedance amplifier to buffer VREFBUF
to any external circuits is recommended.
PD (Pin 22): Power Down Input. When this pin is brought
high, the LTC2348-18 is powered down and subsequent
conversion requests are ignored. If this occurs during a
conversion, the device powers down once the conversion
completes. If this pin is brought high twice without an
intervening conversion, an internal global reset is initi-
ated, equivalent to a power-on-reset event. Logic levels
are determined by OVDD.
LVDS/CMOS (Pin 23): I/O Mode Select. Tie this pin to OVDD
to select LVDS I/O mode, or to ground to select CMOS I/O
mode. Logic levels are determined by OVDD.
CNV (Pin 24): Conversion Start Input. A rising edge on
this pin puts the internal sample-and-holds into the hold
mode and initiates a new conversion. CNV is not gated
by CS, allowing conversions to be initiated independent
of the state of the serial I/O bus.
BUSY (Pin 38): Busy Output. The BUSY signal indicates
that a conversion is in progress. This pin transitions low-
to-high at the start of each conversion and stays high until
the conversion is complete. Logic levels are determined
by OVDD.
VDDLBYP (Pin 40): Internal 2.5V Regulator Bypass Pin. The
voltage on this pin is generated via an internal regulator
operating off of VDD. This pin must be bypassed to GND
close to the pin with a 2.2μF ceramic capacitor. Do not
connect this pin to any external circuitry.
VDD (Pins 42, 43): 5V Power Supply. The range of VDD
is 4.75V to 5.25V. Connect Pins 42 and 43 together and
bypass the VDD network to GND with a shared 0.1μF
ceramic capacitor close to the pins.
LTC2348-18
14
234818fa
For more information www.linear.com/LTC2348-18
pin FuncTions
CMOS I/O Mode
SDO0 to SDO7 (Pins 25, 26, 27, 28, 33, 34, 35, and 36):
CMOS Serial Data Outputs, Channels 0 to 7. The most
recent conversion result along with channel configuration
information is clocked out onto the SDO pins on each ris-
ing edge of SCKI. Output data formatting is described in
the Digital Interface section. Leave unused SDO outputs
unconnected. Logic levels are determined by OVDD.
SCKI (Pin 29): CMOS Serial Clock Input. Drive SCKI with
the serial I/O clock. SCKI rising edges latch serial data in
on SDI and clock serial data out on SDO0 to SDO7. For
standard SPI bus operation, capture output data at the
receiver on rising edges of SCKI. SCKI is allowed to idle
either high or low. Logic levels are determined by OVDD.
OVDD (Pin 31): I/O Interface Power Supply. In CMOS I/O
mode, the range of OVDD is 1.71V to 5.25V. Bypass OVDD
to GND (Pin 30) close to the pin with a 0.1μF ceramic
capacitor.
SCKO (Pin 32): CMOS Serial Clock Output. SCKI rising
edges trigger transitions on SCKO that are skew-matched
to the serial output data streams on SDO0 to SDO7. The
resulting SCKO frequency is half that of SCKI. Rising and
falling edges of SCKO may be used to capture SDO data at
the receiver (FPGA) in double data rate (DDR) fashion. For
standard SPI bus operation, SCKO is not used and should
be left unconnected. SCKO is forced low at the falling edge
of BUSY. Logic levels are determined by OVDD.
SDI (Pin 37): CMOS Serial Data Input. Drive this pin with the
desired 24-bit SoftSpan configuration word (see Table1a),
latched on the rising edges of SCKI. If all channels will be
configured to operate only in SoftSpan 7, tie SDI to OVDD.
Logic levels are determined by OVDD.
CS (Pin 39): Chip Select Input. The serial data I/O bus is
enabled when CS is low and is disabled and Hi-Z when
CS is high. CS also gates the external shift clock, SCKI.
Logic levels are determined by OVDD.
LVDS I/O Mode
SDO0, SDO7, SDI (Pins 25, 36, and 37): CMOS Serial
Data I/O. In LVDS I/O mode, these pins are Hi-Z.
SDI+, SDI– (Pins 26 and 27): LVDS Positive and Negative
Serial Data Input. Differentially drive SDI+/SDI– with the
desired 24-bit SoftSpan configuration word (see Table
1a), latched on both the rising and falling edges of SCKI+/
SCKI–. The SDI+/SDI– input pair is internally terminated
with a 100Ω differential resistor when CS is low.
SCKI+, SCKI– (Pins 28 and 29): LVDS Positive and Negative
Serial Clock Input. Differentially drive SCKI+/SCKI– with
the serial I/O clock. SCKI+/SCKI– rising and falling edges
latch serial data in on SDI+/SDI– and clock serial data out
on SDO+/SDO–. Idle SCKI+/SCKI– low, including when
transitioning CS. The SCKI+/SCKI– input pair is internally
terminated with a 100Ω differential resistor when CS = 0.
OVDD (Pin 31): I/O Interface Power Supply. In LVDS I/O
mode, the range of OVDD is 2.375V to 5.25V. Bypass OVDD
to GND (Pin 30) close to the pin with a 0.1μF ceramic
capacitor.
SCKO+, SCKO– (Pins 32 and 33): LVDS Positive and
Negative Serial Clock Output. SCKO+/SCKO– outputs a
copy of the input serial I/O clock received on SCKI+/SCKI–,
skew-matched with the serial output data stream on SDO+/
SDO–. Use the rising and falling edges of SCKO+/SCKO–
to capture SDO+/SDO– data at the receiver (FPGA). The
SCKO+/SCKO– output pair must be differentially terminated
with a 100Ω resistor at the receiver (FPGA).
SDO+, SDO– (Pins 34 and 35): LVDS Positive and Nega-
tive Serial Data Output. The most recent conversion result
along with channel configuration information is clocked
out onto SDO+/SDO– on both rising and falling edges of
SCKI+/SCKI–, beginning with channel 0. The SDO+/SDO–
output pair must be differentially terminated with a 100Ω
resistor at the receiver (FPGA).
CS (Pin 39): Chip Select Input. The serial data I/O bus is
enabled when CS is low, and is disabled and Hi-Z when
CS is high. CS also gates the external shift clock, SCKI+/
SCKI–. The internal 100Ω differential termination resistors
on the SCKI+/SCKI– and SDI+/SDI– input pairs are disabled
when CS is high. Logic levels are determined by OVDD.
LTC2348-18
15
234818fa
For more information www.linear.com/LTC2348-18
conFiguraTion Tables
Table 1a. SoftSpan Configuration Table. Use This Table with Table 1b to Choose Independent Binary SoftSpan Codes SS[2:0] for Each
Channel Based on Desired Analog Input Range. Combine SoftSpan Codes to Form 24-Bit SoftSpan Configuration Word S[23:0]. Use
Serial Interface to Write SoftSpan Configuration Word to LTC2348-18, as shown in Figure 19
BINARY SoftSpan CODE
SS[2:0] ANALOG INPUT RANGE FULL SCALE RANGE BINARY FORMAT OF
CONVERSION RESUL
T
111 ±2.5 • VREFBUF 5 • VREFBUF Tw o ’s Complement
110 ±2.5 • VREFBUF/1.024 5 • VREFBUF/1.024 Tw o ’s Complement
101 0V to 2.5 • VREFBUF 2.5 • VREFBUF Straight Binary
100 0V to 2.5 • VREFBUF/1.024 2.5 • VREFBUF/1.024 Straight Binary
011 ±1.25 • VREFBUF 2.5 • VREFBUF Tw o ’s Complement
010 ±1.25 • VREFBUF/1.024 2.5 • VREFBUF/1.024 Tw o ’s Complement
001 0V to 1.25 • VREFBUF 1.25 • VREFBUF Straight Binary
000 Channel Disabled Channel Disabled All Zeros
Table 1b. Reference Configuration Table. The LTC2348-18 Supports Three Reference Configurations. Analog Input Range Scales with
the Converter Master Reference Voltage, VREFBUF
REFERENCE CONFIGURATION VREFIN VREFBUF BINARY SoftSpan CODE
SS[2:0] ANALOG INPUT RANGE
Internal Reference with
Internal Buffer 2.048V 4.096V
111 ±10.24V
110 ±10V
101 0V to 10.24V
100 0V to 10V
011 ±5.12V
010 ±5V
001 0V to 5.12V
External Reference with
Internal Buffer
(REFIN Pin Externally
Overdriven)
1.25V
(Min Value) 2.5V
111 ±6.25V
110 ±6.104V
101 0V to 6.25V
100 0V to 6.104V
011 ±3.125V
010 ±3.052V
001 0V to 3.125V
2.2V
(Max Value) 4.4V
111 ±11V
110 ±10.742V
101 0V to 11V
100 0V to 10.742V
011 ±5.5V
010 ±5.371V
001 0V to 5.5V
LTC2348-18
16
234818fa
For more information www.linear.com/LTC2348-18
REFERENCE CONFIGURATION VREFIN VREFBUF BINARY SoftSpan CODE
SS[2:0] ANALOG INPUT RANGE
External Reference
Unbuffered
(REFBUF Pin
Externally Overdriven,
REFIN Pin Grounded)
0V 2.5V
(Min Value)
111 ±6.25V
110 ±6.104V
101 0V to 6.25V
100 0V to 6.104V
011 ±3.125V
010 ±3.052V
001 0V to 3.125V
0V 5V
(Max Value)
111 ±12.5V
110 ±12.207V
101 0V to 12.5V
100 0V to 12.207V
011 ±6.25V
010 ±6.104V
001 0V to 6.25V
conFiguraTion Tables
Table 1b. Reference Configuration Table (Continued). The LTC2348-18 Supports Three Reference Configurations. Analog Input Range
Scales with the Converter Master Reference Voltage, VREFBUF
LTC2348-18
17
234818fa
For more information www.linear.com/LTC2348-18
FuncTional block DiagraM
SDO0
SDO7
SCKO
SDI
SCKI
CS
IN0+
IN0–
BUSY
18-BIT
SAR ADC
CMOS
SERIAL
I/O
INTERFACE
234818 BD01
18 BITS
REFERENCE
BUFFER
REFBUFREFINGND
VCC
VEE
VDDLBYP
VDD OVDD
LTC2348-18
CONTROL
LOGIC
2.048V
REFERENCE
2.5V
REGULATOR
LVDS/CMOSPDCNV
S/H
IN1+
IN1–S/H
IN2+
IN2–S/H
IN3+
IN3–S/H
IN4+
IN4–S/H
IN5+
IN5–S/H
IN6+
IN6–S/H
IN7+
IN7–S/H
8-CHANNEL MULTIPLEXER
20k 2×
• • •
SDO+
SDO–
SCKO+
SCKO–
SDI+
SDI–
SCKI+
SCKI–
CS
IN0+
IN0–
BUSY
18-BIT
SAR ADC
LVDS
SERIAL
I/O
INTERFACE
234818 BD02
