LTC2357CLX-18#PBF - ANALOG DEVICES

LTC2357-18

235718f

For more information www.linear.com/LTC2357-18

TYPICAL APPLICATION

FEATURES DESCRIPTION

Buffered Quad, 18-Bit, 350ksps/Ch

Differential ±10.24V ADC with

30VP-P Common Mode Range

The LT C

2357-18 is an 18-bit, low noise 4-channel

simultaneous sampling successive approximation regis-

ter (SAR) ADC with buffered differential, wide common

mode range picoamp inputs. Operating from a 5V low

voltage supply, flexible high voltage supplies, and using

the internal reference and buffer, each channel of this

SoftSpan™ 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 chan-

nels may also be disabled to increase throughput on the

remaining channels.

The integrated picoamp-input analog buffers, wide

input common mode range and 128dB CMRR of the

LTC2357-18 allow the ADC to directly digitize a variety

of signals using minimal board space and power. This

input signal flexibility, combined with ±3.5LSB INL, no

missing codes at 18 bits, and 96.4dB SNR, makes the

LTC2357-18 an ideal choice for many high voltage appli-

cations requiring wide dynamic range.

The LTC2357-18 supports pin-selectable SPI CMOS (1.8V

to 5V) and LVDS serial interfaces. Between one and four

lanes of data output may be employed in CMOS mode,

allowing the user to optimize bus width and throughput.

All registered trademarks and trademarks are the property of their respective owners. Protected

by U.S. Patents, including 7705765, 7961132, 8319673, 9197235.

APPLICATIONS

n Simultaneous Sampling of 4 Buffered Channels

n 350ksps per Channel Throughput

n 500pA/12nA Max Input Leakage at 85°C/125°C

n ±3.5LSB 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:

n ±10.24V, 0V to 10.24V, ±5.12V, 0V to 5.12V

n ±12.5V, 0V to 12.5V, ±6.25V, 0V to 6.25V

n 96.4dB Single-Conversion SNR (Typical)

n −110dB THD (Typical) at fIN = 2kHz

n 128dB CMRR (Typical) at fIN = 200Hz

n Rail-to-Rail Input Overdrive Tolerance

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 175mW Power Dissipation (44mW/Ch 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

235718 TA01a

VCC VDD VDDLBYP OVDD

BUFFERS

FOUR BUFFERED

SIMULTANEOUS

SAMPLING CHANNELS

DIFFERENTIAL INPUTS IN+/IN– WITH

WIDE INPUT COMMON MODE RANGE

FULLY

DIFFERENTIAL

+10V

–10V

TRUE BIPOLAR

+10V

–10V

ARBITRARY

+10V

–10V

UNIPOLAR

+5V

–5V

S/H

MUX

SDO0

SDO3

SCKO

SCKI

SDI

BUSY

CNV

• • •

LVDS/CMOS

IN0+

IN0–

IN3+

IN3–

18-BIT

SAR ADC

CMOS OR LVDS

I/O INTERFACE

0.1µF

REFIN

47µF

0.1µF

GNDREFBUFVEE

LTC2357-18

±10.24V RANGE

TRUE BIPOLAR DRIVE (IN

–

= 0V)

ALL CHANNELS

OUTPUT CODE

–131072

–65536

65536

131072

–2.0

–1.5

–1.0

–0.5

0.5

1.0

1.5

2.0

INL ERROR (LSB)

235718 TA01b

LTC2357-18

235718f

For more information www.linear.com/LTC2357-18

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 IN3+,

IN0– to IN3– (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

LTC2357C ................................................ 0°C to 70°C

LTC2357I .............................................–40°C to 85°C

LTC2357H .......................................... –40°C to 125°C

Storage Temperature Range .................. –65°C to 150°C

(Notes 1, 2)

GND

IN2–

IN2+

GND

IN1–

IN1+

GND

IN0–

IN0+

GND

VCC

VEE

GND

REFIN

GND

REFBUF

LVDS/CMOS

CNV

IN3+

IN3–

GND

VEE

GND

VDD

GND

VDDLBYP

BUSY

SDI

GND

SDO–

SDO+/SDO3

SCKO–/SDO2

SCKO+/SCKO

OVDD

GND

SCKI–/SCKI

SCKI+/SDO1

SDI–/SDO0

SDI+

GND

TOP VIEW

LX PACKAGE

48-LEAD (7mm × 7mm) PLASTIC LQFP

TJMAX = 150°C, θJA = 53°C/W

ORDER INFORMATION

TRAY PART MARKING* PACKAGE DESCRIPTION TEMPERATURE RANGE

LTC2357CLX-18#PBF LTC2357LX-18 48-Lead (7mm × 7mm) Plastic LQFP 0°C to 70°C

LTC2357ILX-18#PBF LTC2357LX-18 48-Lead (7mm × 7mm) Plastic LQFP –40°C to 85°C

LTC2357HLX-18#PBF LTC2357LX-18 48-Lead (7mm × 7mm) Plastic LQFP –40°C to 125°C

Consult ADI 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/

http://www.linear.com/product/LTC2357-18#orderinfo

LTC2357-18

235718f

For more information www.linear.com/LTC2357-18

ELECTRICAL CHARACTERISTICS

CONVERTER CHARACTERISTICS

SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS

VIN+ Absolute Input Range

(IN0+ to IN3+)

(Note 7) lVEE + 4 VCC – 4 V

VIN– Absolute Input Range

(IN0– to IN3–)

(Note 7) lVEE + 4 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)

–2.5 • VREFBUF

–2.5 • VREFBUF/1.024

–1.25 • VREFBUF

–1.25 • VREFBUF/1.024

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

VCM Input Common Mode

Voltage Range

(Note 7) lVEE + 4 VCC – 4 V

VIN+ – VIN– Input Differential

Overdrive Tolerance

(Note 8) l−(VCC − VEE) (VCC − VEE) V

IOVERDRIVE Input Overdrive

Current Tolerance

VIN+ > VCC, VIN– > VCC (Note 8)

VIN+ < VEE, VIN– < VEE (Note 8)

10 mA

IIN Analog Input Leakage

Current

C-Grade and I-Grade

H-Grade

500

IIN+ – IIN–Analog Input Leakage

Offset Current

VIN+ = VIN–

VIN+ = VIN–, C-Grade and I-Grade

VIN+ = VIN–, H-Grade

–100

–1.2

±1

100

1.2

RIN Analog Input Resistance For Each Pin >1000 GΩ

CIN Analog Input

Capacitance

3 pF

CMRR Input Common Mode

Rejection Ratio

VIN+ = VIN− = 18VP-P 200Hz Sine l105 128 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

1.4

2.8

2.1

4.2

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)

–3.5

–5

–4

–6

±1

±1.5

±1

3.5

LSB

DNL Differential Linearity Error (Note 11) l−0.9 ±0.2 0.9 LSB

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)

LTC2357-18

235718f

For more information www.linear.com/LTC2357-18

DYNAMIC ACCURACY

INTERNAL REFERENCE 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

92.7

86.9

89.3

83.6

96.2

90.3

92.5

86.6

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

92.8

87.0

89.5

83.6

96.4

90.4

92.5

86.6

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

–110

–111

–112

–113

–101

–99

–102

–99

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

101

103

112

113

114

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 6 MHz

Aperture Delay 1 ns

Aperture Delay Matching 150 ps

Aperture Jitter 3 psRMS

Transient Response Full-Scale Step, 0.005% Settling 420 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

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)

CONVERTER 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

ZSE Zero-Scale Error (Note 12) l−900 ±80 900 μV

Zero-Scale Error Drift ±4 μV/°C

FSE Full-Scale Error VREFBUF = 4.096V (REFBUF Overdriven) (Note 12) l−0.1 ±0.025 0.1 %FS

Full-Scale Error Drift VREFBUF = 4.096V (REFBUF Overdriven) (Note 12) ±2.5 ppm/°C

LTC2357-18

235718f

For more information www.linear.com/LTC2357-18

REFERENCE BUFFER CHARACTERISTICS

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, 4 Channels Enabled (Notes 15, 16)

VREFBUF = 5V, Acquisition or Nap Mode (Note 15)

l1.4

0.36

1.5 mA

The l denotes the specifications which apply over the full

operating temperature range, otherwise specifications are at TA = 25°C. (Note 9)

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

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)

LTC2357-18

235718f

For more information www.linear.com/LTC2357-18

SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS

VCC Supply Voltage l7.5 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 350ksps Sample Rate, 4 Channels Enabled (Note 17)

Acquisition Mode (Note 17)

Nap Mode

Power Down Mode

3.4

5.7

1.5

3.9

6.9

1.8

μA

IVEE Supply Current 350ksps Sample Rate, 4 Channels Enabled (Note 17)

Acquisition Mode (Note 17)

Nap Mode

Power Down Mode

–4.1

–7.1

–2

–15

–3.2

–5.6

–1.4

–4

μA

CMOS I/O Mode

OVDD Supply Voltage l1.71 5.25 V

IVDD Supply Current 350ksps Sample Rate, 4 Channels Enabled

350ksps Sample Rate, 4 Channels Enabled, VREFBUF = 5V (Note 15)

