1
LTC1594L/LTC1598L
APPLICATIO S
U
DESCRIPTIO
U
FEATURES
TYPICAL APPLICATION
U
The LTC
®
1594L/LTC1598L are 3V micropower, 12-bit
sampling A/D converters that feature 4- and 8-channel
multiplexers, respectively. They typically draw only 160µA
of supply current when converting and automatically
power down to a typical supply current of 1nA between
conversions. The LTC1594L is available in a 16-pin SO
package and the LTC1598L is packaged in a 24-pin
SSOP. Both operate on a 3V supply. The 12-bit, switched-
capacitor, successive approximation ADCs include a
sample-and-hold.
On-chip serial ports allow efficient data transfer to a wide
range of microprocessors and microcontrollers over three
or four wires. This, coupled with micropower consump-
tion, makes remote location possible and facilitates trans-
mitting data through isolation barriers.
The circuit can be used in ratiometric applications or with
an external reference. The high impedance analog inputs
and the ability to operate with reduced spans (to 1.5V full
scale) allow direct connection to sensors and transducers
in many applications, eliminating the need for gain stages.
4- and 8-Channel,
3V Micropower Sampling
12-Bit Serial I/O A/D Converters
■
12-Bit Resolution on 3V Supply
■
Low Supply Current: 160µA Typ
■
Auto Shutdown to 1nA
■
Guaranteed ±3/4LSB Max DNL
■
Guaranteed 2.7V Operation
(5V Versions Available: LTC1594/LTC1598)
■
Multiplexer: 4-Channel MUX (LTC1594L)
8-Channel MUX (LTC1598L)
■
Separate MUX Output and ADC Input Pins
■
MUX and ADC May Be Controlled Separately
■
Sampling Rate: 10.5ksps
■
I/O Compatible with QSPI, SPI and MICROWIRE
TM
, etc.
■
Small Package: 16-Pin Narrow SO (LTC1594L)
24-Pin SSOP (LTC1598L)
■
Pen Screen Digitizing
■
Battery-Operated Systems
■
Remote Data Acquisition
■
Isolated Data Acquisition
■
Battery Monitoring
■
Temperature Measurement MICROWIRE is a trademark of National Semiconductor Corporation.
, LTC and LT are registered trademarks of Linear Technology Corporation.
SAMPLE FREQUENCY (kHz)
0.1
1
SUPPLY CURRENT (µA)
10
100
1000
1 10 100
1594L/98L TA02
T
A
= 25°C
V
CC
= 2.7V
V
REF
= 2.5V
f
CLK
= 200kHz
Supply Current vs Sample Rate
12µW, 8-Channel, 12-Bit ADC Samples at 200Hz and Runs Off a 3V Supply
ANALOG
INPUTS
0V TO 3V
RANGE
1k 1µF
OPTIONAL
ADC FILTER
20
21
22
23
24
1
2
3
8
4, 9
MPU
SERIAL DATA LINK
MICROWIRE AND
SPI COMPATABLE
10
6
5, 14
7
11
12
13
18 17 16 15, 19 1µF
3V
1594L/98L TA01
CH0
CH1
CH2
CH3
CH4
CH5
CH6
CH7
COM GND
CSADC
CSMUX
CLK
D
IN
D
OUT
NC
NC
ADCIN
LTC1598L
MUXOUT V
REF
V
CC
+
–
12-BIT
SAMPLING
ADC
8-CHANNEL
MUX
2
LTC1594L/LTC1598L
(Notes 1, 2)
Supply Voltage (V
CC
) to GND................................... 12V
Voltage
Analog Reference .................... –0.3V to (V
CC
+ 0.3V)
Analog Inputs .......................... –0.3V to (V
CC
+ 0.3V)
Digital Inputs .........................................–0.3V to 12V
Digital Output .......................... –0.3V to (V
CC
+ 0.3V)
ABSOLUTE MAXIMUM RATINGS
W
WW
U
Power Dissipation.............................................. 500mW
Operating Temperature Range
LTC1594LCS/LTC1598LCG ..................... 0°C to 70°C
LTC1594LIS/LTC1598LIG ................. –40°C to 85°C
Storage Temperature Range ................. –65°C to 150°C
Lead Temperature (Soldering, 10 sec).................. 300°C
PACKAGE/ORDER INFORMATION
W
UU
Consult factory for Military grade parts.