18 BITS
REFERENCE
BUFFER
REFBUFREFINGND
VCC
VEE
VDDLBYP
VDD OVDD
LTC2348-18
CONTROL
LOGIC
2.048V
REFERENCE
2.5V
REGULATOR
LVDS/CMOSPDCNV
S/H
IN1+
IN1–S/H
IN2+
IN2–S/H
IN3+
IN3–S/H
IN4+
IN4–S/H
IN5+
IN5–S/H
IN6+
IN6–S/H
IN7+
IN7–S/H
8-CHANNEL MULTIPLEXER
20k 2×
LVDS I/O Mode
CMOS I/O Mode
LTC2348-18
18
234818fa
For more information www.linear.com/LTC2348-18
TiMing DiagraM
LVDS I/O Mode
S23
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 25 2624
S22 S21 S20 S19 S18 S17 S16 S15 S14 S13 S12 S11 S10 S9 S8 S7 S6 S5 S3S4 S1S2 S0
D17 D16 D15 D14 D13 D12 D11 D10 D9 D8 D7 D6 D5 D4 D3 D2 D1 D0 SS1SS2 SS0 D17 D16 D15
CNV
(CMOS)
CS = PD = 0
234818 TD02
CONVERT
DON’T CARE
ACQUIRE
BUSY
(CMOS)
SCKO
(LVDS)
SDO
(LVDS)
SCKI
(LVDS)
SDI
(LVDS)
DON’T CARE
SAMPLE N
• • •
• • •
• • •
• • •
• • •
• • •
SAMPLE
N + 1
SoftSpan CONFIGURATION WORD FOR CONVERSION N + 1
CHANNEL 0
CONVERSION N
CHANNEL 1
CONVERSION N
CHANNEL 7
CONVERSION N
CONVERSION RESULT CHANNEL ID SoftSpan
186 187 188 189 190 191 192
D0 SS1SS2 SS0 D17
CHANNEL 0
CONVERSION N
CONVERSION
RESULT
CHANNEL ID SoftSpan
C2 C1 C0 C2 C1 C0
CMOS I/O Mode
S23
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
S22 S21 S20 S19 S18 S17 S16 S15 S14 S13 S12 S11 S10 S9 S8 S7 S6 S5 S3S4 S1S2 S0
D17 D16 D15 D14 D13 D12 D11 D10 D9 D8 D7 D6 D5 D4 D3 D2 D1 D0 C2 C1 C0
C2 C1 C0
SS1SS2 SS0 D17
CNV
CS = PD = 0
CONVERT
DON’T CARE
ACQUIREBUSY
SDO7
SCKO
SDO0
SCKI
SDI
DON’T CARE
SAMPLE N SAMPLE N + 1
SoftSpan CONFIGURATION WORD FOR CONVERSION N + 1
CHANNEL 0
CONVERSION N
CHANNEL 1
CONVERSION N
CHANNEL 7
CONVERSION N
CHANNEL 0
CONVERSION N
234818 TD01
CONVERSION RESULT CHANNEL ID SoftSpan CONVERSION RESULT
D17 D16 D15 D14 D13 D12 D11 D10 D9 D8 D7 D6 D5 D4 D3 D2 D1 D0 SS1SS2 SS0 D17DON’T CARE
CONVERSION RESULT CHANNEL ID SoftSpan CONVERSION RESULT
• • •
LTC2348-18
19
234818fa
For more information www.linear.com/LTC2348-18
applicaTions inForMaTion
OVERVIEW
The LTC2348-18 is an 18-bit, low noise 8-channel si-
multaneous sampling successive approximation register
(SAR) ADC with differential, wide common mode range
inputs. The ADC operates from a 5V low voltage supply and
flexible high voltage supplies, nominally ±15V. Using the
integrated low-drift reference and buffer (VREFBUF=4.096V
nominal), each channel of this SoftSpan ADC can be in-
dependently configured on a conversion-by-conversion
basis to accept ±10.24V, 0V to 10.24V, ±5.12V, or 0V to
5.12V signals. The input signal range may be expanded
up to ±12.5V using an external 5V reference. Individual
channels may also be disabled to increase throughput on
the remaining channels.
The wide input common mode range and high CMRR
(118dB typical, VIN+ = VIN– = 18VP-P 200Hz Sine) of the
LTC2348-18 analog inputs allow the ADC to directly digitize
a variety of signals, simplifying signal chain design. The
absolute common mode input range is determined by
the choice of high voltage supplies, which may be biased
asymmetrically around ground and include the ability for
either the positive or negative supply to be tied directly to
ground. This input signal flexibility, combined with ±3LSB
INL, no missing codes at 18-bits, and 96.7dB SNR, makes
the LTC2348-18 an ideal choice for many high voltage
applications requiring wide dynamic range.
The LTC2348-18 supports pin-selectable SPI CMOS (1.8V
to 5V) and LVDS serial interfaces, enabling it to com-
municate equally well with legacy microcontrollers and
modern FPGAs. In CMOS mode, applications may employ
between one and eight lanes of serial output data, allowing
the user to optimize bus width and data throughput. The
LTC2348-18 typically dissipates 140mW when converting
eight analog input channels simultaneously at 200ksps per
channel throughput. Optional nap and power down modes
may be employed to further reduce power consumption
during inactive periods.
CONVERTER OPERATION
The LTC2348-18 operates in two phases. During the ac-
quisition phase, the sampling capacitors in each channel’s
sample-and-hold (S/H) circuit connect to their respective
analog input pins and track the differential analog input
voltage (VIN+ – VIN–). A rising edge on the CNV pin transi-
tions all channels’ S/H circuits from track mode to hold
mode, simultaneously sampling the input signals on all
channels and initiating a conversion. During the conversion
phase, each channel’s sampling capacitors are connected,
one channel at a time, to an 18-bit charge redistribution
capacitor D/A converter (CDAC). The CDAC is sequenced
through a successive approximation algorithm, effectively
comparing the sampled input voltage with binary-weighted
fractions of the channel’s SoftSpan full-scale range (e.g.,
VFSR/2, VFSR/4 … VFSR/262144) using a differential
comparator. At the end of this process, the CDAC output
approximates the channel’s sampled analog input. Once
all channels have been converted in this manner, the ADC
control logic prepares the 18-bit digital output codes from
each channel for serial transfer.
TRANSFER FUNCTION
The LTC2348-18 digitizes each channel’s full-scale voltage
range into 218 levels. In conjunction with the ADC master
reference voltage, VREFBUF, a channel’s SoftSpan configu-
ration determines its input voltage range, full-scale range,
LSB size, and the binary format of its conversion result, as
shown in Tables 1a and 1b. For example, employing the
internal reference and buffer (VREFBUF = 4.096V nominal),
SoftSpan 7 configures a channel to accept a ±10.24V
bipolar analog input voltage range, which corresponds
to a 20.48V full-scale range with a 78.125μV LSB. Other
SoftSpan configurations and reference voltages may be
employed to convert both larger and smaller bipolar and
unipolar input ranges. Conversion results are output in
two’s complement binary format for all bipolar SoftSpan
ranges, and in straight binary format for all unipolar
SoftSpan ranges. The ideal two’s complement transfer
function is shown in Figure 2, while the ideal straight
binary transfer function is shown in Figure 3.
LTC2348-18
20
234818fa
For more information www.linear.com/LTC2348-18
INPUT VOLTAGE (V)
0V
OUTPUT CODE (TWO’S COMPLEMENT)
–1
LSB
234818 F02
011...111
011...110
000...001
000...000
100...000
100...001
111...110
1
LSB
BIPOLAR
ZERO
111...111
FSR/2 – 1LSB–FSR/2
FSR = +FS – –FS
1LSB = FSR/262144
Figure 2. LTC2348-18 Tw o ’s Complement Transfer Function
INPUT VOLTAGE (V)
OUTPUT CODE (STRAIGHT BINARY)
234818 F03
111...111
111...110
100...001
100...000
000...000
000...001
011...110
UNIPOLAR
ZERO
011...111
FSR – 1LSB0V
FSR = +FS
1LSB = FSR/262144
Figure 3. LTC2348-18 Straight Binary Transfer Function
ANALOG INPUTS
Each channel of the LTC2348-18 simultaneously samples
the voltage difference (VIN+ – VIN–) between its analog
input pins over a wide common mode input range while
attenuating unwanted signals common to both input
pins by the common-mode rejection ratio (CMRR) of the
ADC. Wide common mode input range coupled with high
CMRR allows the IN+/IN– analog inputs to swing with
an arbitrary relationship to each other, provided each
pin remains between (VCC – 4V) and VEE. This unique
feature of the LTC2348-18 enables it to accept a wide
variety of signal swings, including traditional classes of
analog input signals such as pseudo-differential unipolar,
applicaTions inForMaTion
pseudo-differential true bipolar, and fully differential,
simplifying signal chain design.
The wide operating range of the high voltage supplies
offers further input common mode flexibility. As long as
the voltage difference limits of 10V ≤ VCC – VEE ≤ 38V
are observed, VCC and VEE may be independently biased
anywhere within their own individual allowed operating
ranges, including the ability for either of the supplies to be
tied directly to ground. This feature enables the common
mode input range of the LTC2348-18 to be tailored to the
specific application’s requirements.
In all SoftSpan ranges, each channel’s analog inputs can
be modeled by the equivalent circuit shown in Figure 4.
At the start of acquisition, the 40pF sampling capacitors
(CIN) connect to the analog input pins IN+/IN– through the
sampling switches, each of which has approximately 600Ω
(RIN) of on-resistance. The initial voltage on both sampling
capacitors at the start of acquisition is approximately equal
to the sampled common-mode voltage (VIN++VIN–)/2
from the prior conversion. The external circuitry connected
to IN+ and IN– must source or sink the charge that flows
through RIN as the sampling capacitors settle from their
initial voltages to the new input pin voltages over the
course of the acquisition interval. During conversion, nap,
and power down modes, the analog inputs draw only a
small leakage current. The diodes at the inputs provide
ESD protection.