Acquisition Mode

Nap Mode

Power Down Mode (C-Grade and I-Grade)

Power Down Mode (H-Grade)

14.3

12.6

1.8

1.6

14.3

2.5

2.2

275

500

μA

µA

IOVDD Supply Current 350ksps Sample Rate, 4 Channels Enabled (CL = 25pF)

Acquisition or Nap Mode

Power Down Mode

1.8

2.6

μA

PDPower Dissipation 350ksps Sample Rate, 4 Channels Enabled

Acquisition Mode

Nap Mode

Power Down Mode (C-Grade and I-Grade)

Power Down Mode (H-Grade)

175

179

0.56

207

223

1.9

LVDS I/O Mode

OVDD Supply Voltage l2.375 5.25 V

IVDD Supply Current 350ksps Sample Rate, 4 Channels Enabled

350ksps Sample Rate, 4 Channels Enabled, VREFBUF = 5V (Note 15)

Acquisition Mode

Nap Mode

Power Down Mode (C-Grade and I-Grade)

Power Down Mode (H-Grade)

16.4

14.9

3.4

3.2

18.1

16.7

4.2

275

500

μA

µA

IOVDD Supply Current 350ksps Sample Rate, 4 Channels Enabled (RL = 100Ω)

Acquisition or Nap Mode (RL = 100Ω)

Power Down Mode

7.5

8.7

8.2

μA

PDPower Dissipation 350ksps Sample Rate, 4 Channels Enabled

Acquisition Mode

Nap Mode

Power Down Mode (C-Grade and I-Grade)

Power Down Mode (H-Grade)

200

204

0.56

232

252

1.9

The l denotes the specifications which apply over the full operating temperature

range, otherwise specifications are at TA = 25°C. (Note 9)

POWER REQUIREMENTS

LTC2357-18

235718f

For more information www.linear.com/LTC2357-18

The l denotes the specifications which apply over the full operating

temperature range, otherwise specifications are at TA = 25°C. (Note 9)

ADC TIMING CHARACTERISTICS

SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS

fSMPL Maximum Sampling Frequency 4 Channels Enabled

3 Channels Enabled

2 Channels Enabled

1 Channel Enabled

350

425

550

800

ksps

tCYC Time Between Conversions 4 Channels Enabled, fSMPL = 350ksps

3 Channels Enabled, fSMPL = 425ksps

2 Channels Enabled, fSMPL = 550ksps

1 Channel Enabled, fSMPL = 800ksps

2855

2350

1815

1250

tCONV Conversion Time N Channels Enabled, 1 ≤ N ≤ 4 l450•N 500•N 550•N ns

tACQ Acquisition Time

(tACQ = tCYC – tCONV – tBUSYLH)

4 Channels Enabled, fSMPL = 350ksps

3 Channels Enabled, fSMPL = 425ksps

2 Channels Enabled, fSMPL = 550ksps

1 Channel Enabled, fSMPL = 800ksps

625

670

685

670

835

830

795

730

tCNVH CNV High Time l40 ns

tCNVL CNV Low Time l750 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 18, 19) l10 ns

tSCKIH SCKI High Time l4 ns

tSCKIL SCKI Low Time l4 ns

tSSDISCKI SDI Setup Time from SCKI↑(Note 18) l2 ns

tHSDISCKI SDI Hold Time from SCKI↑(Note 18) l1 ns

tDSDOSCKI SDO Data Valid Delay from SCKI↑CL = 25pF (Note 18) l7.5 ns

tHSDOSCKI SDO Remains Valid Delay from SCKI↑CL = 25pF (Note 18) l1.5 ns

tSKEW SDO to SCKO Skew (Note 18) l–1 0 1 ns

tDSDOBUSYL SDO Data Valid Delay from BUSY↓CL = 25pF (Note 18) l0 ns

tEN Bus Enable Time After CS↓(Note 18) l15 ns

tDIS Bus Relinquish Time After CS↑(Note 18) l15 ns

LVDS I/O Mode

tSCKI SCKI Period (Note 20) l4 ns

tSCKIH SCKI High Time (Note 20) l1.5 ns

tSCKIL SCKI Low Time (Note 20) l1.5 ns

tSSDISCKI SDI Setup Time from SCKI (Notes 11, 20) l1.2 ns

tHSDISCKI SDI Hold Time from SCKI (Notes 11, 20) l–0.2 ns

tDSDOSCKI SDO Data Valid Delay from SCKI (Notes 11, 20) l6 ns

tHSDOSCKI SDO Remains Valid Delay from SCKI (Notes 11, 20) 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

LTC2357-18

235718f

For more information www.linear.com/LTC2357-18

CMOS Timings

0.8 • OVDD

0.2 • OVDD

50% 50%

235718 F01a

0.2 • OVDD

0.8 • OVDD

0.2 • OVDD

0.8 • OVDD

tDELAY

tWIDTH

tDELAY

LVDS Timings (Differential)

+200mV

–200mV

0V 0V

235718 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 GND.

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 GND or above VDD or

OVDD, they will be clamped by internal diodes. This product can handle

currents of up to 100mA below GND or above VDD or OVDD without

latch-up.

Note 6: –16.5V ≤ VEE ≤ 0V, 7.5V ≤ 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. Driving an analog input above VCC on any

channel up to 10mA will not affect conversion results on other channels.

Driving an analog input below VEE may corrupt conversion results on other

channels. Refer to Applications Information section for further details.

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 = 350ksps,

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: Portions of the analog input circuitry are powered down during

conversion, reducing IVCC and IVEE. Refer to Applications Information

section for more details.

Note 18: Parameter tested and guaranteed at OVDD = 1.71V, OVDD = 2.5V,

and OVDD = 5.25V.

Note 19: A tSCKI period of 10ns minimum allows a shift clock frequency of

up to 100MHz for rising edge capture.

Note 20: VICM = 1.2V, VID = 350mV for LVDS differential input pairs.

ADC TIMING CHARACTERISTICS

LTC2357-18

235718f

For more information www.linear.com/LTC2357-18

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

TA = 25°C, VCC=+15V, VEE=–15V, VDD=5V,

OVDD=2.5V, Internal Reference and Buffer (VREFBUF =4.096V), fSMPL = 350ksps, unless otherwise noted.

±10.24V RANGE

TRUE BIPOLAR DRIVE (IN

–

= 0V)

ALL CHANNELS

OUTPUT CODE

–131072

–65536

65536

131072

–2.0

–1.5

–1.0

–0.5

0.5

1.0

1.5

2.0

INL ERROR (LSB)

235718 G01

±10.24V RANGE

FULLY DIFFERENTIAL DRIVE (IN

–

= –IN

)

ALL CHANNELS

OUTPUT CODE

–131072

–65536

65536

131072

–2.0

–1.5

–1.0

–0.5

0.5

1.0

1.5

2.0

INL ERROR (LSB)

235718 G02

ALL RANGES

ALL CHANNELS

OUTPUT CODE

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)

235718 G03

TRUE BIPOLAR DRIVE (IN

–

= 0V)

ONE CHANNEL

±10.24V AND ±10V

RANGES

±5.12V AND ±5V

RANGES

OUTPUT CODE

–131072

–65536

65536

131072

–2.0

–1.5

–1.0

–0.5

0.5

1.0

1.5

2.0

INL ERROR (LSB)

235718 G04

FULLY DIFFERENTIAL DRIVE (IN

–

= –IN

)

ONE CHANNEL

±10.24V, ±10V

RANGES

±5.12V, AND ±5V

RANGES

OUTPUT CODE

–131072

–65536

65536

131072

–2.0

–1.5

–1.0

–0.5

0.5

1.0

1.5

2.0

INL ERROR (LSB)

235718 G05

UNIPOLAR DRIVE (IN

–

= 0V)

ONE CHANNEL

0V TO 5.12V RANGE

0V TO 10.24V AND

0V TO 10V RANGES

OUTPUT CODE

65536

131072

196608

262144

–2.0

–1.5

–1.0

–0.5

0.5

1.0

1.5

2.0

INL ERROR (LSB)

235718 G06

±10.24V RANGE

TRUE BIPOLAR DRIVE (IN

–

= 0V)

ARBITRARY DRIVE

/IN

–

COMMON MODE

SWEPT –10.24V TO 10.24V

OUTPUT CODE

–131072

–65536

65536

131072

–2.0

–1.5

–1.0

–0.5

0.5

1.0

1.5

2.0

INL ERROR (LSB)

235718 G07

±10.24V RANGE

σ = 1.35

CODE

–6

–4

–2

10000

20000

30000

40000

50000

60000

70000

80000

90000

COUNTS

235718 G08

±10.24V RANGE

σ = 1.4

CODE

131044

131047

131050

131053

131056

10000

20000

30000

40000

50000

60000

70000

80000

90000

COUNTS

235718 G09

LTC2357-18

235718f

For more information www.linear.com/LTC2357-18

TYPICAL PERFORMANCE CHARACTERISTICS

32k Point FFT fSMPL = 350ksps,

fIN=2kHz

SNR, SINAD vs VREFBUF,

fIN=2kHz

THD, Harmonics vs VREFBUF,

fIN=2kHz

SNR, SINAD

vs Input Frequency

THD vs Input Frequency

and Source Resistance

THD, Harmonics vs Input

Common Mode, fIN = 2kHz

32k Point FFT fSMPL=350ksps,

fIN=2kHz

32k Point FFT fSMPL=350ksps,

fIN=2kHz

32k Point Arbitrary Two-Tone FFT

fSMPL=350ksps, IN+=–7dBFS 2kHz

Sine, IN–=–7dBFS 3.1kHz Sine

TA = 25°C, VCC=+15V, VEE=–15V, VDD=5V,

OVDD=2.5V, Internal Reference and Buffer (VREFBUF =4.096V), fSMPL = 350ksps, unless otherwise noted.