ORDER PART
NUMBER ORDER PART
NUMBER
T
JMAX
= 150°C, θ
JA
= 110°C/ W
1
2
3
4
5
6
7
8
9
10
11
12
TOP VIEW
G PACKAGE
24-LEAD PLASTIC SSOP
24
23
22
21
20
19
18
17
16
15
14
13
CH5
CH6
CH7
GND
CLK
CSMUX
D
IN
COM
GND
CSADC
D
OUT
NC
CH4
CH3
CH2
CH1
CH0
V
CC
MUXOUT
ADCIN
V
REF
V
CC
CLK
NC
T
JMAX
= 125°C, θ
JA
= 120°C/ W
TOP VIEW
S PACKAGE
16-LEAD PLASTIC SO
1
2
3
4
5
6
7
8
16
15
14
13
12
11
10
9
CH0
CH1
CH2
CH3
ADCIN
VREF
COM
GND
VCC
MUXOUT
DIN
CSMUX
CLK
VCC
DOUT
CSADC
RECOM ENDED OPERATING CONDITIONS
UU
UU WW
SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS
V
CC
Supply Voltage (Note 3) 2.7 3.6 V
f
CLK
Clock Frequency V
CC
= 2.7V (Note 4) 200 kHz
t
CYC
Total Cycle Time f
CLK
= 200kHz 95 µs
t
hDI
Hold Time, D
IN
After CLK↑V
CC
= 2.7V 450 ns
t
suCS
Setup Time CS↓ Before First CLK↑ (See Operating Sequence) V
CC
= 2.7V 2 µs
t
suDI
Setup Time, D
IN
Stable Before CLK↑V
CC
= 2.7V 600 ns
t
WHCLK
CLK High Time V
CC
= 2.7V 1.5 µs
t
WLCLK
CLK Low Time V
CC
= 2.7V 1.5 µs
t
WHCS
CS High Time Between Data Transfer Cycles f
CLK
= 200kHz 25 µs
t
WLCS
CS Low Time During Data Transfer f
CLK
= 200kHz 70 µs
LTC1594LCS
LTC1594LIS LTC1598LCG
LTC1598LIG
The ● denotes the specifications which apply over
the full operating temperature range, otherwise specifications are at TA = 25°C. (Note 5)
3
LTC1594L/LTC1598L
CONVERTER AND MULTIPLEXER CHARACTERISTICS
UW U
PARAMETER CONDITIONS MIN TYP MAX MIN TYP MAX UNITS
Resolution (No Missing Codes) ●12 12 Bits
Integral Linearity Error (Note 6) ●±3±3 LSB
Differential Linearity Error ●±3/4 ±1 LSB
Offset Error ●±3±3 LSB
Gain Error ●±8±8 LSB
REF Input Range (Notes 7, 8) 1.5V to V
CC
+ 0.05V V
Analog Input Range (Notes 7, 8) –0.05V to V
CC
+ 0.05V V
MUX Channel Input Leakage Current Off Channel ●±200 ±200 nA
MUXOUT Leakage Current Off Channel ●±200 ±200 nA
ADCIN Input Leakage Current (Note 9) ●±1±1µA
SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS
S/(N + D) Signal-to-Noise Plus Distortion Ratio 1kHz Input Signal 68 dB
THD Total Harmonic Distortion (Up to 5th Harmonic) 1kHz Input Signal –78 dB
SFDR Spurious-Free Dynamic Range 1kHz Input Signal 80 dB
Peak Harmonic or Spurious Noise 1kHz Input Signal –80 dB
DYNAMIC ACCURACY
UW
DIGITAL AND DC ELECTRICAL CHARACTERISTICS
U
SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS
V
IH
High Level Input Voltage V
CC
= 3.6V ●2.0 V
V
IL
Low Level Input Voltage V
CC
= 2.7V ●0.8 V
I
IH
High Level Input Current V
IN
= V
CC
●2.5 µA
I
IL
Low Level Input Current V
IN
= 0V ●–2.5 µA
V
OH
High Level Output Voltage V
CC
= 2.7V, I
O
= 10µA●2.4 2.64 V
V
CC
= 2.7V, I
O
= 360µA●2.1 2.30 V
V
OL
Low Level Output Voltage V
CC
= 2.7V, I
O
= 400µA●0.4 V
I
OZ
Hi-Z Output Leakage CS = High ●±3µA
I
SOURCE
Output Source Current V
OUT
= 0V –10 mA
I
SINK
Output Sink Current V
OUT
= V
CC
15 mA
R
REF
Reference Input Resistance CS = V
IH
2700 MΩ
CS = V
IL
60 kΩ
I
REF
Reference Current CS = V
CC
●0.001 2.5 µA
t
CYC
≥ 760µs, f
CLK
≤ 25kHz 50 µA
t
CYC