IN+
RIN
600Ω
RIN
600Ω
CIN
40pF
CIN
40pF
VCC
VCC
VEE
VEE
BIAS
VOLTAGE
IN–234818 F04
Figure 4. Equivalent Circuit for Differential Analog
Inputs, Single Channel Shown
LTC2348-18
21
234818fa
For more information www.linear.com/LTC2348-18
applicaTions inForMaTion
Bipolar SoftSpan Input Ranges
For channels configured in SoftSpan ranges 7, 6, 3,
or 2, the LTC2348-18 digitizes the differential analog
input voltage (VIN+ – VIN–) over a bipolar span of
±2.5•VREFBUF, ±2.5•VREFBUF/1.024, ±1.25•VREFBUF, or
±1.25•VREFBUF/1.024, respectively, as shown in Table
1a. These SoftSpan ranges are useful for digitizing input
signals where IN+ and IN– swing above and below each
other. Traditional examples include fully differential input
signals, where IN+ and IN– are driven 180 degrees out-of-
phase with respect to each other centered around a common
mode voltage (VIN++VIN–)/2, and pseudo-differential true
bipolar input signals, where IN+ swings above and below
a ground reference level, driven on IN–. Regardless of the
chosen SoftSpan range, the wide common mode input
range and high CMRR of the IN+/IN– analog inputs allow
them to swing with an arbitrary relationship to each other,
provided each pin remains between (VCC – 4V) and VEE.
The output data format for all bipolar SoftSpan ranges is
two’s complement.
Unipolar SoftSpan Input Ranges
For channels configured in SoftSpan ranges 5, 4, or 1, the
LTC2348-18 digitizes the differential analog input voltage
(VIN+ – VIN–) over a unipolar span of 0V to 2.5•VREFBUF, 0V
to 2.5•VREFBUF/1.024, or 0V to 1.25•VREFBUF, respectively,
as shown in Table 1a. These SoftSpan ranges are useful
for digitizing input signals where IN+ remains above IN–.
A traditional example includes pseudo-differential unipolar
input signals, where IN+ swings above a ground reference
level, driven on IN–. Regardless of the chosen SoftSpan
range, the wide common mode input range and high
CMRR of the IN+/IN– analog inputs allow them to swing
with an arbitrary relationship to each other, provided each
pin remains between (VCC – 4V) and VEE. The output data
format for all unipolar SoftSpan ranges is straight binary.
INPUT DRIVE CIRCUITS
The initial voltage on each channel’s sampling capacitors
at the start of acquisition must settle to the new input
pin voltages during the acquisition interval. The external
circuitry connected to IN+ and IN– must source or sink
the charge that flows through RIN as this settling occurs.
The LTC2348-18 sampling network RC time constant of
24ns implies an 18-bit settling time to a full-scale step of
approximately 13•(RIN• CIN)=312ns. The impedance and
self-settling of external circuitry connected to the analog
input pins will increase the overall settling time required.
Low impedance sources can directly drive the inputs of
the LTC2348-18 without gain error, but high impedance
sources should be buffered to ensure sufficient settling
during acquisition and to optimize the linearity and distor-
tion performance of the ADC. Settling time is an important
consideration even for DC input signals, as the voltages on
the sampling capacitors will differ from the analog input
pin voltages at the start of acquisition.
Most applications should use a buffer amplifier to drive the
analog inputs of the LTC2348-18. The amplifier provides
low output impedance, enabling fast settling of the analog
signal during the acquisition phase. It also provides isola-
tion between the signal source and the charge flow at the
analog inputs when entering acquisition.
Input Filtering
The noise and distortion of an input buffer amplifier and
other supporting circuitry must be considered since they
add to the ADC noise and distortion. Noisy input signals
should be filtered prior to the buffer amplifier with a low-
bandwidth filter to minimize noise. The simple one-pole
RC lowpass filter shown in Figure 5 is sufficient for many
applications.
At the output of the buffer, a lowpass RC filter network
formed by the 600Ω sampling switch on-resistance (RIN)
and the 40pF sampling capacitance (CIN) limits the input
bandwidth on each channel to 7MHz, which is fast enough
to allow for sufficient transient settling during acquisition
LTC2348-18
22
234818fa
For more information www.linear.com/LTC2348-18
while simultaneously filtering driver wideband noise. A
buffer amplifier with low noise density should be selected
to minimize SNR degradation over this bandwidth. An
additional filter network may be placed between the buf-
fer output and ADC input to further minimize the noise
contribution of the buffer and reduce disturbances to the
buffer from ADC acquisition transients. A simple one-pole
lowpass RC filter is sufficient for many applications. It is
important that the RC time constant of this filter be small
enough to allow the analog inputs to completely settle to
18-bit resolution within the ADC acquisition time (tACQ),
as insufficient settling can limit INL and THD performance.
Also note that the minimum acquisition time varies with
sampling frequency (fSMPL) and the number of enabled
channels.
High quality capacitors and resistors should be used in
the RC filters since these components can add distortion.
NPO/COG and silver mica type dielectric capacitors have
excellent linearity. Carbon surface mount resistors can
generate distortion from self-heating and from damage
that may occur during soldering. Metal film surface mount
resistors are much less susceptible to both problems.
applicaTions inForMaTion
Buffering Arbitrary and Fully Differential Analog Input
Signals
The wide common mode input range and high CMRR of
the LTC2348-18 allow each channel’s IN+ and IN– pins
to swing with an arbitrary relationship to each other,
provided each pin remains between (VCC – 4V) and VEE.
This unique feature of the LTC2348-18 enables it to accept
a wide variety of signal swings, simplifying signal chain
design. In many applications, connecting a channel’s IN+
and IN– pins directly to the existing signal chain circuitry
will not allow the channel’s sampling network to settle to
18-bit resolution within the ADC acquisition time (tACQ).
In these cases, it is recommended that two unity-gain
buffers be inserted between the signal source and the ADC
input pins, as shown in Figure 6a. Table 2 lists several
amplifier and lowpass filter combinations recommended
for use in this circuit. The LT1469 combines fast settling,
high linearity, and low offset with 5nV/√Hz input-referred
noise density, enabling it to achieve the full ADC data sheet
SNR and THD specifications, as shown in the FFT plots in
Figures6b to 6e. In applications where slightly degraded
LTC2348-18
234818 F05
ONLY CHANNEL 0 SHOWN FOR CLARITY
0V
TRUE BIPOLAR
INPUT SIGNAL
IN0+
IN0–
BUFFER
AMPLIFIER
LOWPASS
SIGNAL FILTER
10nF
160Ω
BW = 100kHz
Figure 5. True Bipolar Signal Chain with Input Filtering
Table 2. Recommended Amplifier and Filter Combinations for the Buffer Circuits in Figures 6a and 9. AC Performance Measured
Using Circuit in Figure 6a, ±10.24V Range
AMPLIFIER RFILT
(Ω)
CFILT
(pF) INPUT SIGNAL DRIVE SNR
(dB)
THD
(dB)
SINAD
(dB)
SFDR
(dB)
½ LT1469 49.9 1000 FULL
Y DIFFERENTIAL 96.7 −119 96.6 120
½ LT1355 100 270 FULLY DIFFERENTIAL 96.5 −119 96.4 120
½ LT1469 49.9 1000 TRUE BIPOLAR 96.7 −109 96.5 110
½ LT1355 100 270 TRUE BIPOLAR 96.5 −106 96.1 108
½ LT1469 0 0 TRUE BIPOLAR 95.7 −109 95.5 110
½ LT1355 0 0 TRUE BIPOLAR 95.9 −106 95.5 108
½ LT1358 100 270 TRUE BIPOLAR 96.7 −108 96.5 110
LTC2348-18
23
234818fa
For more information www.linear.com/LTC2348-18
applicaTions inForMaTion
SFDR = 119dB
±10.24V RANGE
6.2kHz
SNR = 96.7dB
FREQUENCY (kHz)
0
20
40
60
80
100
–180
–160
–140
–120
–100
–80
–60
–40
–20
0
AMPLITUDE (dBFS)
Arbitrary Drive
234818 F06b
Figure 6b. Tw o -Tone Test. IN+ = –7dBFS 2kHz Sine, IN– = –7dBFS
3.1kHz Sine, 32k Point FFT, fSMPL = 200ksps. Circuit Shown in
Figure 6a with LT1469 Amplifiers, RFILT = 49.9Ω, CFILT = 1000pF
SNR = 96.7dB
THD = –119dB
SINAD = 96.6dB
SFDR = 120dB
±10.24V RANGE
FREQUENCY (kHz)
0
20
40
60
80
100
–180
–160
–140
–120
–100
–80
–60
–40
–20
0
AMPLITUDE (dBFS)
Fully Differential Drive
234818 F06c
Figure 6c. IN+/IN– = –1dBFS 2kHz Fully Differential Sine,
VCM=0V, 32k Point FFT, fSMPL = 200ksps. Circuit Shown in
Figure 6a with LT1469 Amplifiers, RFILT = 49.9Ω, CFILT = 1000pF
Figure 6a. Buffering Arbitrary, Fully Differential, True Bipolar, and Unipolar Signals. See Table 2 For
Recommended Amplifier and Filter Combinations
SNR = 96.7dB
THD = –109dB
SINAD = 96.5dB
SFDR = 110dB
±10.24V RANGE
FREQUENCY (kHz)
0
20
40
60
80
100
–180
–160
–140
–120
–100
–80
–60
–40
–20
0
AMPLITUDE (dBFS)
True Bipolar Drive
234818 F06d
Figure 6d. IN+ = –1dBFS 2kHz True Bipolar Sine, IN– = 0V, 32k
Point FFT, fSMPL = 200ksps. Circuit Shown in Figure 6a with
LT1469 Amplifiers, RFILT = 49.9Ω, CFILT = 1000pF
SNR = 90.8dB
THD = –111dB
SINAD = 90.7dB
SFDR = 112dB
0V to 10.24V RANGE
FREQUENCY (kHz)
0
20
40
60
80
100
–180
–160
–140
–120
–100
–80
–60
–40
–20
0
AMPLITUDE (dBFS)
Unipolar Drive
234818 F06e
Figure 6e. IN+ = –1dBFS 2kHz Unipolar Sine, IN– = 0V,
32k Point FFT, fSMPL = 200ksps. Circuit Shown in Figure 6a
with LT1469 Amplifiers, RFILT = 49.9Ω, CFILT = 1000pF
–15V
–15V
15V
15V
LTC2348-18
234818 F06a
ONLY CHANNEL 0 SHOWN FOR CLARITY
+5V
0V
–5V
FULLY
DIFFERENTIAL
+10V
0V
–10V
TRUE BIPOLAR
+10V
0V
–10V
ARBITRARY
+10V
0V
–10V
UNIPOLAR
–
+
–
+
0.1µF
0.1µF
0.1µF
47µF
IN0+
IN0–
AMPLIFIER
AMPLIFIER
VCC
REFINREFBUFVEE
OPTIONAL
LOWPASS FILTERS
CFILT
CFILT
RFILT
RFILT
IN+
IN–
LTC2348-18
24
234818fa
For more information www.linear.com/LTC2348-18
SNR and THD performance is acceptable, it is possible
to drive the LTC2348-18 using the lower-power LT1355.