±10.24V RANGE

TRUE BIPOLAR DRIVE (IN

–

= 0V)

SNR = 96.4dB

THD = –110dB

SINAD = 96.2dB

SFDR = 112dB

FREQUENCY (kHz)

100

125

150

175

–180

–160

–140

–120

–100

–80

–60

–40

–20

AMPLITUDE (dBFS)

235718 G10

±10.24V RANGE

FULLY DIFFERENTIAL DRIVE (IN

–

= –IN

)

SNR = 96.4dB

THD = –118dB

SINAD = 96.4dB

SFDR = 121dB

FREQUENCY (kHz)

100

125

150

175

–180

–160

–140

–120

–100

–80

–60

–40

–20

AMPLITUDE (dBFS)

235718 G11

±10.24V RANGE

ARBITRARY DRIVE

SFDR = 119dB

SNR = 96.6dB

6.2kHz

FREQUENCY (kHz)

100

125

150

175

–180

–160

–140

–120

–100

–80

–60

–40

–20

AMPLITUDE (dBFS)

235718 G12

±5.12V RANGE

TRUE BIPOLAR DRIVE (IN

–

= 0V)

SNR = 92.7dB

THD = –111dB

SINAD = 92.6dB

SFDR = 115dB

FREQUENCY (kHz)

100

125

150

175

–180

–160

–140

–120

–100

–80

–60

–40

–20

AMPLITUDE (dBFS)

235718 G13

SNR

SINAD

±2.5 • V

REFBUF

RANGE

TRUE BIPOLAR DRIVE (IN

–

= 0V)

REFBUF VOLTAGE (V)

2.5

3.5

4.5

100

SNR, SINAD (dBFS)

235718 G14

THD

2ND

3RD

±2.5 • V

REFBUF

RANGE

TRUE BIPOLAR DRIVE (IN

–

= 0V)

REFBUF VOLTAGE (V)

2.5

3.5

4.5

–130

–125

–120

–115

–110

–105

–100

THD, HARMONICS (dBFS)

235718 G15

SNR

SINAD

±10.24V RANGE

TRUE BIPOLAR DRIVE (IN

–

= 0V)

FREQUENCY (Hz)

100

10k

100k

100

SNR, SINAD (dBFS)

235718 G16

±10.24V RANGE

TRUE BIPOLAR DRIVE (IN

–

= 0V)

50Ω Source

1kΩ

Source

10kΩ

Source

FREQUENCY (Hz)

100

10k

100k

–130

–120

–110

–100

–90

–80

–70

–60

THD (dBFS)

235718 G17

2ND

THD

3RD

–11V ≤ V

≤ 11V

COMMON MODE LIMITS

INPUT COMMON MODE (V)

–15

–10

–5

–160

–140

–120

–100

–80

–60

–40

–20

THD, HARMONICS (dBFS)

235718 G18

±10.24V RANGE

P–P

FULLY DIFFERENTIAL DRIVE

LTC2357-18

235718f

For more information www.linear.com/LTC2357-18

TYPICAL PERFORMANCE CHARACTERISTICS

SNR, SINAD vs Temperature,

fIN=2kHz

THD, Harmonics vs Temperature,

fIN = 2kHz INL, DNL vs Temperature

Analog Input Leakage

Current vs Temperature

Positive 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

TA = 25°C, VCC=+15V, VEE=–15V, VDD=5V,

OVDD=2.5V, Internal Reference and Buffer (VREFBUF =4.096V), fSMPL = 350ksps, unless otherwise noted.

SNR

SINAD

±10.24V RANGE

TRUE BIPOLAR DRIVE (IN

–

= 0V)

INPUT LEVEL (dBFS)

–40

–30

–20

–10

96.4

96.6

96.8

97.0

97.2

97.4

SNR, SINAD (dBFS)

235718 G19

±10.24V RANGE

= IN

–

= 18V

P–P

SINE

ALL CHANNELS

FREQUENCY (Hz)

100

10k

100k

100

120

140

160

CMRR (dB)

235718 G20

±10.24V RANGE

IN0

= 0V

IN0

–

= 18V

P–P

SINE

ALL CHANNELS CONVERTING

CH3

CH1

FREQUENCY (Hz)

100

10k

100k

–125

–120

–115

–110

–105

–100

–95

–90

CROSSTALK (dB)

235718 G21

SNR

SINAD

±10.24V RANGE

TRUE BIPOLAR DRIVE (IN

–

= 0V)

TEMPERATURE (°C)

–55

–35

–15

105

125

94.0

94.5

95.0

95.5

96.0

96.5

97.0

97.5

98.0

SNR, SINAD (dBFS)

235718 G22

THD

2ND

3RD

±10.24V RANGE

TRUE BIPOLAR DRIVE (IN

–

= 0V)

TEMPERATURE (°C)

–55

–35

–15

105

125

–125

–120

–115

–110

–105

–100

–95

THD, HARMONICS (dBFS)

235718 G23

MAX INL

MIN INL

MAX DNL

MIN DNL

±10.24V RANGE

TRUE BIPOLAR DRIVE (IN

–

= 0V)

ALL CHANNELS

TEMPERATURE (°C)

–55

–35

–15

105

125

–2.0

–1.5

–1.0

–0.5

0.5

1.0

1.5

2.0

INL, DNL ERROR (LSB)

235718 G24

8 ANALOG INPUT PIN TRACES

FOR EACH INPUT VOLTAGE

TEMPERATURE (°C)

–55

–35

–15

105

125

0.1

100

10k

ANALOG INPUT LEAKAGE CURRENT (pA)

235718 G25

IN = 0V

IN = +10V

IN = –10V

±10.24V RANGE

REFBUF OVERDRIVEN

REFBUF

= 4.096V

ALL CHANNELS

TEMPERATURE (°C)

–55

–35

–15

105

125

–0.100

–0.075

–0.050

–0.025

0.000

0.025

0.050

0.075

0.100

FULL-SCALE ERROR (%)

235718 G26

±10.24V RANGE

ALL CHANNELS

TEMPERATURE (°C)

–55

–35

–15

105

125

–5

–4

–3

–2

–1

ZERO-SCALE ERROR (LSB)

235718 G27

LTC2357-18

235718f

For more information www.linear.com/LTC2357-18

TYPICAL PERFORMANCE CHARACTERISTICS

Supply Current vs Sampling Rate

Power Dissipation vs Sampling

Rate, N-Channels Enabled

Step Response

(Large-Signal Settling)

Supply Current vs Temperature

Power-Down Current

vs Temperature

Offset Error

vs Input Common Mode

Internal Reference Output

vs Temperature

TA = 25°C, VCC=+15V, VEE=–15V, VDD=5V,

OVDD=2.5V, Internal Reference and Buffer (VREFBUF =4.096V), fSMPL = 350ksps, unless otherwise noted.

PSRR vs Frequency

Step Response

(Fine Settling)

OVDD

VDD

VEE

VCC

TEMPERATURE (°C)

–55

–35

–15

105

125

–6

–4

–2

SUPPLY CURRENT (mA)

235718 G28

OVDD

VDD

–I

VEE

VCC

TEMPERATURE (°C)

–55

–35

–15

105

125

0.01

0.1

100

1000

POWER-DOWN CURRENT (µA)

235718 G29

= IN

–

= 0V

FREQUENCY (Hz)

100

10k

100k

100

110

120

130

140

150

PSRR (dB)

235718 G30

= 38V, V

= 0V

= 4V TO 34V

±10.24V RANGE

= 21.5V, V

= –16.5V

= –12.5V TO 17.5V

INPUT COMMON MODE (V)

–17

–2.0

–1.5

–1.0

–0.5

0.5

1.0

1.5

2.0

OFFSET ERROR (LSB)

235718 G31

15 UNITS

TEMPERATURE (°C)

–55

–35

–15

105

125

2.045

2.046

2.047

2.048

2.049

2.050

2.051

INTERNAL REFERENCE OUTPUT (V)

235718 G32

OVDD

VDD

VEE

VCC

WITH NAP MODE

CNVL

= 770ns

SAMPLING RATE (ksps)

100

150

200

250

300

350

–6

–4

–2

SUPPLY CURRENT (mA)