≥ 60µs, f
CLK
≤ 200kHz ●50 70 µA
I
CC
Supply Current CS = V
CC
, CLK = V
CC
, D
IN
= V
CC
●0.001 ±3µA
t
CYC
≥ 760µs, f
CLK
≤ 25kHz 160 µA
t
CYC
≥ 60µs, f
CLK
≤ 200kHz ●160 320 µA
LTC1594LCS/LTC1598LCG LTC1594LIS/LTC1598LIG
The ● denotes the specifications which
apply over the full operating temperature range, otherwise specifications are at TA = 25°C. (Note 5)
The ● denotes the specifications which
apply over the full operating temperature range, otherwise specifications are at TA = 25°C. (Note 5)
TA = 25°C, fSMPL = 10.5kHz. (Note 5)
4
LTC1594L/LTC1598L
SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS
t
SMPL
Analog Input Sample Time See Figure 1 in Applications Information 1.5 CLK Cycles
f
SMPL(MAX)
Maximum Sampling Frequency See Figure 1 in Applications Information ●10.5 kHz
t
CONV
Conversion Time See Figure 1 in Applications Information 12 CLK Cycles
t
dDO
Delay Time, CLK↓ to D
OUT
Data Valid See Test Circuits ●600 1500 ns
t
dis
Delay Time, CS↑ to D
OUT
Hi-Z See Test Circuits ●220 600 ns
t
en
Delay Time, CLK↓ to D
OUT
Enabled See Test Circuits ●180 500 ns
t
hDO
Time Output Data Remains Valid After CLK↓C
LOAD
= 100pF 520 ns
t
f
D
OUT
Fall Time See Test Circuits ●60 180 ns
t
r
D
OUT
Rise Time See Test Circuits ●80 180 ns
t
ON
Enable Turn-On Time See Figure 1 in Applications Information ●540 1200 ns
t
OFF
Enable Turn-Off Time See Figure 2 in Applications Information ●190 500 ns
t
OPEN
Break-Before-Make Interval ●125 350 ns
C
IN
Input Capacitance Analog Inputs On-Channel 20 pF
Off-Channel 5 pF
Digital Input 5 pF
AC CHARACTERISTICS
Note 1: Absolute Maximum Ratings are those values beyond which the life
of a device may be impaired.
Note 2: All voltage values are with respect to GND.
Note 3: These devices are specified at 3V. Consult factory for 5V
specified devices (LTC1594/LTC1598).
Note 4: Increased leakage currents at elevated temperatures cause the S/H
to droop, therefore it is recommended that f
CLK
≥ 200kHz at 85°C,
f
CLK
≥ 75kHz at 70°C and f
CLK
≥ 1kHz at 25°C.
Note 5: V
CC
= 2.7V, V
REF
= 2.5V and CLK = 200kHz unless otherwise
specified. CSADC and CSMUX pins are tied together during the test.
Note 6: Linearity error is specified between the actual end points of the
A/D transfer curve.
Note 7: Two on-chip diodes are tied to each reference and analog input
which will conduct for reference or analog input voltages one diode drop
below GND or one diode drop above V
CC
. This spec allows 50mV forward
bias of either diode for 2.7V ≤ V
CC
≤ 3.6V. This means that as long as the
reference or analog input does not exceed the supply voltage by more than
50mV, the output code will be correct. To achieve an absolute 0V to 3V
input voltage range, it will therefore require a minimum supply voltage of
2.950V over initial tolerance, temperature variations and loading.
Note 8: Recommended operating condition.
Note 9: Channel leakage current is measured after the channel selection.