The LT1355 combines fast settling, good linearity, and
moderate offset with 10nV/√Hz input-referred noise den-
sity, enabling it to drive the LTC2348-18 with only 0.2dB
SNR loss and 3dB THD loss compared with the LT1469.
As shown in Table2, both the LT1469 and LT1355 may be
used without a lowpass filter at a loss of ≤1dB SNR due
to increased wideband noise. For sampling frequencies
with minimum acquisition times (tACQ) under 500ns, use
either the LT1469 or LT1355 without lowpass filtering, or
the LT1358 with lowpass filtering, for the best settling,
linearity, and THD performance.
The two-tone test shown in Figure 6b demonstrates the
arbitrary input drive capability of the LTC2348-18. This test
simultaneously drives IN+ with a −7dBFS 2kHz single-ended
sine wave and IN− with a −7dBFS 3.1kHz single-ended sine
wave. Together, these signals sweep the analog inputs
across a wide range of common mode and differential
mode voltage combinations, similar to the more general
arbitrary input signal case. They also have a simple spec-
tral representation. An ideal differential converter with no
common-mode sensitivity will digitize this signal as two
−7dBFS spectral tones, one at each sine wave frequency.
The FFT plot in Figure 6b demonstrates the LTC2348-18
response approaches this ideal, with 119dB of SFDR
limited by the converter's second harmonic distortion
response to the 3.1kHz sine wave on IN–.
The ability of the LTC2348-18 to accept arbitrary signal
swings over a wide input common mode range with high
CMRR can simplify application solutions. In practice,
many sensors produce a differential sensor voltage riding
on top of a large common mode signal. Figure 7a depicts
one way of using the LTC2348-18 to digitize signals of
this type. The amplifier stage provides a differential gain
of approximately 10V/V to the desired sensor signal while
the unwanted common mode signal is attenuated by the
ADC CMRR. The circuit employs the ±5V SoftSpan range of
the ADC. Figure7b shows measured CMRR performance
of this solution, which is competitive with the best com-
mercially available instrumentation amplifiers. Figure7c
shows measured AC performance of this solution.
In Figure 8, another application circuit is shown which
uses two channels of the LTC2348-18 to simultaneously
sense the voltage on and bidirectional current through a
sense resistor over a wide common mode range. In many
applications of this type, the impedance of the external
circuitry is low enough that the ADC sampling network
can fully settle without buffering.
–5V
–5V
31V
31V
LTC2348-18
234818 F07a
ONLY CHANNEL 0 SHOWN FOR CLARITY
24V
0V
ARBITRARY +
–
+
–
0.1µF
0.1µF
0.1µF
47µF
IN0+
IN0–
½ LT1124
½ LT1124
VCC
REFINREFBUFVEE
LOWPASS FILTERS
BW ~ 500kHz
6.6nF
6.6nF
49.9Ω
49.9Ω
549Ω
18pF
18pF
IN+
IN–
2.49k
2.49k
COMMON MODE
INPUT RANGE
DIFFERENTIAL MODE
INPUT RANGE: ±500mV
Figure 7a. Digitize Differential Signals Over a Wide Common Mode Range
applicaTions inForMaTion
LTC2348-18
25
234818fa
For more information www.linear.com/LTC2348-18
±5V RANGE
IN
+
= IN
–
= 1V
P–P
SINE
IN
+
= IN
–
= 24V
P–P
SINE
OP–AMPS SLEW f
IN
> 30kHz
FREQUENCY (Hz)
10
100
1k
10k
100k
50
60
70
80
90
100
110
120
CMRR (dB)
CMRR vs Input Frequency
234818 F07b
Figure 7b. CMRR vs Input Frequency. Circuit Shown in Figure 7a
SNR = 89.8dB
THD = –116dB
SINAD = 89.8dB
SFDR = 119dB
FULLY DIFFERENTIAL DRIVE (IN
–
= –IN
+
)
±5V RANGE
FREQUENCY (kHz)
0
10
20
30
40
50
–180
–160
–140
–120
–100
–80
–60
–40
–20
0
AMPLITUDE (dBFS)
234818 F07c
Figure 7c. IN+/IN–=450mV 2kHz Fully Differential Sine,
0V≤VCM≤24V, 32k Point FFT, fSMPL = 100ksps. Circuit
Shown in Figure 7a
–15V
15V
LTC2348-18
234818 F08
ONLY CHANNELS 0 AND 1 SHOWN FOR CLARITY
0.1µF
0.1µF
0.1µF
47µF
–10.24V ≤ VS1 ≤ 10.24V
–10.24V ≤ VS2 ≤ 10.24V
VS1 – VS2
RSENSE
ISENSE =
IN0+
IN0–
IN1+
IN1–
VCC
REFINREFBUFVEE
ISENSE
RSENSE
VS2
VS1
Figure 8. Simultaneously Sense Voltage (CH0) and
Current (CH1) Over a Wide Common Mode Range
Buffering Single-Ended Analog Input Signals
While the circuit shown in Figure 6a is capable of buffering
single-ended input signals, the circuit shown in Figure 9 is
preferable when the single-ended signal reference level is
inherently low impedance and doesn't require buffering.
This circuit eliminates one driver and lowpass filter, reduc-
ing part count, power dissipation, and SNR degradation
due to driver noise. Using the recommended driver and
filter combinations in Table 2, the performance of this
circuit with single-ended input signals is on par with the
performance of the circuit in Figure 6a.
–15V
–15V
15V
15V
LTC2348-18
234818 F09
ONLY CHANNEL 0 SHOWN FOR CLARITY
TRUE BIPOLAR
+10V
0V
–10V
+10V
0V
–10V
UNIPOLAR
–
+
0.1µF
0.1µF
0.1µF
47µF
IN0+
IN0–
AMPLIFIER
VCC
REFINREFBUFVEE
OPTIONAL
LOWPASS FILTER
CFILT
RFILT
IN+
IN–
Figure 9. Buffering Single-Ended Input Signals. See Table 2 For Recommended Amplifier and Filter Combinations
applicaTions inForMaTion
LTC2348-18
26
234818fa
For more information www.linear.com/LTC2348-18
ADC REFERENCE
As shown previously in Table 1b, the LTC2348-18 supports
three reference configurations. The first uses both the in-
ternal bandgap reference and reference buffer. The second
externally overdrives the internal reference but retains the
internal buffer, which isolates the external reference from
ADC conversion transients. This configuration is ideal
for sharing a single precision external reference across
multiple ADCs. The third disables the internal buffer and
overdrives the REFBUF pin externally.
Internal Reference with Internal Buffer
The LTC2348-18 has an on-chip, low noise, low drift
(20ppm/°C maximum), temperature compensated band-
gap reference that is factory trimmed to 2.048V. The
reference output connects through a 20kΩ resistor to
the REFIN pin, which serves as the input to the on-chip
reference buffer, as shown in Figure 10a. When employing
the internal bandgap reference, the REFIN pin should be
bypassed to GND (Pin 20) close to the pin with a 0.1μF
ceramic capacitor to filter wideband noise. The reference
buffer amplifies VREFIN to create the converter master
reference voltage VREFBUF = 2•VREFIN on the REFBUF pin,
nominally 4.096V when using the internal bandgap refer-
ence. Bypass REFBUF to GND (Pin 20) close to the pin with
at least a 47μF ceramic capacitor (X7R, 10V, 1210 size or
X5R, 10V, 0805 size) to compensate the reference buffer,
absorb transient conversion currents, and minimize noise.
234818 F10a
47µF 6.5k
20k
LTC2348-18
REFERENCE
BUFFER
REFBUF
REFIN
GND
BANDGAP
REFERENCE
6.5k
0.1µF
Figure 10a. Internal Reference with Internal Buffer Configuration
External Reference with Internal Buffer
If more accuracy and/or lower drift is desired, REFIN can
be easily overdriven by an external reference since 20kΩ
of resistance separates the internal bandgap reference
output from the REFIN pin, as shown in Figure 10b. The
valid range of external reference voltage overdrive on the
REFIN pin is 1.25V to 2.2V, resulting in converter mas-
ter reference voltages VREFBUF between 2.5V and 4.4V,
respectively. Linear Technology offers a portfolio of high
performance references designed to meet the needs of
many applications. With its small size, low power, and high
accuracy, the LTC6655-2.048 is well suited for use with the
LTC2348-18 when overdriving the internal reference. The
LTC6655-2.048 offers 0.025% (maximum) initial accuracy
applicaTions inForMaTion
234818 F10b
47µF 6.5k
20k
LTC2348-18
REFERENCE
BUFFER
REFBUF
REFIN
GND
BANDGAP
REFERENCE
6.5k
2.7µF
LTC6655-2.048
Figure 10b. External Reference with Internal Buffer Configuration
LTC2348-18
27
234818fa
For more information www.linear.com/LTC2348-18
and 2ppm/°C (maximum) temperature coefficient for high
precision applications. The LTC6655-2.048 is fully speci-
fied over the H-grade temperature range, complementing
the extended temperature range of the LTC2348-18 up to
125°C. Bypassing the LTC6655-2.048 with a 2.7µF to 100µF
ceramic capacitor close to the REFIN pin is recommended.
External Reference with Disabled Internal Buffer
The internal reference buffer supports VREFBUF = 4.4V
maximum. By grounding REFIN, the internal buffer may
be disabled allowing REFBUF to be overdriven with an
external reference voltage between 2.5V and 5V, as shown
in Figure 10c. Maximum input signal swing and SNR are
achieved by overdriving REFBUF using an external 5V
reference. The buffer feedback resistors load the REFBUF
pin with 13kΩ even when the reference buffer is disabled.
The LTC6655-5 offers the same small size, accuracy, drift,
and extended temperature range as the LTC6655-2.048,
and achieves a typical SNR of 97.5dB when paired with
the LTC2348-18. Bypass the LTC6655-5 to GND (Pin 20)
close to the REFBUF pin with at least a 47μF ceramic ca-
pacitor (X7R, 10V, 1210 size or X5R, 10V, 0805 size) to
absorb transient conversion currents and minimize noise.