235718 G33

WITH NAP MODE

CNVL

= 750ns

N = 4

N = 1

N = 2

N = 3

SAMPLING RATE (ksps)

100

200

300

400

500

600

700

800

100

120

140

160

180

200

POWER DISSIPATION (mW)

235718 G34

±10.24V RANGE

= 350.00045kHz SQUARE WAVE

–

= 0V

SETTLING TIME (ns)

–100

100

200

300

400

500

600

700

800

900

–131072

–98304

–65536

–32768

32768

65536

98304

131072

OUTPUT CODE (LSB)

235718 G35

±10.24V RANGE

= 350.00045kHz

SQUARE WAVE

–

= 0V

SETTLING TIME (ns)

–100

100

200

300

400

500

600

700

800

900

–250

–200

–150

–100

–50

100

150

200

250

DEVIATION FROM FINAL VALUE (LSB)

235718 G36

LTC2357-18

235718f

For more information www.linear.com/LTC2357-18

PIN FUNCTIONS

Pins that are the Same for All Digital I/O Modes

IN0+/IN0– to IN3+/IN3– (Pins 14/13, 10/9, 4/3, and

48/47): Positive and Negative Analog Inputs, Channels 0

to 3. The converter simultaneously samples and digitizes

(VIN+ – VIN–) for all channels. Wide input common mode

range (VEE + 4V≤ VCM ≤ VCC – 4V) and high common

mode rejection allow the inputs to accept a wide variety

of signal swings. Full-scale input range is determined by

the channel’s SoftSpan configuration.

GND (Pins 1, 2, 5, 6, 7, 8, 11, 12, 15, 18, 20, 25, 30,

36, 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 7.5V 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.

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. Do not load this pin when internal refer-

ence is used.

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 ref-

erence buffer may be disabled by grounding its input at

REFIN. With the buffer disabled, overdrive REFBUF with

an external 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

200µA. 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 LTC2357-18 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

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.

LTC2357-18

235718f

For more information www.linear.com/LTC2357-18

PIN FUNCTIONS

CMOS I/O Mode

SDI+, SDO– (Pins 26 and 35): LVDS Input and Output. In

CMOS I/O mode these pins are Hi-Z.

SDO0 to SDO3 (Pins 27, 28, 33, and 34): CMOS Serial

Data Outputs, Channels 0 to 3. The most recent conver-

sion result along with channel configuration information

is clocked out onto the SDO pins on each rising edge of

SCKI. Output data formatting is described in the Digital

Interface section. Leave unused SDO outputs uncon-

nected. 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 SDO3. 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 SDO3. 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 12-bit SoftSpan configuration word (see

Table1a), latched on the rising edges of SCKI. If all chan-

nels 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

SDI+/SDI– (Pins 26/27): LVDS Positive and Negative

Serial Data Input. Differentially drive SDI+/SDI– with the

desired 12-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 28/29): 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 tran-

sitioning CS. The SCKI+/SCKI– input pair is internally ter-

minated with a 100Ω differential resistor when CS is low.

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 32/33): 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 termi-

nated with a 100Ω resistor at the receiver (FPGA).

SDO+/SDO– (Pins 34/35): LVDS Positive and Negative

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).

SDI (Pin 37): CMOS Serial Data. In LVDS I/O mode, this

pin is Hi-Z.

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 resis-

tors on the SCKI

/SCKI

–

and SDI

/SDI

–

input pairs are

disabled when CS is high. Logic levels are determined

by OVDD.

LTC2357-18

235718f

For more information www.linear.com/LTC2357-18

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 12-Bit SoftSpan Configuration Word S[11:0]. Use

Serial Interface to Write SoftSpan Configuration Word to LTC2357-18, as shown in Figure 18

BINARY SoftSpan CODE

SS[2:0] ANALOG INPUT RANGE FULL SCALE RANGE BINARY FORMAT OF

CONVERSION RESULT

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 LTC2357-18 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

LTC2357-18

235718f

For more information www.linear.com/LTC2357-18

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 LTC2357-18 Supports Three Reference Configurations. Analog Input Range

Scales with the Converter Master Reference Voltage, VREFBUF

LTC2357-18

235718f

For more information www.linear.com/LTC2357-18

FUNCTIONAL BLOCK DIAGRAM

LVDS I/O Mode

CMOS I/O Mode

SDO0

SDO3

SCKO

SDI

SCKI

BUSY

18-BIT

SAR ADC

CMOS

SERIAL

I/O

INTERFACE

235718 BD01

18 BITS

REFERENCE

BUFFER

REFBUFREFINGND

VCC

VEE

VDDLBYP

VDD OVDD

LTC2357-18

CONTROL

LOGIC

2.048V

REFERENCE

2.5V

REGULATOR

LVDS/CMOSPDCNV

20k

2×

• • •

BUFFERS

IN0+

IN0–S/H

S/H

IN1+

IN1–

S/H

IN2+

IN2–

S/H

IN3+

IN3–

4-CHANNEL MULTIPLEXER

SDO+

SDO–

SCKO+

SCKO–

SDI+

SDI–

SCKI+

SCKI–

BUSY

18-BIT

SAR ADC

LVDS

SERIAL

I/O

INTERFACE

235718 BD02

18 BITS

REFERENCE

BUFFER

REFBUFREFINGND

VCC

VEE

VDDLBYP

VDD OVDD

LTC2357-18

CONTROL

LOGIC

2.048V

REFERENCE

2.5V

REGULATOR

LVDS/CMOSPDCNV

20k

2×

BUFFERS

IN0+

IN0–S/H

S/H

IN1+

IN1–

S/H

IN2+

IN2–

S/H

IN3+

IN3–

4-CHANNEL MULTIPLEXER

LTC2357-18

235718f

For more information www.linear.com/LTC2357-18

TIMING DIAGRAM

LVDS I/O Mode

CMOS I/O Mode

1 2 3 4 5 6 7 8 9 10 11 12

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 0 C1 C0

0 C1 C0

SS1SS2 SS0 D17

CNV

CS = PD = 0

CONVERT

DON’T CARE

ACQUIRE

BUSY

SDO3

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 3

CONVERSION N

CHANNEL 0

CONVERSION N

235718 TD01

CONVERSION RESULT CHAN 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 CHAN ID SoftSpan CONVERSION RESULT

• • •

13 14 15 16 17 18 19 20 21 22 23 24

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

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

235718 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 3

CONVERSION N

CONVERSION RESULT CHAN ID SoftSpan

90 91 92 93 94 95 96

D0 SS1SS2 SS0 D17

CHANNEL 0

CONVERSION N

CONVERSION

RESULT

CHAN ID SoftSpan

0 C1 C0 0 C1 C0

LTC2357-18

235718f

For more information www.linear.com/LTC2357-18

APPLICATIONS INFORMATION

OVERVIEW

The LTC2357-18 is an 18-bit, low noise 4-channel

simultaneous sampling successive approximation regis-

ter (SAR) ADC with buffered differential, wide common

mode range picoamp 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 independently 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 integrated picoamp-input analog buffers, wide input

common mode range, and 128dB CMRR of the LTC2357-18

allow the ADC to directly digitize a variety of signals using

minimal board space and power. This input signal flex-

ibility, combined with ±3.5LSB INL, no missing codes at

18 bits, and 96.4dB SNR, makes the LTC2357-18 an ideal

choice for many high voltage applications requiring wide

dynamic range.

The absolute common mode input range (VEE+4V to

VCC–4V) is determined by the choice of high voltage

supplies. These supplies may be biased asymmetrically

around ground and include the ability for VEE to be tied

directly to ground.

The LTC2357-18 supports pin-selectable SPI CMOS

(1.8V to 5V) and LVDS serial interfaces, enabling it to

communicate equally well with legacy microcontrollers

and modern FPGAs. In CMOS mode, applications may

employ between one and four lanes of serial output data,

allowing the user to optimize bus width and data through-

put. The LTC2357-18 typically dissipates 175mW when

converting four channels simultaneously at 350ksps per

channel. Optional nap and power down modes may be

employed to further reduce power consumption during

inactive periods.

CONVERTER OPERATION

The LTC2357-18 operates in two phases. During the acqui-

sition phase, the sampling capacitors in each channel’s

sample-and-hold (S/H) circuit connect to their respective

analog input buffers, which track the differential analog

input voltage (VIN+ – VIN–). A rising edge on the CNV pin

transitions 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 algo-

rithm, 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 LTC2357-18 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 (V

REFBUF

= 4.096V nomi-

nal), 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

LTC2357-18

235718f

For more information www.linear.com/LTC2357-18

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.