TYPICAL PERFORMANCE CHARACTERISTICS
UW
Supply Current vs Sample Rate Reference Current vs Temperature
Supply Current vs Temperature
SAMPLE FREQUENCY (kHz)
0.1
1
SUPPLY CURRENT (µA)
10
100
1000
1 10 100
1594L/98L G01
T
A
= 25°C
V
CC
= 2.7V
V
REF
= 2.5V
f
CLK
= 200kHz
TEMPERATURE (°C)
–55
60
SUPPLY CURRENT (µA)
100
180
220
260
–15 25 45 125
1594L/98L G02
140
–35 5 65 85 105
T
A
= 25°C
V
CC
= 2.7V
V
REF
= 2.5V
f
CLK
= 200kHz
f
SMPL
= 10.5kHz
TEMPERATURE (°C)
–55
43
REFERENCE CURRENT (µA)
44
46
47
48
53
50
–15 25 45 125
1594L/98L G03
45
51
52
49
–35 5 65 85 105
V
CC
= 2.7V
V
REF
= 2.5V
f
CLK
= 200kHz
f
SMPL
= 10.5kHz
The ● denotes the specifications which apply over the full operating temperature range,
otherwise specifications are at TA = 25°C.(Note 5)
5
LTC1594L/LTC1598L
TYPICAL PERFORMANCE CHARACTERISTICS
UW
Change in Linearity
vs Reference Voltage
Change in Gain
vs Reference Voltage
Change in Offset
vs Reference Voltage
REFERENCE VOLTAGE (V)
0.5
0
CHANGE IN OFFSET (LSB = 1/4096 × V
REF
)
0.5
1.0
1.5
2.0
2.5
3.0
1.0 1.5 2.0 2.5
1594L/98L G04
3.0
T
A
= 25°C
V
CC
= 2.7V
f
CLK
= 200kHz
f
SMPL
= 10.5kHz
REFERENCE VOLTAGE (V)
1.0
0
CHANGE IN LINEARITY (LSB)
0.05
0.15
0.20
0.25
0.50
0.35
1.4 1.8 2.0 2.8
1594L/98L G06
0.10
0.40
0.45
0.30
1.2 1.6 2.2 2.4 2.6
T
A
= 25°C
V
CC
= 2.7V
f
CLK
= 200kHz
f
SMPL
= 10.5kHz
REFERENCE VOLTAGE (V)
1.0
0
CHANGE IN GAIN (LSB)
–1
–3
–4
–5
–10
–7
1.4 1.8 2.0 2.8
1594L/98L G07
–2
–8
–9
–6
1.2 1.6 2.2 2.4 2.6
T
A
= 25°C
V
CC
= 2.7V
f
CLK
= 200kHz
f
SMPL
= 10.5kHz
INPUT FREQUENCY (kHz)
1
0
EFFECTIVE NUMBER OF BITS (ENOBs)
S/(N + D) (dB)
3
5
7
10
10 100
1594L/98L G09
1
4
6
9
12
11
8
62
56
74
68
50
2
T
A
= 25°C
V
CC
= 2.7V
f
CLK
= 200kHz
f
SMPL
= 10.5kHz
Effective Bits and S/(N + D)
vs Input Frequency
Differential Nonlinearity vs Code
CODE
0
–1
DIFFERENTIAL NONLINEARITY ERROR (LSB)
–0.5
0
0.5
1
512 1024 1536 2048
1594L/98L G08
2560 3072 3584 4096
T
A
= 25°C
V
CC
= 2.7V
V
REF
= 2.5V
f
CLK
= 200kHz
Spurious Free Dynamic Range
vs Input Frequency
INPUT FREQUENCY (kHz)
1
0
SPURIOUS-FREE DYNAMIC RANGE (dB)
20
40
60
80
10 100
1594L/98L G10
100
10
30
50
70
90
T
A
= 25°C
V
CC
= 2.7V
V
REF
= 2.5V
f
SMPL
= f
SMPL(MAX)
Frequency Response
INPUT FREQUENCY (Hz)
80
ATTENUATION (%)
60
40
50
20
0
90
70
30
10
1k 100k 1M 10M
1594L/98L G12
100 10k
(MUX + A
DC
)
T
A
= 25°C
V
CC
= 2.7V
V
REF
= 2.5V
f
SMPL
= f
SMPL(MAX)
Change in Offset vs Temperature
TEMPERATURE (°C)
0
CHANGE IN OFFSET (LSB)
0.15
30
1594L/98L G05
0
–0.10
10 20 40
–0.15
–0.20
0.20
0.10
0.05
–0.05
50 60 70
VCC = 2.7V
VREF = 2.5V
fCLK = 200kHz
fSMPL = fSMPL(MAX)
S/(N + D) vs Input Level
INPUT LEVEL (dB)
–45
SIGNAL-TO-NOISE PLUS DISTORTION (dB)
40
50
60
–5