The LTC2348-18 converter draws a charge (QCONV) from
the REFBUF pin during each conversion cycle. On short
time scales most of this charge is supplied by the external
REFBUF bypass capacitor, but on longer time scales all of
the charge is supplied by either the reference buffer, or
when the internal reference buffer is disabled, the external
reference. This charge draw corresponds to a DC current
equivalent of IREFBUF = QCONV• fSMPL, which is proportional
to sample rate. In applications where a burst of samples
is taken after idling for long periods of time, as shown in
Figure11, IREFBUF quickly transitions from approximately
0.4mA to 1.5mA (VREFBUF = 5V, fSMPL=200kHz). This
current step triggers a transient response in the external
reference that must be considered, since any deviation in
VREFBUF affects converter accuracy. If an external reference
is used to overdrive REFBUF, the fast settling LTC6655
family of references is recommended.
Internal Reference Buffer Transient Response
For optimum performance in applications employing burst
sampling, the external reference with internal reference
buffer configuration should be used. The internal reference
buffer incorporates a proprietary design that minimizes
movements in VREFBUF when responding to a burst of
conversions following an idle period. Figure 12 compares
applicaTions inForMaTion
CNV
IDLE
PERIOD
IDLE
PERIOD
234818 F11
Figure 11. CNV Waveform Showing Burst Sampling
234818 F10c
47µF 6.5k
20k
LTC2348-18
REFERENCE
BUFFER
REFBUF
REFIN
GND
BANDGAP
REFERENCE
6.5k
LTC6655-5
Figure 10c. External Reference with Disabled
Internal Buffer Configuration
LTC2348-18
28
234818fa
For more information www.linear.com/LTC2348-18
the burst conversion response of the LTC2348-18 with an
input near full scale for two reference configurations. The
first configuration employs the internal reference buffer
with REFIN externally overdriven by an LTC6655-2.048,
while the second configuration disables the internal ref-
erence buffer and overdrives REFBUF with an external
LTC6655-4.096. In both cases REFBUF is bypassed to
GND with a 47µF ceramic capacitor.
INTERNAL REFERENCE BUFFER
EXTERNAL REFERENCE ON REFBUF
IN
–
= 0V
IN
+
= 10V
±10.24V RANGE
TIME (µs)
0
100
200
300
400
500
–5
0
5
10
15
20
DEVIATION FROM FINAL VALUE (LSB)
234818 F12
Figure 12. Burst Conversion Response of the LTC2348-18,
fSMPL=200ksps
DYNAMIC PERFORMANCE
Fast Fourier transform (FFT) techniques are used to test
the ADC’s frequency response, distortion, and noise at the
rated throughput. By applying a low distortion sine wave
and analyzing the digital output using an FFT algorithm,
the ADC’s spectral content can be examined for frequen-
cies outside the fundamental. The LTC2348-18 provides
guaranteed tested limits for both AC distortion and noise
measurements.
Signal-to-Noise and Distortion Ratio (SINAD)
The signal-to-noise and distortion ratio (SINAD) is the
ratio between the RMS amplitude of the fundamental input
frequency and the RMS amplitude of all other frequency
components at the A/D output. The output is band-limited
to frequencies below half the sampling frequency, exclud-
ing DC. Figure 13 shows that the LTC2348-18 achieves a
typical SINAD of 96.5dB in the ±10.24V range at a 200kHz
sampling rate with a true bipolar 2kHz input signal.
Signal-to-Noise Ratio (SNR)
The signal-to-noise ratio (SNR) is the ratio between the
RMS amplitude of the fundamental input frequency and
the RMS amplitude of all other frequency components
except the first five harmonics and DC. Figure 13 shows
that the LTC2348-18 achieves a typical SNR of 96.7dB in
the ±10.24V range at a 200kHz sampling rate with a true
bipolar 2kHz input signal.
Total Harmonic Distortion (THD)
Total harmonic distortion (THD) is the ratio of the RMS sum
of all harmonics of the input signal to the fundamental itself.
The out-of-band harmonics alias into the frequency band
between DC and half the sampling frequency (fSMPL/2).
THD is expressed as:
THD =20log V2
2+V3
2+V4
2...VN
2
V1
where V1 is the RMS amplitude of the fundamental fre-
quency and V2 through VN are the amplitudes of the second
through Nth harmonics, respectively. Figure 13 shows
that the LTC2348-18 achieves a typical THD of –109dB
(N=6) in the ±10.24V range at a 200kHz sampling rate
with a true bipolar 2kHz input signal.
SNR = 96.7dB
THD = –109dB
SINAD = 96.5dB
SFDR = 110dB
TRUE BIPOLAR DRIVE (IN– = 0V)
±10.24V RANGE
FREQUENCY (kHz)
0
20
40
60
80
100
–180
–160
–140
–120
–100
–80
–60
–40
–20
0
AMPLITUDE (dBFS)
234818 F13
Figure 13. 32k Point FFT fSMPL = 200ksps, fIN = 2kHz
applicaTions inForMaTion
LTC2348-18
29
234818fa
For more information www.linear.com/LTC2348-18
POWER CONSIDERATIONS
The LTC2348-18 requires four power supplies: the posi-
tive and negative high voltage power supplies (VCC and
VEE), the 5V core power supply (VDD) and the digital input/
output (I/O) interface power supply (OVDD). As long as
the voltage difference limits of 10V ≤ VCC – VEE ≤ 38V
are observed, VCC and VEE may be independently biased
anywhere within their own individual allowed operating
ranges, including the ability for either of the supplies to be
tied directly to ground. This feature enables the common
mode input range of the LTC2348-18 to be tailored to the
specific application’s requirements. The flexible OVDD sup-
ply allows the LTC2348-18 to communicate with CMOS
logic operating between 1.8V and 5V, including 2.5V and
3.3V systems. When using LVDS I/O mode, the range of
OVDD is 2.375V to 5.25V.
Power Supply Sequencing
The LTC2348-18 does not have any specific power supply
sequencing requirements. Care should be taken to adhere
to the maximum voltage relationships described in the
Absolute Maximum Ratings section. The LTC2348-18 has
an internal power-on-reset (POR) circuit which resets the
converter on initial power-up and whenever VDD drops
below 2V. Once the supply voltage re-enters the nominal
supply voltage range, the POR reinitializes the ADC. No
conversions should be initiated until at least 10ms after
a POR event to ensure the initialization period has ended.
When employing the internal reference buffer, allow 200ms
for the buffer to power up and recharge the REFBUF bypass
capacitor. Any conversion initiated before these times will
produce invalid results.
TIMING AND CONTROL
CNV Timing
The LTC2348-18 sampling and conversion is controlled by
CNV. A rising edge on CNV transitions all channels’ S/H
circuits from track mode to hold mode, simultaneously
sampling the input signals on all channels and initiating
a conversion. Once a conversion has been started, it
cannot be terminated early except by resetting the ADC,
as discussed in the Reset Timing section. For optimum
performance, drive CNV with a clean, low jitter signal and
avoid transitions on data I/O lines leading up to the rising
edge of CNV. Additionally, to minimize channel-to-channel
crosstalk, avoid high slew rates on the analog inputs for
100ns before and after the rising edge of CNV. Converter
status is indicated by the BUSY output, which transitions
low-to-high at the start of each conversion and stays high
until the conversion is complete. Once CNV is brought high
to begin a conversion, it should be returned low between
40ns and 60ns later or after the falling edge of BUSY to
minimize external disturbances during the internal conver-
sion process. The CNV timing required to take advantage
of the reduced power nap mode of operation is described
in the Nap Mode section.
Internal Conversion Clock
The LTC2348-18 has an internal clock that is trimmed to
achieve a maximum conversion time of 550•N ns with N
channels enabled. With a minimum acquisition time of
570ns when converting eight channels simultaneously,
throughput performance of 200ksps is guaranteed without
any external adjustments.
applicaTions inForMaTion
CNV
tCONV
tACQ
BUSY
NAP NAP MODE
tCNVL
234818 F14
Figure 14. Nap Mode Timing for the LTC2348-18
LTC2348-18
30
234818fa
For more information www.linear.com/LTC2348-18
Nap Mode
The LTC2348-18 can be placed into nap mode after a con-
version has been completed to reduce power consumption
between conversions. In this mode a portion of the device
circuitry is turned off, including circuits associated with
sampling the analog input signals. Nap mode is enabled
by keeping CNV high between conversions, as shown in
Figure 14. To initiate a new conversion after entering nap
mode, bring CNV low and hold for at least 420ns before
bringing it high again. The converter acquisition time (tACQ)
is set by the CNV low time (tCNVL) when using nap mode.
Power Down Mode
When PD is brought high, the LTC2348-18 is powered
down and subsequent conversion requests are ignored. If
this occurs during a conversion, the device powers down
once the conversion completes. In this mode, the device
draws only a small regulator standby current resulting in a
typical power dissipation of 0.36mW. To exit power down
mode, bring the PD pin low and wait at least 10ms before
initiating a conversion. When employing the internal refer-
ence buffer, allow 200ms for the buffer to power up and
recharge the REFBUF bypass capacitor. Any conversion
initiated before these times will produce invalid results.
Reset Timing
A global reset of the LTC2348-18, equivalent to a power-
on-reset event, may be executed without needing to cycle
the supplies. This feature is useful when recovering from
system-level events that require the state of the entire sys-
tem to be reset to a known synchronized value. To initiate
a global reset, bring PD high twice without an intervening
conversion, as shown in Figure 15. The reset event is trig-
gered on the second rising edge of PD, and asynchronously
applicaTions inForMaTion
ends based on an internal timer. Reset clears all serial data
output registers and restores the internal SoftSpan configu-
ration register default state of all channels in SoftSpan7.
If reset is triggered during a conversion, the conversion
is immediately halted. The normal power down behavior
associated with PD going high is not affected by reset. Once
PD is brought low, wait at least 10ms before initiating a
conversion. When employing the internal reference buffer,
allow 200ms for the buffer to power up and recharge the
REFBUF bypass capacitor. Any conversion initiated before
these times will produce invalid results.
Power Dissipation vs Sampling Frequency
When nap mode is employed, the power dissipation of
the LTC2348-18 decreases as the sampling frequency is
reduced, as shown in Figure 16. This decrease in aver-
age power dissipation occurs because a portion of the
LTC2348-18 circuitry is turned off during nap mode, and
the fraction of the conversion cycle (tCYC) spent napping
increases as the sampling frequency (fSMPL) is decreased.