INPUT VOLTAGE (V)

OUTPUT CODE (TWO’S COMPLEMENT)

–1

LSB

235718 F02

011...111

011...110

000...001

000...000

100...000

100...001

111...110

LSB

BIPOLAR

ZERO

111...111

FSR/2 – 1LSB–FSR/2

FSR = +FS – –FS

1LSB = FSR/262144

Figure2. LTC2357-18 Two’s Complement Transfer Function

INPUT VOLTAGE (V)

OUTPUT CODE (STRAIGHT BINARY)

235718 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. LTC2357-18 Straight Binary Transfer Function

BUFFERED ANALOG INPUTS

Each channel of the LTC2357-18 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 arbi-

trary relationship to each other, provided each pin remains

between (VEE+4V) and (VCC–4V). This feature of the

APPLICATIONS INFORMATION

LTC2357-18 enables it to accept a wide variety of signal

swings, including traditional classes of analog input sig-

nals such as pseudo-differential unipolar, pseudo-differ-

ential true bipolar, and fully differential, simplifying signal

chain design. For conversion of signals extending to VEE,

the unbuffered LTC2348-18 ADC is recommended.

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 individually allowed operating

ranges, including the ability for VEE to be tied directly to

ground. This feature enables the common mode input

range of the LTC2357-18 to be tailored to specific applica-

tion 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 sampling capacitors (CSAMP)

connect to the integrated buffers BUFFER+/BUFFER–

through the sampling switches. The sampled voltage

is reset during the conversion process and is therefore

re-acquired for each new conversion.

The diodes between the inputs and the VCC and VEE sup-

plies provide input ESD protection. While within the sup-

ply voltages, the analog inputs of the LTC2357-18 draw

only 5pA typical DC leakage current and the ESD protec-

tion diodes don’t turn on. This offers a significant advan-

tage over external op amp buffers, which often have diode

protection that turns on during transients and corrupts the

voltage on any filter capacitors at their inputs.

IN+

RSAMP

750Ω

RSAMP

750Ω

CSAMP

30pF

CSAMP

30pF

VCC

VEE

BIAS

VOLTAGE

IN–235718 F04

BUFFER+

BUFFER–

Figure4. Equivalent Circuit for Differential Analog

Inputs, Single Channel Shown

LTC2357-18

235718f

For more information www.linear.com/LTC2357-18

APPLICATIONS INFORMATION

Bipolar SoftSpan Input Ranges

For channels configured in SoftSpan ranges 7, 6, 3,

or 2, the LTC2357-18 digitizes the differential analog

input voltage (VIN+ – VIN–) over a bipolar span of

±2.5•V

REFBUF

, ±2.5•V

REFBUF

/1.024, ±1.25•V

REFBUF

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+4V). 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

LTC2357-18 digitizes the differential analog input voltage

(VIN+ – VIN–) over a unipolar span of 0V to 2.5•VREFBUF,

0V to 2.5•V

REFBUF

/1.024, or 0V to 1.25•V

REFBUF

, respec-

tively, as shown in Table 1a. These SoftSpan ranges are

useful for digitizing input signals where IN+ remains above

–

. 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+4V). The

output data format for all unipolar SoftSpan ranges is

straight binary.

INPUT DRIVE CIRCUITS

The CMOS buffer input stage offers a very high degree

of transient isolation from the sampling process. Most

sensors, signal conditioning amplifiers and filter networks

with less than 10kΩ of impedance can drive the passive

3pF analog input capacitance directly. For higher imped-

ances and slow-settling circuits, add a 680pF capaci-

tor at the pins to maintain the full DC accuracy of the

LTC2357-18.

The very high input impedance of the unity gain buffers in

the LTC2357-18 greatly reduces the input drive require-

ments and makes it possible to include optional RC filters

with kΩ impedance and arbitrarily slow time constants

for anti-aliasing or other purposes. Micropower op amps

with limited drive capability are also well suited to drive

the high impedance analog inputs directly.

The LTC2357-18 features proprietary circuitry to achieve

exceptional internal crosstalk isolation between channels

(–109dB typical). The PC board wiring to the analog inputs

should be short and shielded to prevent external capaci-

tive crosstalk between channels. The capacitance between

adjacent package pins is 0.16pF. Low source resistance

and/or high source capacitance help reduce external

capacitively coupled crosstalk. Single ended input drive

also enjoys additional external crosstalk isolation because

every other input pin is grounded, or at a low impedance

DC source, and serves as a shield between channels.

INPUT OVERDRIVE TOLERANCE

Driving an analog input above VCC on any channel up to

10mA will not affect conversion results on other chan-

nels. Approximately 70% of this overdrive current will

flow out of the VCC pin and the remaining 30% will flow

out of VEE. This current flowing out of VEE will produce

heat across the VCC – VEE voltage drop and must be taken

into account for the total Absolute Maximum power dis-

sipation of 500mW. Driving an analog input below V

may corrupt conversion results on other channels. This

product can handle input currents of up to 100mA below

VEE or above VCC without latch-up.

Keep in mind that driving the inputs above VCC or below

VEE may reverse the normal current flow from the external

power supplies driving these pins.

LTC2357-18

235718f

For more information www.linear.com/LTC2357-18

APPLICATIONS INFORMATION

each pin remains between (V

– 4V) and (V

+4V). This

feature of the LTC2357-18 enables it to accept a wide

variety of signal swings, simplifying signal chain design.

The two-tone test shown in Figure 6b demonstrates

the arbitrary input drive capability of the LTC2357-18.

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 spectral 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 LTC2357-18 response approaches this ideal, with

121dB of SFDR limited by the converter's second har-

monic distortion response to the 3.1kHz sine wave on IN

–

The ability of the LTC2357-18 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 LTC2357-18 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. Figure7b shows measured CMRR performance

of this solution, which is competitive with the best com-

mercially available instrumentation amplifiers. Figure7c

shows measured AC performance of this solution.

In Figure 8, another application circuit is shown which

uses two channels of the LTC2357-18 to simultaneously

sense the voltage and bidirectional current through a

sense resistor over a wide common mode range.

Input Filtering

The true high impedance analog inputs can accommodate

a very wide range of passive or active signal conditioning

filters. The buffered ADC inputs have an analog bandwidth

of 6MHz, and impose no particular bandwidth require-

ment on external filters. The external input filters can

therefore be optimized independent of the ADC to reduce

signal chain noise and interference. A common filter con-

figuration is the simple anti-aliasing and noise reducing

RC filter with its pole at half the sampling frequency. For

example, 175kHz with R=1.33kΩ and C=680pF as shown

in Figure 5.

–15V

15V

LTC2357-18

235718 F05

ONLY CHANNEL 0 SHOWN FOR CLARITY

TRUE BIPOLAR

+10V

–10V

+10V

–10V

UNIPOLAR

0.1µF

47µF

IN0+

IN0–

VCC

REFINREFBUFVEE

OPTIONAL

LOWPASS FILTER

680pF

R = 1.33k

IN+

IN–

Figure5. Filtering Single-Ended Input Signals

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.

Arbitrary and Fully Differential Analog Input Signals

The wide common mode input range and high CMRR of

the LTC2357-18 allow each channel’s IN+ and IN– pins to

swing with an arbitrary relationship to each other, provided

LTC2357-18

235718f

For more information www.linear.com/LTC2357-18

APPLICATIONS INFORMATION

±10.24V RANGE

SFDR = 119dB

SNR = 96.6dB

6.2kHz

FREQUENCY (kHz)

100

125

150

175

–180

–160

–140

–120

–100

–80

–60

–40

–20

AMPLITUDE (dBFS)

235718 F06b

Figure 6b. Two-Tone Test. IN+ = –7dBFS 2kHz Sine,

IN–=–7dBFS 3.1kHz Sine, 32k Point FFT, fSMPL = 350ksps.

CircuitShown in Figure 6a

±10.24V RANGE

SNR = 96.4dB

THD = –118dB

SINAD = 96.4dB

SFDR = 121dB

FREQUENCY (kHz)

100

125

150

175

–180

–160

–140

–120

–100

–80

–60

–40

–20

AMPLITUDE (dBFS)

235718 F06c

Figure 6c. IN+/IN– = –1dBFS 2kHz Fully Differential Sine,

VCM=0V, 32k Point FFT, fSMPL = 350ksps. Circuit Shown in

Figure 6a

–15V

15V

LTC2357-18

235718 F06a

ONLY CHANNEL 0 SHOWN FOR CLARITY

FULLY

DIFFERENTIAL

+10V

–10V

TRUE BIPOLAR

+10V

–10V

ARBITRARY

+10V

–10V

UNIPOLAR

0.1µF

47µF

+5V

–5V IN0+

IN0–

VCC

REFINREFBUFVEE

IN+

IN–

Figure 6a. Input Arbitrary, Fully Differential, True Bipolar, and Unipolar Signals

±10.24V RANGE

SNR = 96.4dB

THD = –110dB

SINAD = 96.2dB

SFDR = 112dB

FREQUENCY (kHz)

100

125

150

175

–180

–160

–140

–120

–100

–80

–60

–40

–20

AMPLITUDE (dBFS)

235718 F06d

Figure 6d. IN+ = –1dBFS 2kHz True Bipolar Sine, IN– = 0V, 32k

Point FFT, fSMPL = 350ksps. Circuit Shown in Figure 6a

0V TO 10.24V RANGE

SNR = 90.7dB

THD = –113dB

SINAD = 90.7dB

SFDR = 115dB

FREQUENCY (kHz)