1594L/98/ G11
30
20
0–35 –25 –15
–40 0
–30 –20 –10
10
80
70
T
A
= 25°C
V
CC
= 2.7V
V
REF
= 2.5V
f
IN
= 1kHz
f
SMPL
= f
SMPL(MAX)
6
LTC1594L/LTC1598L
TYPICAL PERFORMANCE CHARACTERISTICS
UW
Power Supply Feedthrough
vs Ripple Frequency
FREQUENCY (kHz)
0
–120
MAGNITUDE (dB)
–100
–80
–60
–40
1.0 2.0 3.0 4.0
1594L/98L G13
–20
0
0.5 1.5 2.5 3.5
T
A
= 25°C
V
CC
= 2.7V
V
REF
= 2.5V
f
IN
= 3.05kHz
f
CLK
= 120kHz
f
SMPL
= 7.5kHz
4096 Point FFT Plot
RIPPLE FREQUENCY (Hz)
–80
FEEDTHROUGH (dB)
–60
–40
–50
–20
0
–90
–70
–30
–10
1k 100k 1M 10M
1594L/98L G15
–100 10k
T
A
= 25°C
V
CC
= 2.7V (V
RIPPLE
= 1mV)
V
REF
= 2.5V
f
CLK
= 200kHz
Intermodulation Distortion
FREQUENCY (kHz)
0
–120
MAGNITUDE (dB)
–100
–80
–60
–40
1.0 2.0 3.0 4.0
1594L/98L G14
–20
0
0.5 1.5 2.5 3.5
T
A
= 25°C
V
CC
= 2.7V
V
REF
= 2.5V
f1 = 2.05kHz
f2 = 3.05kHz
f
SMPL
= 7.5kHz
Maximum Clock Frequency
vs Source Resistance
SOURCE RESISTANCE (Ω)
10
CLOCK FREQUENCY (kHz)
120
130
150
170
100 1000
1594L/98L G16
200
140
160
180
190
–INPUT
+INPUTV
IN
R
SOURCE–
T
A
= 25°C
V
CC
= 2.7V
V
REF
= 2.5V
Sample-and-Hold Acquisition Time
vs Source Resistance
SOURCE RESISTANCE (Ω)
1
100
S & H ACQUISITION TIME (ns)
1000
10000
100 100010 10000
1594L/98L G17
TA = 25°C
VCC = 2.7V
VREF = 2.5V
–INPUT
+INPUTVIN
RSOURCE+
Input Channel Leakage Current
vs Temperature
Minimum Clock Frequency for
0.1LSB Error vs Temperature
TEMPERATURE (°C)
0
CLOCK FREQUENCY (kHz)
80
100
120
40
1594L/98L G18
60
40
010 20 30 60 70
50
20
2
V
CC
= 2.7V
V
REF
= 2.5V
TEMPERATURE (°C)
–55
LEAKAGE CURRENT (nA)
10
100
1000
105
1594L/98L G19
1
0.1
0.01 –15 25 65
–35 125
545 85
V
CC
= 2.7V
V
REF
= 2.5V
ON CHANNEL OFF CHANNEL
7
LTC1594L/LTC1598L
PIN FUNCTIONS
UUU
LTC1594L
CH0 (Pin 1): Analog Multiplexer Input.
CH1 (Pin 2): Analog Multiplexer Input.
CH2 (Pin 3): Analog Multiplexer Input.
CH3 (Pin 4): Analog Multiplexer Input.
ADCIN (Pin 5): ADC Input. This input is the positive analog
input to the ADC. Connect this pin to MUXOUT for normal
operation.
V
REF
(Pin 6): Reference Input. The reference input defines
the span of the ADC.
COM (Pin 7): Negative Analog Input. This input is the
negative analog input to the ADC and must be free of noise
with respect to GND.
GND (Pin 8): Analog Ground. GND should be tied directly
to an analog ground plane.
CSADC (Pin 9): ADC Chip Select Input. A logic high on this
input powers down the ADC and three-states D
OUT
. A logic
low on this input enables the ADC to sample the selected
channel and start the conversion. For normal operation,
drive this pin in parallel with CSMUX.
LTC1598L
CH5 (Pin 1): Analog Multiplexer Input.
CH6 (Pin 2): Analog Multiplexer Input.
CH7 (Pin 3): Analog Multiplexer Input.