I
OVDD
I
VDD
I
VEE
I
VCC
WITH NAP MODE
t
CNVL
= 1µs
SAMPLING FREQUENCY (kHz)
0
40
80
120
160
200
–4
–2
0
2
4
6
8
10
12
14
16
18
SUPPLY CURRENT (mA)
234818 F16
Figure 16. Power Dissipation of the LTC2348-18
Decreases with Decreasing Sampling Frequency
CNV
tCONV
tCNVH
tPDH
tPDL
BUSY
RESET RESET TIME
SET INTERNALLY
SECOND RISING EDGE OF
PD TRIGGERS RESET
PD tWAKE
234818 F15
Figure 15. Reset Timing for the LTC2348-18
LTC2348-18
31
234818fa
For more information www.linear.com/LTC2348-18
applicaTions inForMaTion
DIGITAL INTERFACE
The LTC2348-18 features CMOS and LVDS serial interfaces,
selectable using the LVDS/CMOS pin. The flexible OVDD
supply allows the LTC2348-18 to communicate with any
CMOS logic operating between 1.8V and 5V, including
2.5V and 3.3V systems, while the LVDS interface supports
low noise digital designs. In CMOS mode, applications
may employ between one and eight lanes of serial data
output, allowing the user to optimize bus width and data
throughput. Together, these I/O interface options enable
the LTC2348-18 to communicate equally well with legacy
microcontrollers and modern FPGAs.
Serial CMOS I/O Mode
As shown in Figure 17, in CMOS I/O mode the serial data
bus consists of a serial clock input, SCKI, serial data
input, SDI, serial clock output, SCKO, and eight lanes of
serial data output, SDO0 to SDO7. Communication with
the LTC2348-18 across this bus occurs during predefined
data transaction windows. Within a window, the device
accepts 24-bit SoftSpan configuration words for the next
conversion on SDI and outputs 24-bit packets containing
conversion results and channel configuration information
from the previous conversion on SDO0 to SDO7. New
data transaction windows open 10ms after powering up
or resetting the LTC2348-18, and at the end of each con-
version on the falling edge of BUSY. In the recommended
use case, the data transaction should be completed with
a minimum tQUIET time of 20ns prior to the start of the
next conversion, as shown in Figure 17. New SoftSpan
configuration words are only accepted within this recom-
mended data transaction window, but SoftSpan changes
take effect immediately with no additional analog input
settling time required before starting the next conversion.
It is still possible to read conversion data after starting the
next conversion, but this will degrade conversion accuracy
and therefore is not recommended.
C2 C1 C0
C2 C1 C0
S23
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
S22 S21 S20 S19 S18 S17 S16 S15 S14 S13 S12 S11 S10 S9 S8 S7 S6 S5 S3S4 S1S2 S0
D17 D16 D15 D14 D13 D12 D11 D10 D9 D8 D7 D6 D5 D4 D3 D2 D1 D0 SS1SS2 SS0 D17
CNV
CS = PD = 0
DON’T CARE
tACQ
BUSY
SDO7
SCKO
SDO0
SCKI
SDI
DON’T CARE
SAMPLE N
RECOMMENDED DATA TRANSACTION WINDOW
SAMPLE N + 1
SOFTSPAN CONFIGURATION WORD FOR CONVERSION N + 1
CHANNEL 0
24-BIT PACKET
CONVERSION N
CHANNEL 1
24-BIT PACKET
CONVERSION N
CHANNEL 7
24-BIT PACKET
CONVERSION N
CHANNEL 0
24-BIT PACKET
CONVERSION N
234818 TD01
CONVERSION RESULT CHANNEL ID SOFTSPAN CONVERSION RESULT
D17 D16 D15 D14 D13 D12 D11 D10 D9 D8 D7 D6 D5 D4 D3 D2 D1 D0 SS1SS2 SS0 D17DON’T CARE
CONVERSION RESULT CHANNEL ID SOFTSPAN CONVERSION RESULT
• • •
tSCKI tSCKIH
tBUSYLH
tSCKIL
tHSDOSCKI
tDSDOSCKI
tSSDISCKI
tQUIET
tHSDISCKI
tDSDOBUSYL
tCNVH
tSKEW
tCONV
tCNVL
tCYC
Figure 17. Serial CMOS I/O Mode
LTC2348-18
32
234818fa
For more information www.linear.com/LTC2348-18
Just prior to the falling edge of BUSY and the opening of
a new data transaction window, SCKO is forced low and
SDO0 to SDO7 are updated with the latest conversion
results from analog input channels 0 to 7, respectively.
Rising edges on SCKI serially clock conversion results
and analog input channel configuration information out
on SDO0 to SDO7 and trigger transitions on SCKO that
are skew-matched to the data on SDO0 to SDO7. The
resulting SCKO frequency is half that of SCKI. SCKI
rising edges also latch SoftSpan configuration words
provided on SDI, which are used to program the internal
24-bit SoftSpan configuration register. See the section
Programming the SoftSpan Configuration Register in
CMOS I/O Mode for further details. SCKI is allowed to
idle either high or low in CMOS I/O mode. As shown in
Figure 18, the CMOS bus is enabled when CS is low and
is disabled and Hi-Z when CS is high, allowing the bus
to be shared across multiple devices.
The data on SDO0 to SDO7 are grouped into 24-bit
packets consisting of an 18-bit conversion result, 3-bit
analog channel ID, and 3-bit SoftSpan code, all presented
MSB first. As suggested in Figures 17 and 18, each SDO
lane outputs these packets for all analog input channels
in a sequential, circular manner. For example, the first
24-bit packet output on SDO0 corresponds to analog
input channel 0, followed by the packets for channels 1
through 7. The data output on SDO0 then wraps back
to channel 0, and this pattern repeats indefinitely. Other
SDO lanes follow a similar circular pattern, except the
first packet presented on each lane corresponds to its
associated analog input channel.
When interfacing the LTC2348-18 with a standard SPI
bus, capture output data at the receiver on rising edges of
SCKI. SCKO is not used in this case. Multiple SDO lanes
are also usually not useful in this case. In other applica-
tions, such as interfacing the LTC2348-18 with an FPGA
or CPLD, rising and falling edges of SCKO may be used
to capture serial output data on SDO0 to SDO7 in double
data rate (DDR) fashion. Capturing data using SCKO adds
robustness to delay variations over temperature and supply.
Full Eight Lane Serial CMOS Output Data Capture
As shown in Table 3, full 200ksps per channel throughput
can be achieved with a 45MHz SCKI frequency by capturing
the first packet (24 SCKI cycles total) from all eight serial
data output lanes SDO0 to SDO7. This configuration also
allows conversion results from all channels to be captured
using as few as 18 SCKI cycles if the 3-bit analog channel
ID and 3-bit SoftSpan code are not needed and the device
SoftSpan configuration is not being changed. Multi-lane
data capture is usually best suited for use with FPGA
or CPLD capture hardware, but may be useful in other
application-specific cases.
applicaTions inForMaTion
CS
PD = 0
DON’T CARE
BUSY
SDO7
SCKO
SDO0
SCKI
SDI
DON’T CARE
DON’T CARE
DON’T CARE
234818 F18
Hi-Z
Hi-Z CHANNEL 0 PACKET CHANNEL 1 PACKET CHANNEL 2 PACKET CHANNEL 3 PACKET
(PARTIAL)
Hi-Z
Hi-Z
Hi-Z
Hi-Z
CHANNEL 7 PACKET CHANNEL 0 PACKET CHANNEL 1 PACKET CHANNEL 2 PACKET
(PARTIAL)
• • •
NEW SoftSpan CONFIGURATION WORD
(OVERWRITES INTERNAL CONFIG REGISTER)
TWO ALL-ZERO WORDS AND ONE PARTIAL WORD
(INTERNAL CONFIG REGISTER RETAINS CURRENT VALUE)
tEN tDIS
Figure 18. Internal SoftSpan Configuration Register Behavior. Serial CMOS Bus Response to CS
LTC2348-18
33
234818fa
For more information www.linear.com/LTC2348-18
Fewer Than Eight Lane Serial CMOS Output Data Capture
Applications that cannot accommodate the full eight lanes
of serial data capture may employ fewer lanes without
reconfiguring the LTC2348-18. For example, capturing
the first two packets (48 SCKI cycles total) from SDO0,
SDO2, SDO4, and SDO6 provides data for analog input
channels 0 and 1, 2 and 3, 4 and 5, and 6 and 7, respec-
tively, using four output lanes. Similarly, capturing the first
four packets (96 SCKI cycles total) from SDO0 and SDO4
provides data for analog input channels 0 to 3 and 4 to
7, respectively, using two output lanes. If only one lane
can be accommodated, capturing the first eight packets
(192 SCKI cycles total) from SDO0 provides data for all
analog input channels. As shown in Table 3, full 200ksps
per channel throughput can be achieved with a 90MHz
SCKI frequency in the four lane case, but the maximum
CMOS SCKI frequency of 100MHz limits the throughput
to less than 200ksps per channel in the two lane and one
lane cases. Finally, note that in choosing the number of
lanes and which lanes to use for data capture, the user is
not restricted to the specific cases mentioned above. Other
choices may be more optimal in particular applications.
Programming the SoftSpan Configuration Register in
CMOS I/O Mode
The internal 24-bit SoftSpan configuration register con-
trols the SoftSpan range for all analog input channels of
the LTC2348-18. The default state of this register after
power-up or resetting the device is all ones, configuring
each channel to convert in SoftSpan 7, the ±2.5 • VREFBUF
range (see Table 1a). The state of this register may be
modified by providing a new 24-bit SoftSpan configuration
word on SDI during the data transaction window shown
in Figure 17. New SoftSpan configuration words are only
accepted within this recommended data transaction win-
dow, but SoftSpan changes take effect immediately with
no additional analog input settling time required before
starting the next conversion. Setting a channel’s SoftSpan
code to SS[2:0] = 000 immediately disables the channel,
resulting in a corresponding reduction in tCONV on the next
conversion. Similarly, enabling a previously disabled chan-
nel requires no additional analog input settling time before
starting the next conversion. The mapping between the
serial SoftSpan configuration word, the internal SoftSpan
configuration register, and each channel’s 3-bit SoftSpan
code is illustrated in Figure 19.
If fewer than 24 SCKI rising edges are provided during a
data transaction window, the partial word received on SDI
will be ignored and the SoftSpan configuration register will
not be updated. If exactly 24 SCKI rising edges are provided,
the SoftSpan configuration register will be updated to
match the received SoftSpan configuration word, S[23:0].