100

125

150

175

–180

–160

–140

–120

–100

–80

–60

–40

–20

AMPLITUDE (dBFS)

235718 F06e

Figure 6e. IN+ = –1dBFS 2kHz Unipolar Sine, IN– = 0V,

32k Point FFT, fSMPL = 350ksps. Circuit Shown in Figure 6a

Arbitrary Drive Fully Differential Drive

True Bipolar Drive Unipolar Drive

LTC2357-18

235718f

For more information www.linear.com/LTC2357-18

–7V

31V

BUFFERED

ANALOG

INPUTS

LTC2357-18

235718 F07a

ONLY CHANNEL 0 SHOWN FOR CLARITY

24V

ARBITRARY

–

0.1µF

47µF

IN0+

IN0–

LTC2057HV

VCC

REFINREFBUFVEE

BW = 10kHz

2.2nF

3.65k

549Ω

IN+

IN–

2.49k

COMMON MODE

INPUT RANGE

DIFFERENTIAL MODE

INPUT RANGE: ±500mV

INTERNAL HI-Z BUFFERS

ALLOW OPTIONAL

kΩ PASSIVE FILTERS

GAIN = 10

Figure 7a. Amplify Differential Signals with Gain of 10

Over a Wide Common Mode Range with Buffered Analog Inputs

APPLICATIONS INFORMATION

±5V RANGE

= IN

–

= 5V

P–P

SINE

FREQUENCY (Hz)

100

10k

100

110

120

130

140

150

160

CMRR (dB)

235718 F07b

Figure 7b. CMRR vs Input Frequency.

Circuit Shown in Figure 7a

SNR = 91.9dB

THD = –105dB

SINAD = 91.7dB

SFDR = 107dB

±5V RANGE

FULLY DIFFERENTIAL DRIVE (IN

–

= –IN

)

FREQUENCY (kHz)

100

125

150

175

–180

–160

–140

–120

–100

–80

–60

–40

–20

AMPLITUDE (dBFS)

235718 F07c

Figure 7c. IN+/IN–=450mV 200Hz Fully

Differential Sine, 0V≤VCM≤24V, 32k Point FFT,

fSMPL=350ksps. Circuit Shown in Figure 7a

–15V

15V

LTC2357-18

235718 F08

ONLY CHANNELS 0 AND 1 SHOWN FOR CLARITY

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

LTC2357-18

235718f

For more information www.linear.com/LTC2357-18

ADC REFERENCE

As shown previously in Table 1b, the LTC2357-18 sup-

ports three reference configurations. The first uses both

the internal bandgap reference and reference buffer. The

second externally overdrives the internal reference but

retains the internal buffer, which isolates the external ref-

erence from ADC conversion transients. This configura-

tion is ideal for sharing a single precision external refer-

ence across multiple ADCs. The third disables the internal

buffer and overdrives the REFBUF pin externally.

Internal Reference with Internal Buffer

The LTC2357-18 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 9a. 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 V

REFBUF

= 2•V

REFIN

on the REFBUF

pin, nominally 4.096V when using the internal bandgap

reference. 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.

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 9b. 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

APPLICATIONS INFORMATION

235718 F09a

47µF 6.5k

20k

LTC2357-18

REFBUF

REFIN

GND

BANDGAP

REFERENCE

6.5k

0.1µF

REFERENCE

BUFFER

Figure 9a. Internal Reference with Internal

Buffer Configuration

235718 F09b

47µF 6.5k

20k

LTC2357-18

REFBUF

REFIN

GND

BANDGAP

REFERENCE

6.5k

2.7µF

LTC6655-2.048

REFERENCE

BUFFER

Figure 9b. External Reference with Internal

Buffer Configuration

235718 F09c

47µF 6.5k

20k

LTC2357-18

REFBUF

REFIN

GND

BANDGAP

REFERENCE

6.5k

LTC6655-5

REFERENCE

BUFFER

Figure 9c. External Reference with Disabled

Internal Buffer Configuration

LTC2357-18

235718f

For more information www.linear.com/LTC2357-18

with the LTC2357-18 when overdriving the internal ref-

erence. The LTC6655-2.048 offers 0.025% (maximum)

initial accuracy and 2ppm/°C (maximum) temperature

coefficient for high precision applications. The LTC6655-

2.048 is fully specified over the H-grade temperature

range, complementing the extended temperature range

of the LTC2357-18 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 9c. Maximum input signal swing and SNR are

achieved by overdriving REFBUF using an external 5V ref-

erence. 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 97.9dB when paired with

the LTC2357-18. Bypass the LTC6655-5 to GND (Pin 20)

close to the REFBUF pin with at least a 47μF ceramic

capacitor (X7R, 10V, 1210 size or X5R, 10V, 0805 size) to

absorb transient conversion currents and minimize noise.

The LTC2357-18 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 exter-

nal REFBUF bypass capacitor, but on longer time scales

all of the charge is supplied by either the reference buf-

fer, 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 Figure10, IREFBUF quickly transitions

from approximately 0.4mA to 1.4mA (VREFBUF = 5V,

fSMPL=350ksps). This current step triggers a transient

response in the external reference that must be consid-

ered, 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 ref-

erence 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 11

compares the burst conversion response of the LTC2357-

18 with an input near full scale for two reference con-

figurations. The first configuration employs the internal

reference buffer with REFIN externally overdriven by an

LTC6655-2.048, while the second configuration disables

the internal reference buffer and overdrives REFBUF with

an external LTC6655-4.096. In both cases REFBUF is

bypassed to GND with a 47µF ceramic capacitor.

EXTERNAL REFERENCE ON REFBUF

INTERNAL REFERENCE BUFFER

±10.24V RANGE

= 10V

–

= 0V

TIME (µs)

100

200

300

400

500

–5

DEVIATION FROM FINAL VALUE (LSB)

235718 F11

Figure11. Burst Conversion Response of the LTC2357-18,

fSMPL=350ksps

APPLICATIONS INFORMATION

CNV

IDLE

PERIOD

IDLE

PERIOD

235718 F10

Figure10. CNV Waveform Showing Burst Sampling

LTC2357-18

235718f

For more information www.linear.com/LTC2357-18

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 LTC2357-18 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 12 shows that the LTC2357-18 achieves a

typical SINAD of 96.2dB in the ±10.24V range at a 350ksps

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 12 shows

that the LTC2357-18 achieves a typical SNR of 96.4dB in

the ±10.24V range at a 350ksps 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 funda-

mental itself. The out-of-band harmonics alias into the

frequency band between DC and half the sampling fre-

quency (fSMPL/2). THD is expressed as:

THD =20log V22+V32+V42...VN2

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 12

shows that the LTC2357-18 achieves a typical THD of

–110dB (N=6) in the ±10.24V range at a 350ksps sam-

pling rate with a true bipolar 2kHz input signal.

±10.24V RANGE

TRUE BIPOLAR DRIVE (IN

–

= 0V)

SNR = 96.4dB

THD = –110dB

SINAD = 96.2dB

SFDR = 112dB

FREQUENCY (kHz)

100

125

150

175

–180

–160

–140

–120

–100

–80

–60

–40

–20

AMPLITUDE (dBFS)

235718 F12

Figure12. 32k Point FFT fSMPL = 350ksps, fIN = 2kHz

POWER CONSIDERATIONS

The LTC2357-18 requires four power supplies: the posi-

tive and negative high voltage power supplies (VCC and

), the 5V core power supply (V

) 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 VEE to be tied directly to

ground. This feature enables the common mode input

range of the LTC2357-18 to be tailored to the specific

application’s requirements. The flexible OVDD supply

allows the LTC2357-18 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 LTC2357-18 does not have any specific power sup-

ply sequencing requirements. Care should be taken to

adhere to the maximum voltage relationships described

in the Absolute Maximum Ratings section. The LTC2357-

18 has an internal power-on-reset (POR) circuit which

APPLICATIONS INFORMATION

LTC2357-18

235718f

For more information www.linear.com/LTC2357-18

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 LTC2357-18 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 initiat-

ing 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

conversion 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 LTC2357-18 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 625ns when converting four channels simultaneously,

throughput performance of 350ksps is guaranteed with-

out any external adjustments. Also note that the minimum

acquisition time varies with sampling frequency (fSMPL)

and the number of enabled channels.

Nap Mode

The LTC2357-18 can be placed into nap mode after a

conversion has been completed to reduce power con-

sumption 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 conver-

sions, as shown in Figure 13. To initiate a new conversion

after entering nap mode, bring CNV low and hold for at

least 750ns 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 LTC2357-18 is powered

down and subsequent conversion requests are ignored. If

this occurs during a conversion, the device powers down

APPLICATIONS INFORMATION

CNV

tCONV

tACQ

BUSY

NAP NAP MODE

tCNVL

235718 F13

Figure13. Nap Mode Timing for the LTC2357-18

LTC2357-18

235718f

For more information www.linear.com/LTC2357-18

once the conversion completes. In this mode, the device

draws only a small regulator standby current resulting in a

typical power dissipation of 0.56mW. 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 LTC2357-18, 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 interven-

ing conversion, as shown in Figure 14. The reset event

is triggered on the second rising edge of PD, and asyn-

chronously ends based on an internal timer. Reset clears

all serial data output registers and restores the internal

SoftSpan configuration register default state of all channels

in SoftSpan7. 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 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.