GND (Pin 4): Analog Ground. GND should be tied directly
to an analog ground plane.
CLK (Pin 5): Shift Clock. This clock synchronizes the serial
data transfer to both MUX and ADC. It also determines the
conversion speed of the ADC.
CSMUX (Pin 6): MUX Chip Select Input. A logic high on
this input allows the MUX to receive a channel address. A
logic low enables the selected MUX channel and connects
it to the MUXOUT pin for A/D conversion. For normal
operation, drive this pin in parallel with CSADC.
D
IN
(Pin 7): Digital Data Input. The multiplexer address is
shifted into this input.
D
OUT
(Pin 10): Digital Data Output. The A/D conversion
result is shifted out of this output.
V
CC
(Pin 11): Power Supply Voltage. This pin provides
power to the ADC. It must be bypassed directly to the
analog ground plane.
CLK (Pin 12): Shift Clock. This clock synchronizes the
serial data transfer to both MUX and ADC.
CSMUX (Pin 13): MUX Chip Select Input. A logic high on
this input allows the MUX to receive a channel address. A
logic low enables the selected MUX channel and connects
it to the MUXOUT pin for A/D conversion. For normal
operation, drive this pin in parallel with CSADC.
D
IN
(Pin 14): Digital Data Input. The multiplexer address
is shifted into this input.
MUXOUT (Pin 15): MUX Output. This pin is the output of
the multiplexer. Tie to ADCIN for normal operation.
V
CC
(Pin 16): Power Supply Voltage. This pin should be
tied to Pin 11.
COM (Pin 8): Negative Analog Input. This input is the
negative analog input to the ADC and must be free of noise
with respect to GND.
GND (Pin 9): Analog Ground. GND should be tied directly
to an analog ground plane.
CSADC (Pin 10): ADC Chip Select Input. A logic high on
this input deselects and powers down the ADC and three-
states D
OUT
. A logic low on this input enables the ADC to
sample the selected channel and start the conversion. For
normal operation drive this pin in parallel with CSMUX.
D
OUT
(Pin 11): Digital Data Output. The A/D conversion
result is shifted out of this output.
NC (Pin 12): No Connection.
NC (Pin 13): No Connection.
CLK (Pin 14): Shift Clock. This input should be tied to Pin 5.
8
LTC1594L/LTC1598L
PIN FUNCTIONS
UUU
V
CC
(Pin 15): Power Supply Voltage. This pin provides
power to the A/D Converter. It must be bypassed directly
to the analog ground plane.
V
REF
(Pin 16): Reference Input. The reference input
defines the span of the ADC.
ADCIN (Pin 17): ADC Input. This input is the positive
analog input to the ADC. Connect this pin to MUXOUT for
normal operation.
MUXOUT (Pin 18): MUX Output. This pin is the output of
the multiplexer. Tie to ADCIN for normal operation.
V
CC
(Pin 19): Power Supply Voltage. This pin should be
tied to Pin 15.
CH0 (Pin 20): Analog Multiplexer Input.
CH1 (Pin 21): Analog Multiplexer Input.
CH2 (Pin 22): Analog Multiplexer Input.
CH3 (Pin 23): Analog Multiplexer Input.
CH4 (Pin 24): Analog Multiplexer Input.
TEST CIRCUITS
Voltage Waveforms for DOUT Rise and Fall Times, tr, tf
Load Circuit for tdDO, tr and tf
DOUT
1.4V
3k
100pF
TEST POINT
1594L/98L TC01
BLOCK DIAGRA S
W
LTC1594L
CH0
CH1
CH2
CH3
1
2
3
4
7COM GND
8
1594L BD
9
13
12
14
10
CSADC
CSMUX
CLK
D
IN
D
OUT
ADCINMUXOUT
15 5 6 16
V
REF
V
CC
LTC1594L
+
–
4-CHANNEL
MUX
12-BIT
SAMPLING
ADC
LTC1598L
1598L BD
CH0
CH1
CH2
CH3
CH4
CH5
CH6
CH7
+
–
8 COM
GND
4, 9
10
6
5, 14
7
11
CSADC
CSMUX
CLK
D
IN
D
OUT
12
13
NC
NC
ADCINMUXOUT
18 17 16 15, 19
V
REF
V
CC
LTC1598L
20
21
22
23
24
1
2
3
8-CHANNEL
MUX
12-BIT
SAMPLING
ADC
D
OUT
V
OL
V
OH
t
r
t
f1594L/98L TC02
9
LTC1594L/LTC1598L
TEST CIRCUITS
Voltage Waveforms for ten
Voltage Waveforms for DOUT Delay Times, tdDO
CLK
D
OUT
V
IL
t
dDO
V
OL
V
OH
1594L/98L TC03
D
OUT
WAVEFORM 1
(SEE NOTE 1)
V
IH
t
dis
90%
10%
D
OUT
WAVEFORM 2
(SEE NOTE 2)
CSADC = CSMUX = CS
NOTE 1: WAVEFORM 1 IS FOR AN OUTPUT WITH INTERNAL CONDITIONS SUCH
THAT THE OUTPUT IS HIGH UNLESS DISABLED BY THE OUTPUT CONTROL.