The one exception to this behavior occurs when S[23:0] is
all zeros. In this case, the SoftSpan configuration register
will not be updated, allowing applications to retain the
current SoftSpan configuration state by idling SDI low. If
more than 24 SCKI rising edges are provided during a data
transaction window, each complete 24-bit word received
on SDI will be interpreted as a new SoftSpan configuration
word and applied to the SoftSpan configuration register
as described above. Any partial words are ignored.
applicaTions inForMaTion
Table 3. Required SCKI Frequency to Achieve Various Throughputs in Common Output Bus Configurations with Eight Channels
Enabled. Shaded Entries Denote Throughputs That Are Not Achievable In a Given Configuration. Calculated Using fSCKI = (Number of
SCKI Cycles)/(tACQ(MIN) – tQUIET)
I/O MODE NUMBER OF SDO
LANES
NUMBER OF SCKI
CYCLES
REQUIRED fSCKI (MHz) TO ACHIEVE THROUGHPUT OF
200ksps/CHANNEL
(tACQ = 570ns)
100ksps/CHANNEL
(tACQ = 5570ns)
50ksps/CHANNEL
(tACQ = 15570ns)
CMOS
8 18 35 4 2
8 24 45 5 2
4 48 90 9 4
2 96 Not Achievable 18 7
1 192 Not Achievable 35 13
LVDS 1 96 180 (360Mbps) 18 (36Mbps) 7 (14Mbps)
LTC2348-18
34
234818fa
For more information www.linear.com/LTC2348-18
Typically, applications will update the SoftSpan configura-
tion register in the manner shown in Figures 17 and 18.
After the opening of a new data transaction window at the
falling edge of BUSY, the user supplies a 24-bit SoftSpan
configuration word on SDI during the first 24 SCKI cycles.
This new word overwrites the internal configuration register
contents following the 24th SCKI rising edge. The user then
holds SDI low for the remainder of the data transaction
window causing the register to retain its contents regardless
of the number of additional SCKI cycles applied. SoftSpan
settings may be retained across multiple conversions by
holding SDI low for the entire data transaction window,
regardless of the number of SCKI cycles applied.
Serial LVDS I/O Mode
In LVDS I/O mode, information is transmitted using posi-
tive and negative signal pairs (LVDS+/LVDS−) with bits
differentially encoded as (LVDS+ − LVDS−). These signals
are typically routed using differential transmission lines
with 100Ω characteristic impedance. Logical 1’s and 0’s
are nominally represented by differential +350mV and
−350mV, respectively. For clarity, all LVDS timing diagrams
and interface discussions adopt the logical rather than
physical convention.
As shown in Figure 20, in LVDS I/O mode the serial data
bus consists of a serial clock differential input, SCKI, serial
data differential input, SDI, serial clock differential output,
SCKO, and serial data differential output, SDO. Communi-
cation with the LTC2348-18 across this bus occurs during
predefined data transaction windows. Within a window,
the device accepts 24-bit SoftSpan configuration words
for the next conversion on SDI and outputs 24-bit packets
containing conversion results and channel configuration
information from the previous conversion on SDO. New
data transaction windows open 10ms after powering up
or resetting the LTC2348-18, and at the end of each con-
version on the falling edge of BUSY. In the recommended
use case, the data transaction should be completed with
a minimum tQUIET time of 20ns prior to the start of the
next conversion, as shown in Figure 20. New SoftSpan
configuration words are only accepted within this recom-
mended data transaction window, but SoftSpan changes
S23
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
S22 S21 S20 S19 S18 S17 S16 S15 S14 S13 S12 S11 S10 S9 S8 S7 S6 S5 S3S4 S1S2 S0DON’T CARE
SCKI
SDI
SoftSpan CONFIGURATION WORD
CMOS I/O MODE
SoftSpan CONFIGURATION WORD
INTERNAL 24-BIT SoftSpan CONFIGURATION REGISTER
(SAME FOR CMOS AND LVDS)
LVDS I/O MODE
1234567891011121314151617181920212223 0
tSCKI
S23
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
S22 S21 S20 S19 S18 S17 S16 S15 S14 S13 S12 S11 S10 S9 S8 S7 S6 S5 S3S4 S1S2 S0DON’T CARE
SCKI
(LVDS)
SDI
(LVDS)
tSCKI
CHANNEL 7 SoftSpan
CODE SS[2:0]
CHANNEL 6 SoftSpan
CODE SS[2:0]
CHANNEL 5 SoftSpan
CODE SS[2:0]
CHANNEL 4 SoftSpan
CODE SS[2:0]
CHANNEL 3 SoftSpan
CODE SS[2:0]
CHANNEL 2 SoftSpan
CODE SS[2:0]
CHANNEL 1 SoftSpan
CODE SS[2:0]
CHANNEL 0 SoftSpan
CODE SS[2:0]
234818 F19
tSCKIH
tSCKIL
tSSDISCKI
tHSDISCKI
tSCKIH
tSCKIL tHSDISCKI
tHSDISCKI
tSSDISCKI
tSSDISCKI
Figure 19. Mapping Between Serial SoftSpan Configuration Word, Internal SoftSpan
Configuration Register, and SoftSpan Code for Each Analog Input Channel
applicaTions inForMaTion
LTC2348-18
35
234818fa
For more information www.linear.com/LTC2348-18
take effect immediately with no additional analog input
settling time required before starting the next conversion.
It is still possible to read conversion data after starting the
next conversion, but this will degrade conversion accuracy
and therefore is not recommended.
Just prior to the falling edge of BUSY and the opening
of a new data transaction window, SDO is updated with
the latest conversion results from analog input channel
0. Both rising and falling edges on SCKI serially clock
conversion results and analog input channel configuration
information out on SDO. SCKI is also echoed on SCKO,
skew-matched to the data on SDO. Whenever possible, it
is recommended that rising and falling edges of SCKO be
used to capture DDR serial output data on SDO, as this will
yield the best robustness to delay variations over supply
and temperature. SCKI rising and falling edges also latch
SoftSpan configuration words provided on SDI, which are
used to program the internal 24-bit SoftSpan configura-
tion register. See the section Programming the SoftSpan
Configuration Register in LVDS I/O Mode for further
details. As shown in Figure 21, the LVDS bus is enabled
when CS is low and is disabled and Hi-Z when CS is high,
allowing the bus to be shared across multiple devices.
Due to the high speeds involved in LVDS signaling, LVDS
bus sharing must be carefully considered. Transmission
line limitations imposed by the shared bus may limit the
maximum achievable bus clock speed. LVDS inputs are
internally terminated with a 100Ω differential resistor when
CS is low, while outputs must be differentially terminated
with a 100Ω resistor at the receiver (FPGA). SCKI must
idle in the low state in LVDS I/O mode, including when
transitioning CS.
The data on SDO are grouped into 24-bit packets consist-
ing of an 18-bit conversion result, 3-bit analog channel
ID, and 3-bit SoftSpan code, all presented MSB first. As
suggested in Figures 20 and 21, SDO outputs these pack-
ets for all analog input channels in a sequential, circular
manner. For example, the first 24-bit packet output on
SDO corresponds to analog input channel 0, followed by
the packets for channels 1 through 7. The data output
on SDO then wraps back to channel 0, and this pattern
repeats indefinitely.
S23
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 25 2624
S22 S21 S20 S19 S18 S17 S16 S15 S14 S13 S12 S11 S10 S9 S8 S7 S6 S5 S3S4 S1S2 S0
D17 D16 D15 D14 D13 D12 D11 D10 D9 D8 D7 D6 D5 D4 D3 D2 D1 D0 SS1SS2 SS0 D17 D16 D15
CNV
(CMOS)
CS = PD = 0
234818 F20
DON’T CARE
tACQ
BUSY
(CMOS)
SCKO
(LVDS)
SDO
(LVDS)
SCKI
(LVDS)
SDI
(LVDS)
DON’T CARE
SAMPLE N
tCNVL
tCYC
RECOMMENDED DATA TRANSACTION WINDOW
SAMPLE N + 1
SoftSpan CONFIGURATION WORD FOR CONVERSION N + 1
CHANNEL 0
24-BIT PACKET
CONVERSION N
CHANNEL 1
24-BIT PACKET
CONVERSION N
CHANNEL 7
24-BIT PACKET
CONVERSION N
CONVERSION RESULT CHANNEL ID SoftSpan
186185 187 188 189 190 191 192
D0 SS1SS2 SS0 D17
CHANNEL 0
24-BIT PACKET
CONVERSION N
CONVERSION
RESULT
CHANNEL ID SoftSpan
tSKEW
tCNVH
tSCKIH
tSCKIL
tBUSYLH
tDSDOSCKI
tHSDOSCKI
tDSDOBUSYL
tSSDISCKI tSSDISCKI
tHSDISCKI tHSDISCKI
tQUIET
tSCKI
tCONV
C2 C1 C0C2 C1 C0
Figure 20. Serial LVDS I/O Mode
applicaTions inForMaTion
LTC2348-18
36
234818fa
For more information www.linear.com/LTC2348-18
CS
(CMOS)
PD = 0
DON’T CARE
BUSY
(CMOS)
SCKO
(LVDS)
SDO
(LVDS)
SCKI
(LVDS)
SDI
(LVDS)
DON’T CARE
DON’T CARE
DON’T CARE
234818 F21
Hi-Z
Hi-Z CHANNEL 0 PACKET CHANNEL 1 PACKET CHANNEL 2 PACKET CHANNEL 3 PACKET
(PARTIAL)
Hi-Z
Hi-Z
NEW SoftSpan CONFIGURATION WORD
(OVERWRITES INTERNAL CONFIG REGISTER)
TWO ALL-ZERO WORDS AND ONE PARTIAL WORD
(INTERNAL CONFIG REGISTER RETAINS CURRENT VALUE)
tEN tDIS
Figure 21. Internal SoftSpan Configuration Register Behavior. Serial LVDS Bus Response to CS
Serial LVDS Output Data Capture
As shown in Table 3, full 200ksps per channel throughput
can be achieved with a 180MHz SCKI frequency by captur-
ing eight packets (96 SCKI cycles total) of DDR data from
SDO. The LTC2348-18 supports LVDS SCKI frequencies
up to 250MHz.
Programming the SoftSpan Configuration Register in
LVDS I/O Mode
The internal 24-bit SoftSpan configuration register con-
trols the SoftSpan range for all analog input channels of
the LTC2348-18. The default state of this register after
power-up or resetting the device is all ones, configuring
each channel to convert in SoftSpan 7, the ±2.5 • VREFBUF
range (see Table 1a). The state of this register may be
modified by providing a new 24-bit SoftSpan configuration
word on SDI during the data transaction window shown
in Figure 20. New SoftSpan configuration words are only
accepted within this recommended data transaction win-
dow, but SoftSpan changes take effect immediately with
no additional analog input settling time required before
starting the next conversion. Setting a channel’s SoftSpan
code to SS[2:0] = 000 immediately disables the channel,
resulting in a corresponding reduction in tCONV on the next
conversion. Similarly, enabling a previously disabled chan-
nel requires no additional analog input settling time before
starting the next conversion. The mapping between the
serial SoftSpan configuration word, the internal SoftSpan
configuration register, and each channel’s 3-bit SoftSpan
code is illustrated in Figure 19.