Power Dissipation vs Sampling Frequency

When nap mode is employed, the power dissipation of

the LTC2357-18 decreases as the sampling frequency is

APPLICATIONS INFORMATION

reduced, as shown in Figure 15. This decrease in aver-

age power dissipation occurs because a portion of the

LTC2357-18 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.

OVDD

VDD

VEE

VCC

WITH NAP MODE

CNVL

= 770ns

SAMPLING FREQUENCY (ksps)

100

150

200

250

300

350

–6

–4

–2

SUPPLY CURRENT (mA)

235718 F15

Figure15. Power Dissipation of the LTC2357-18

Decreases with Decreasing Sampling Frequency

DIGITAL INTERFACE

The LTC2357-18 features CMOS and LVDS serial inter-

faces, selectable using the LVDS/CMOS pin. The flexible

OVDD supply allows the LTC2357-18 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 four lanes of serial data

output, allowing the user to optimize bus width and data

throughput. Together, these I/O interface options enable

the LTC2357-18 to communicate equally well with legacy

microcontrollers and modern FPGAs.

CNV

tCONV

tCNVH

tPDH

tPDL

BUSY

RESET RESET TIME

SET INTERNALLY

SECOND RISING EDGE OF

PD TRIGGERS RESET

PD tWAKE

235718 F14

Figure14. Reset Timing for the LTC2357-18

LTC2357-18

235718f

For more information www.linear.com/LTC2357-18

APPLICATIONS INFORMATION

Serial CMOS I/O Mode

As shown in Figure 16, 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 four lanes of serial

data output, SDO0 to SDO3. Communication with the

LTC2357-18 across this bus occurs during predefined

data transaction windows. Within a window, the device

accepts 12-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 SDO3. New

data transaction windows open 10ms after powering up

or resetting the LTC2357-18, 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 16. 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 accu-

racy and therefore is not recommended.

Just prior to the falling edge of BUSY and the opening of

a new data transaction window, SCKO is forced low and

SDO0 to SDO3 are updated with the latest conversion

results from analog input channels 0 to 3, respectively.

Rising edges on SCKI serially clock conversion results

and analog input channel configuration information out

on SDO0 to SDO3 and trigger transitions on SCKO that are

skew-matched to the data on SDO0 to SDO3. The result-

ing SCKO frequency is half that of SCKI. SCKI rising edges

also latch SoftSpan configuration words provided on SDI,

0 C1 C0

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

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

SDO3

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 3

24-BIT PACKET

CONVERSION N

CHANNEL 0

24-BIT PACKET

CONVERSION N

235718 F16

tHSDISCKI

CONVERSION RESULT CHAN 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 CHAN ID SOFTSPAN CONVERSION RESULT

• • •

tSCKI tSCKIH

tBUSYLH

tSCKIL

tHSDOSCKI

tDSDOSCKI

tSSDISCKI

tQUIET

tDSDOBUSYL

tCNVH

tSKEW

tCONV

tCNVL

tCYC

Figure16. Serial CMOS I/O Mode

LTC2357-18

235718f

For more information www.linear.com/LTC2357-18

which are used to program the internal 12-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 17, 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 SDO3 are grouped into 24-bit pack-

ets consisting of an 18-bit conversion result, followed by

a zero, 2-bit analog channel ID, and 3-bit SoftSpan code,

all presented MSB first. As suggested in Figures 16 and

17, each SDO lane outputs these packets for all analog

input channels in a sequential, circular manner. For exam-

ple, the first 24-bit packet output on SDO0 corresponds

to analog input channel 0, followed by the packets for

channels 1, 2 and 3. 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 LTC2357-18 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

applications, such as interfacing the LTC2357-18 with

an FPGA or CPLD, rising and falling edges of SCKO may

be used to capture serial output data on SDO0 to SDO3

in double data rate (DDR) fashion. Capturing data using

SCKO adds robustness to delay variations over tempera-

ture and supply.

Full Four Lane Serial CMOS Output Data Capture

As shown in Table 2, full 350ksps per channel throughput

can be achieved with a 41MHz SCKI frequency by captur-

ing the first packet (24 SCKI cycles total) from all four

serial data output lanes SDO0 to SDO3. This configuration

also allows conversion results from all channels to be

captured using as few as 18 SCKI cycles if the 2-bit ana-

log channel ID and 3-bit SoftSpan code are not needed.

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

PD = 0

DON’T CARE

BUSY

SDO3

SCKO

SDO0

SCKI

SDI

DON’T CARE

235718 F17

Hi-Z

Hi-Z CHANNEL 0 PACKET CHANNEL 1 PACKET CHANNEL 2 PACKET CHANNEL 3 PACKET

(PARTIAL)

Hi-Z

CHANNEL 3 PACKET CHANNEL 0 PACKET CHANNEL 1 PACKET CHANNEL 2 PACKET

(PARTIAL)

• • •

NEW SoftSpan CONFIGURATION WORD

(OVERWRITES INTERNAL CONFIG REGISTER)

FIVE ALL-ZERO WORDS AND ONE PARTIAL WORD

(INTERNAL CONFIG REGISTER RETAINS CURRENT VALUE)

tEN tDIS

Figure17. Internal SoftSpan Configuration Register Behavior. Serial CMOS Bus Response to CS

LTC2357-18

235718f

For more information www.linear.com/LTC2357-18

Fewer Than Four Lane Serial CMOS Output Data Capture

Applications that cannot accommodate the full four lanes

of serial data capture may employ fewer lanes without

reconfiguring the LTC2357-18. For example, capturing

the first two packets (48 SCKI cycles total) from SDO0

and SDO2 provides data for analog input channels 0 and

1, and 2 and 3, respectively, using two output lanes. If

only one lane can be accommodated, capturing the first

four packets (96 SCKI cycles total) from SDO0 provides

data for all analog input channels. As shown in Table 2,

full 350ksps per channel throughput can be achieved with

41MHz and 81MHz SCKI frequencies in the four lane and

two lane cases, respectively, but the maximum CMOS

SCKI frequency of 100MHz limits the throughput to less

than 350ksps per channel in the one lane case. 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 12-bit SoftSpan configuration register con-

trols the SoftSpan range for all analog input channels of

the LTC2357-18. 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 12-bit SoftSpan configu-

ration word on SDI during the data transaction window

shown in Figure 16. New SoftSpan configuration words

are only accepted within this recommended data transac-

tion window, but SoftSpan changes take effect immedi-

ately 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 channel requires no additional analog input set-

tling time before starting the next conversion. The map-

ping between the serial SoftSpan configuration word, the

internal SoftSpan configuration register

, and each chan-

nel’s 3-bit SoftSpan code is illustrated in Figure 18.

APPLICATIONS INFORMATION

Table2. Required SCKI Frequency to Achieve Various Throughputs in Common Output Bus Configurations with Four 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

350ksps/CHANNEL

(tACQ=625ns)

175ksps/CHANNEL

(tACQ=3480ns)

88ksps/CHANNEL

(tACQ=9190ns)

CMOS

4 18 31 6 2

4 24 41 7 3

2 48 81 14 6

1 96 Not Achievable 28 11

LVDS 1 48 81 (162Mbps) 14 (28Mbps) 6 (12Mbps)

LTC2357-18

235718f

For more information www.linear.com/LTC2357-18

If fewer than 12 SCKI rising edges are provided during

a data transaction window, the partial word received on

SDI will be ignored and the SoftSpan configuration reg-

ister will not be updated. If exactly 12 SCKI rising edges

are provided, the SoftSpan configuration register will be

updated to match the received SoftSpan configuration

word, S[11:0]. The one exception to this behavior occurs

when S[11:0] is all zeros. In this case, the SoftSpan con-

figuration register will not be updated, allowing applica-

tions to retain the current SoftSpan configuration state

by idling SDI low. If more than 12 SCKI rising edges are

provided during a data transaction window, each com-

plete 12-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 configu-

ration register in the manner shown in Figures 16 and

17. After the opening of a new data transaction window

at the falling edge of BUSY, the user supplies a 12-bit

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 ris-

ing 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.

1 2 3 4 5 6 7 8 9 10 11 12

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 12-BIT SoftSpan CONFIGURATION REGISTER

(SAME FOR CMOS AND LVDS)

LVDS I/O MODE

1234567891011 0

tSCKI

1 2 3 4 5 6 7 8 9 10 11 12

S11 S10 S9 S8 S7 S6 S5 S3S4 S1S2 S0DON’T CARE

SCKI

(LVDS)

SDI

(LVDS)

tSCKI

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]

235718 F18

tSCKIH

tSCKIL

tSSDISCKI

tHSDISCKI

tSCKIH

tSSDISCKI

tHSDISCKI

tSCKIL

Figure18. Mapping Between Serial SoftSpan Configuration Word, Internal SoftSpan

Configuration Register, and SoftSpan Code for Each Analog Input Channel

APPLICATIONS INFORMATION

LTC2357-18

235718f

For more information www.linear.com/LTC2357-18

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 dia-

grams and interface discussions adopt the logical rather

than physical convention.