NOTE 2: WAVEFORM 2 IS FOR AN OUTPUT WITH INTERNAL CONDITIONS SUCH
THAT THE OUTPUT IS LOW UNLESS DISABLED BY THE OUTPUT CONTROL.
1594L/98L TC05
Load Circuit for tdis and ten
D
OUT
3k
100pF
TEST POINT
V
CC
t
dis
WAVEFORM 2, t
en
t
dis
WAVEFORM 1
1594L/98L TC04
Voltage Waveforms for tdis
1594L/98L TC06
CSADC
LTC1594L/LTC1598L
1
CLK
D
OUT
t
en
B11
V
OL
2
10
LTC1594L/LTC1598L
APPLICATIONS INFORMATION
WUUU
OVERVIEW
The LTC1594L/LTC1598L are 3V micropower, 12-bit
sampling A/D converters that feature 4- and 8-channel
multiplexers respectively. They typically draw only 160µA
of supply current when sampling at 10.5kHz. Supply
current drops linearly as the sample rate is reduced (see
Supply Current vs Sample Rate). The ADCs automatically
power down when not performing conversions, drawing
only leakage current. The LTC1594L is available in a
16-pin narrow SO package and the LTC1598L is pack-
aged in a 24-pin SSOP. Both devices operate on a single
supply from 2.7V to 3.6V.
The LTC1594L/LTC1598L contain a 12-bit, switched-
capacitor ADC, sample-and-hold, serial port and an
external reference input pin. In addition, the LTC1594L
has a 4-channel multiplexer and the LTC1598L provides
an 8-channel multiplexer (see Block Diagram). They can
measure signals floating on a DC common mode voltage
and can operate with reduced spans to 1.5V. Reducing
the spans allow them to achieve 366µV resolution.
The LTC1594L/LTC1598L provide separate MUX output
and ADC input pins to form an ideal MUXOUT/ADCIN
loop which economizes signal conditioning. The MUX
and ADC of the devices can also be controlled individually
through separate chip selects to enhance flexibility.
SERIAL INTERFACE
For this discussion, we will assume that CSMUX and
CSADC are tied together and will refer to them as simply
CS, unless otherwise specified.
The LTC1594L/LTC1598L communicate with the micro-
processor and other external circuitry via a synchronous,
half duplex, 4-wire interface (see Operating Sequences in
Figures 1 and 2).
CLK
EN D1
D2
CSMUX = CSADC = CS
tCYC
B5
B6
B7
B8B9
B10B11 Hi-Z
DOUT
CH0 TO
CH7
DIN
tCONV
Hi-Z
tsuCS
NULL
BIT
D0
B4 B3 B2 B1 B0*
tSMPL
tON
DON’T CARE
ADCIN =
MUXOUT
COM = GND *AFTER COMPLETING THE DATA TRANSFER, IF FURTHER CLOCKS ARE APPLIED WITH CS LOW,
THE ADC WILL OUTPUT LSB-FIRST DATA THEN FOLLOWED WITH ZEROS INDEFINITELY 1594F/98F F01
Figure 1. LTC1594L/LTC1598L Operating Sequence Example: CH2, GND
11
LTC1594L/LTC1598L
APPLICATIONS INFORMATION
WUUU
Figure 2. LTC1594L/LTC1598L Operating Sequence Example: All Channels Off
CLK
EN D1
D2
t
CYC
Hi-Z
D
OUT
CH0 TO
CH7
D
IN
t
CONV
Hi-Z
t
suCS
NULL
BIT
D0
t
OFF
D0N‘T CARE
ADCIN =
MUXOUT
COM = GND
1594L/98L F02
DUMMY CONVERSION
CSMUX = CSADC = CS
break-before-make interval, t
OPEN
. After a delay of t
ON
(t
OFF
+ t
OPEN
), the selected channel is switched on,
allowing the ADC in the chip to acquire input signal and
start the conversion (see Figures 1 and 2). After 1 null bit,
the result of the conversion is output on the D
OUT
line.