If fewer than 24 SCKI edges (rising plus falling) are
provided during a data transaction window, the partial
word received on SDI will be ignored and the SoftSpan
configuration register will not be updated. If exactly 24
SCKI edges are provided, the SoftSpan configuration
register will be updated to match the received SoftSpan
configuration word, S[23:0]. The one exception to this
behavior occurs when S[23:0] is all zeros. In this case,
the SoftSpan configuration register will not be updated,
allowing applications to retain the current SoftSpan con-
figuration state by idling SDI low. If more than 24 SCKI
edges are provided during a data transaction window, each
complete 24-bit word received on SDI will be interpreted
as a new SoftSpan configuration word and applied to the
SoftSpan configuration register as described above. Any
partial words are ignored.
Typically, applications will update the SoftSpan configura-
tion register in the manner shown in Figures 20 and 21.
After the opening of a new data transaction window at
the falling edge of BUSY, the user supplies a 24-bit DDR
SoftSpan configuration word on SDI during the first 12
SCKI cycles. This new word overwrites the internal con-
figuration register contents following the 12th SCKI falling
edge. The user then holds SDI low for the remainder of
the data transaction window causing the register to retain
its contents regardless of the number of additional SCKI
cycles applied. SoftSpan settings may be retained across
multiple conversions by holding SDI low for the entire
data transaction window, regardless of the number of
SCKI cycles applied.
applicaTions inForMaTion
LTC2348-18
37
234818fa
For more information www.linear.com/LTC2348-18
To obtain the best performance from the LTC2348-18, a
four-layer printed circuit board (PCB) is recommended.
Layout for the PCB should ensure the digital and analog
signal lines are separated as much as possible. In particu-
lar, care should be taken not to run any digital clocks or
signals alongside analog signals or underneath the ADC.
Also minimize the length of the REFBUF to GND (Pin 20)
bypass capacitor return loop, and avoid routing CNV near
signals which could potentially disturb its rising edge.
Supply bypass capacitors should be placed as close as
possible to the supply pins. Low impedance common re-
turns for these bypass capacitors are essential to the low
noise operation of the ADC. A single solid ground plane
is recommended for this purpose. When possible, screen
the analog input traces using ground.
Reference Design
For a detailed look at the reference design for this con-
verter, including schematics and PCB layout, please refer
to DC2094A, the evaluation kit for the LTC2348-18.
boarD layouT
LTC2348-18
38
234818fa
For more information www.linear.com/LTC2348-18
package DescripTion
Please refer to http://www.linear.com/product/LTC2348-18#packaging for the most recent package drawings.
LX48 LQFP 0113 REV A
0° – 7°
11° – 13°
0.45 – 0.75
1.00 REF
11° – 13°
9.00 BSC
A A
7.00 BSC
1
2
7.00 BSC
9.00 BSC
48
1.60
MAX
1.35 – 1.45
0.05 – 0.150.09 – 0.20 0.50
BSC 0.17 – 0.27
GAUGE PLANE
0.25
NOTE:
1. PACKAGE DIMENSIONS CONFORM TO JEDEC #MS-026 PACKAGE OUTLINE
2. DIMENSIONS ARE IN MILLIMETERS
3. DIMENSIONS OF PACKAGE DO NOT INCLUDE MOLD FLASH. MOLD FLASH
SHALL NOT EXCEED 0.25mm ON ANY SIDE, IF PRESENT
4. PIN-1 INDENTIFIER IS A MOLDED INDENTATION, 0.50mm DIAMETER
5. DRAWING IS NOT TO SCALE
SEE NOTE: 4
C0.30 – 0.50
R0.08 – 0.20
7.15 – 7.25
5.50 REF
1
2
5.50 REF
7.15 – 7.25
48
PACKAGE OUTLINE
RECOMMENDED SOLDER PAD LAYOUT
APPLY SOLDER MASK TO AREAS THAT ARE NOT SOLDERED
SECTION A – A
0.50 BSC
0.20 – 0.30
1.30 MIN
LX Package
48-Lead Plastic LQFP (7mm × 7mm)
(Reference LTC DWG # 05-08-1760 Rev A)
e3
LTCXXXX
LX-ES
Q_ _ _ _ _ _
XXYY
TRAY PIN 1
BEVEL PACKAGE IN TRAY LOADING ORIENTATION
COMPONENT
PIN “A1”
LTC2348-18
39
234818fa
For more information www.linear.com/LTC2348-18
Information furnished by Linear Technology Corporation is believed to be accurate and reliable.
However, no responsibility is assumed for its use. Linear Technology Corporation makes no representa-
tion that the interconnection of its circuits as described herein will not infringe on existing patent rights.
revision hisTory
REV DATE DESCRIPTION PAGE NUMBER
A 02/16 Updated the ADC Timing Characteristics section
Inserted new graphs: PSRR vs Frequency and Power Dissipation vs Sampling Rate
Updated Table 2
Updated the Application Information section
Updated Figure 16
Updated Table 3
Updated the Board Layout section
6
12
22
24
30
33
37
LTC2348-18
40
234818fa
For more information www.linear.com/LTC2348-18
LINEAR TECHNOLOGY CORPORATION 2015
LT 0216 REV A • PRINTED IN USA
Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900 ● FAX: (408) 434-0507 ● www.linear.com/LTC2348-18
relaTeD parTs
Typical applicaTion
PART NUMBER DESCRIPTION COMMENTS
ADCs
LTC2348-16 16-Bit, 200ksps, 8-Channel Simultaneous
Sampling, ±1LSB INL, Serial ADC
±10.24V SoftSpan Inputs with Wide Common Mode Range, 94db SNR,
Serial CMOS and LVDS I/O, 7mm × 7mm LQFP-48 Package
LTC2378-20/LTC2377-20/
LTC2376-20
20-Bit, 1Msps/500ksps/250ksps,
±0.5ppm INL Serial, Low Power ADC
2.5V Supply, ±5V Fully Differential Input, 104dB SNR, MSOP-16 and
4mm × 3mm DFN-16 Packages
LTC2338-18/LTC2337-18/
LTC2336-18
18-Bit, 1Msps/500ksps/250ksps, Serial,
Low Power ADC
5V Supply, ±10.24V Fully Differential Input, 100dB SNR, MSOP-16 Package
LTC2328-18/LTC2327-18/
LTC2326-18
18-Bit, 1Msps/500ksps/250ksps, Serial,
Low Power ADC
5V Supply, ±10.24V Pseudo-Differential Input, 95dB SNR,
MSOP-16 Package
LTC2373-18/LTC2372-18 18-Bit, 1Msps/500ksps, 8-Channel, Serial ADC 5V Supply, 8 Channel Multiplexed, Configurable Input Range, 100dB SNR,
DGC, 5mm × 5mm QFN-32 Package
LTC2379-18/LTC2378-18/
LTC2377-18/LTC2376-18
18-Bit,1.6Msps/1Msps/500ksps/250ksps, Serial,
Low Power ADC
2.5V Supply, Differential Input, 101.2dB SNR, ±5V Input Range, DGC, Pin
Compatible Family in MSOP-16 and 4mm × 3mm DFN-16 Packages
LTC2380-16/LTC2378-16/
LTC2377-16/LTC2376-16
16-Bit, 2Msps/1Msps/500ksps/250ksps, Serial,
Low Power ADC
2.5V Supply, Differential Input, 96.2dB SNR, ±5V Input Range, DGC, Pin
Compatible Family in MSOP-16 and 4mm × 3mm DFN-16 Packages
LTC2389-18/LTC2389-16 18-Bit/16-Bit, 2.5Msps, Parallel/Serial ADC 5V Supply, Pin-Configurable Input Range, 99.8dB/96dB SNR, Parallel or
Serial I/O 7mm × 7mm LQFP-48 and QFN-48 Packages
LTC1859/LTC1858/
LTC1857
16-/14-/12-Bit, 8-Channel, 100ksps, Serial ADC ±10V, SoftSpan, Single-Ended or Differential Inputs, Single 5V Supply,
SSOP-28 Package
LTC1609 16-Bit, 200ksps Serial ADC ±10V, Configurable Unipolar/Bipolar Input, Single 5V Supply, SSOP-28 and
SO-20 Packages
DACs
LTC2756/LTC2757 18-Bit, Serial/Parallel IOUT SoftSpan DAC ±1LSB INL/DNL, Software-Selectable Ranges,
SSOP-28/7mm × 7mm LQFP-48 Package
LTC2668 16-Channel 16-/12-Bit ±10V VOUT SoftSpan DACs ±4LSB INL, Precision Reference 10ppm/°C Max, 6mm × 6mm QFN-40 Package
References
LTC6655 Precision Low Drift Low Noise Buffered Reference 5V/2.5V/2.048V/1.25V, 2ppm/°C, 0.25ppm Peak-to-Peak Noise, MSOP-8 Package
LTC6652 Precision Low Drift Low Noise Buffered Reference 5V/2.5V/2.048V/1.25V, 5ppm/°C, 2.1ppm Peak-to-Peak Noise, MSOP-8 Package
Amplifiers
LT1468/LT1469 Single/Dual 90MHz, 22V/µs, 16-Bit Accurate Op Amp Low Input Offset: 75µV/125µV
LT1354/LT1355/LT1356 Single/Dual/Quad 1mA, 12MHz, 400V/µs Op Amp Good DC Precision, Stable with All Capacitive Loads
LT1357/LT1358/LT1359 Single/Dual/Quad 2mA, 25MHz, 800V/µs Op Amp Good DC Precision, Stable with All Capacitive Loads
Digitize Differential Signals Over a Wide Common Mode Range
–5V
–5V
31V
31V
LTC2348-18
234818 F07a
ONLY CHANNEL 0 SHOWN FOR CLARITY
24V
0V
ARBITRARY +
–
+
–
0.1µF
0.1µF
0.1µF
47µF
IN0+
IN0–
½ LT1124
½ LT1124
VCC
REFINREFBUFVEE
LOWPASS FILTERS
BW ~ 500kHz
6.6nF
6.6nF
49.9Ω
49.9Ω
549Ω
18pF
18pF
IN+
IN–
2.49k
2.49k
COMMON MODE
INPUT RANGE
DIFFERENTIAL MODE
INPUT RANGE: ±500mV