As shown in Figure 19, 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.

Communication with the LTC2357-18 across this bus

occurs during predefined data transaction windows.

Within a window, the device accepts 12-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 LTC2357-18,

and at the end of each conversion on the falling edge of

BUSY. In the recommended use case, the data transac-

tion should be completed with a minimum tQUIET time of

20ns prior to the start of the next conversion, as shown

in Figure 19. 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. 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,

S11

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

S10 S9 S8 S7 S6 S5 S4 S3 S2 S1 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

235718 F19

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 3

24-BIT PACKET

CONVERSION N

CONVERSION RESULT CHAN ID SoftSpan

9089 91 92 93 94 95 96

D0 SS1SS2 SS0 D17

CHANNEL 0

24-BIT PACKET

CONVERSION N

CONVERSION

RESULT

CHAN ID SoftSpan

tSKEW

tCNVH

tBUSYLH

tDSDOSCKI

tHSDOSCKI

tDSDOBUSYL

tHSDISCKI tSSDISCKI

tSSDISCKI

tHSDISCKI

tQUIET

tCONV

0 C1 C00 C1 C0

tSCKI tSCKIH

tSCKIL

Figure19. Serial LVDS I/O Mode

APPLICATIONS INFORMATION

LTC2357-18

235718f

For more information www.linear.com/LTC2357-18

(CMOS)

PD = 0

DON’T CARE

BUSY

(CMOS)

SCKO

(LVDS)

SDO

(LVDS)

SCKI

(LVDS)

SDI

(LVDS)

DON’T CARE

235718 F20

Hi-Z

Hi-Z CHANNEL 0 PACKET CHANNEL 1 PACKET CHANNEL 2 PACKET CHANNEL 3 PACKET

(PARTIAL)

Hi-Z

FIVE ALL-ZERO WORDS AND ONE PARTIAL WORD

(INTERNAL CONFIG REGISTER RETAINS CURRENT VALUE)

NEW SoftSpan CONFIGURATION WORD

tEN tDIS

(OVERWRITES INTERNAL CONFIG REGISTER)

Figure20. Internal SoftSpan Configuration Register Behavior. Serial LVDS Bus Response to CS

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 12-bit SoftSpan

configuration register. See the section Programming the

SoftSpan Configuration Register in LVDS I/O Mode for

further details. As shown in Figure 20, 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 sig-

naling, 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 differ-

entially 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, followed by a zero,

2-bit analog channel ID, and 3-bit SoftSpan code, all

presented MSB first. As suggested in Figures 19 and 20,

SDO outputs these packets 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, 2 and

3. The data output on SDO then wraps back to channel 0,

and this pattern repeats indefinitely.

Serial LVDS Output Data Capture

As shown in Table 2, full 350ksps per channel throughput

can be achieved with a 81MHz SCKI frequency by captur-

ing four packets (48 SCKI cycles total) of DDR data from

SDO. The LTC2357-18 supports LVDS SCKI frequencies

up to 250MHz.

Programming the SoftSpan Configuration Register in

LVDS I/O Mode

The internal 12-bit SoftSpan configuration register con-

trols the SoftSpan range for all analog input channels of

the LTC2357-18. The default state of this register after

power-up or resetting the device is all ones, configur-

ing each channel to convert in SoftSpan 7, the ±2.5 •

REFBUF

range

(see Table 1a). The state of this register

may be modified by providing a new 12-bit SoftSpan

APPLICATIONS INFORMATION

LTC2357-18

235718f

For more information www.linear.com/LTC2357-18

To obtain the best performance from the LTC2357-18, 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

returns 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 DC2365, the evaluation kit for the LTC2357-18.

BOARD LAYOUT

configuration word on SDI during the data transaction

window shown in Figure 19. New SoftSpan configuration

words are only accepted within this recommended data

transaction window, 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 reduc-

tion in tCONV on the next conversion. Similarly, enabling a

previously disabled channel requires no additional analog

input settling time before starting the next conversion.

The mapping between the serial SoftSpan configura-

tion word, the internal SoftSpan configuration register,

and each channel’s 3-bit SoftSpan code is illustrated in

Figure18.

If fewer than 12 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 12

SCKI edges are provided, the SoftSpan configuration

configuration word, S[11:0]. The one exception to this

behavior occurs when S[11:0] 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 12 SCKI

edges are provided during a data transaction window,

each complete 12-bit word received on SDI will be inter-

preted 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 19 and 20.

After the opening of a new data transaction window at

the falling edge of BUSY, the user supplies a 12-bit DDR

SoftSpan configuration word on SDI during the first 6

SCKI cycles. This new word overwrites the internal con-

figuration register contents following the 6th 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

LTC2357-18

235718f

For more information www.linear.com/LTC2357-18

Information furnished by Analog Devices is believed to be accurate and reliable. However, no responsibility is assumed by Analog

Devices for its use, nor for any infringements of patents or other rights of third parties that may result from its use. Specifications

subject to change without notice. No license is granted by implication or otherwise under any patent or patent rights of Analog Devices.

PACKAGE DESCRIPTION

Please refer to http://www.linear.com/product/LTC2357-18#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

9.00 BSC

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

7.15 – 7.25

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)

LTCXXXX

LX-ES

Q_ _ _ _ _ _

XXYY

TRAY PIN 1

BEVEL PACKAGE IN TRAY LOADING ORIENTATION

COMPONENT

PIN “A1”

LTC2357-18

235718f

For more information www.linear.com/LTC2357-18

LT 1017 • PRINTED IN USA

www.linear.com/LTC2357-18

 ANALOG DEVICES, INC. 2017

RELATED PARTS

TYPICAL APPLICATION

PART NUMBER DESCRIPTION COMMENTS

ADCs

LTC2358-18/LTC2358-16 Buffered 18-/16-Bit, 200ksps/Ch, 8-Channel

Simultaneous Sampling ±3.5LSB/±1LSB INL ADC

Buffered ±10.24V SoftSpan Inputs with 30VP-P Common Mode Range,

96dB/94dB SNR, Serial CMOS and LVDS I/O, 7mm × 7mm LQFP-48 Package

LTC2333-18/LTC2333-16 18-/16-Bit, 800ksps, 8-Channel Multiplexed,

±3LSB/±1LSB INL, Serial ADC

Buffered ±10.24V SoftSpan Inputs with Wide Common Mode Range, 97dB/94dB

SNR, Serial CMOS and LVDS I/O, 7mm × 7mm LQFP-48 Package

LTC2344-18/LTC2344-16 18-/16-Bit, 400ksps/Ch, 4-Channel Simultaneous

Sampling, ±4LSB/±1.25LSB INL, Serial ADC

±4.096V SoftSpan Inputs with Wide Common Mode Range, 95dB/93dB SNR,

Serial CMOS and LVDS I/O, 7mm × 7mm QFN-32 Package

LTC2345-18/LTC2345-16 18-/16-Bit, 200ksps, 8-Channel Simultaneous

Sampling, ±5LSB/±1.25LSB INL, Serial ADC

±4.096V SoftSpan Inputs with Wide Common Mode Range, 92dB/91dB SNR,

Serial CMOS and LVDS I/O, 7mm × 7mm QFN-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

LTC2387-18/LTC2387-16 18-/16-Bit, 15Msps SAR ADC 5V Supply, Differential Input, 95.7dB/93.8dB SNR, 5mm × 5mm QFN Package

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

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

LT6657 Precision Low Drift Low Noise Buffered Reference 5V/3V/2.5V, 1.5ppm/°C, 0.5ppm Peak-to-Peak Noise, MSOP-8 Package

Amplifiers

LTC2057/LTC2057HV High Voltage, Low Noise Zero-Drift Op Amp Maximum Input Offset: 4.5µV, Supply Voltage Range: 4.75V to 60V

LT6020 Dual, Micropower, 5V/µs, Rail-to-Rail Op Amp Maximum Input Offset: 30µV, Maximum Supply Current: 100µA/Amplifier

LT1354/LT1355/LT1356 Single/Dual/Quad 1mA, 12MHz, 400V/µs Op Amp Good DC Precision, Stable with All Capacitive Loads

Amplify Differential Signals with Gain of 10

Over a Wide Common Mode Range with Buffered Analog Inputs

–7V

31V

BUFFERED

ANALOG

INPUTS

LTC2357-18

235718 TA02

ONLY CHANNEL 0 SHOWN FOR CLARITY

24V

ARBITRARY

–

0.1µF

47µF

IN0+

IN0–

LTC2057HV

VCC

REFINREFBUFVEE

BW = 10kHz

2.2nF

3.65k

549Ω

IN+

IN–

2.49k

COMMON MODE

INPUT RANGE

DIFFERENTIAL MODE

INPUT RANGE: ±500mV

INTERNAL HI-Z BUFFERS

ALLOW OPTIONAL

kΩ PASSIVE FILTERS

GAIN = 10