The selected channel remains on, until the next falling
edge of CS. At the end of the data exchange, CS should
be brought high. This resets the LTC1594L/LTC1598L
and initiates the next data exchange.
D
IN1
D
IN2
D
OUT1
D
OUT2
CS
SHIFT MUX
ADDRESS IN
t
SMPL
+ 1 NULL BIT
SHIFT A/D CONVERSION
RESULT OUT
1594L/98L AI01
Break-Before-Make
The LTC1594L/LTC1598L provide a break-before-make
interval from switching off all the channels simulta-
neously to switching on the next selected channel once
CS is pulled low. In other words, once CS is pulled low,
Data Transfer
The CLK synchronizes the data transfer with each bit
being transmitted on the falling CLK edge and captured
on the rising CLK edge in both transmitting and receiving
systems.
The LTC1594L/LTC1598L first receive input data and
then transmit back the A/D conversion results (half
duplex). Because of the half duplex operation, D
IN
and
D
OUT
may be tied together allowing transmission over
just 3 wires: CS, CLK and DATA (D
IN
/D
OUT
).
Data transfer is initiated by a rising chip select (CS)
signal. After CS rises, the input data on the D
IN
pin is
latched into a 4-bit register on the rising edge of the clock.
More than four input bits can be sent to the D
IN
pin
without problems, but only the last four bits clocked in
before CS falls will be stored into the 4-bit register. This
4-bit input data word will select the channel in the
muliplexer (see Input Data Word and Tables 1 and 2). To
ensure correct operation, the CS must be pulled low
before the next rising edge of the clock.
Once the CS is pulled low, all channels are simulta-
neously switched off after a delay of t
OFF
to ensure a
12
LTC1594L/LTC1598L
after a delay of t
OFF
, all the channels are switched off to
ensure a break-before-make interval. After this interval,
the selected channel is switched on allowing signal
transmission. The selected channel remains on until the
next falling edge of CS and the process repeats itself with
the “EN” bit being logic high. If the “EN” bit is logic low,
all the channels are switched off simultaneously after a
delay of t
OFF
from CS being pulled low and all the
channels remain off until the next falling edge of CS.
Input Data Word
When CS is high, the LTC1594L/LTC1598L clock data
into the D
IN
inputs on the rising edge of the clock and
store the data into a 4-bit register. The input data words
are defined as follows:
D0EN D2 D1
CHANNEL SELECTION
1594L/98L AI02
“EN” Bit
The first bit in the 4-bit register is an “EN” bit. If the “EN”
bit is a logic high, as illustrated in Figure 1, it enables the
selected channel after a delay of t
ON
when the CS is pulled
low. If the “EN” bit is logic low, as illustrated in Figure 2,
it disables all channels after a delay of t
OFF
when the CS
is pulled low.
Multiplexer (MUX) Address
The 3 bits of input word following the “EN” bit select the
channel in the MUX for the requested conversion. For a
given channel selection, the converter will measure the
voltage of the selected channel with respect to the voltage
on the COM pin. Tables 1 and 2 show the various bit
combinations for the LTC1594L/LTC1598L channel
selection.
Table 1. Logic Table for the LTC1594L Channel Selection
CHANNEL STATUS EN D2 D1 DO
All Off 0 X X X
CH0 1 0 0 0
CH1 1 0 0 1
CH2 1 0 1 0
CH3 1 0 1 1
APPLICATIONS INFORMATION
WUUU
Table 2. Logic Table for the LTC1598L Channel Selection
CHANNEL STATUS EN D2 D1 DO
All Off 0 X X X
CH0 1 0 0 0
CH1 1 0 0 1
CH2 1 0 1 0
CH3 1 0 1 1
CH4 1 1 0 0
CH5 1 1 0 1
CH6 1 1 1 0
CH7 1 1 1 1
Transfer Curve
The LTC1594L/LTC1598L are permanently configured
for unipolar only. The input span and code assignment
for this conversion type is illustrated below.
Transfer Curve
0V
1LSB
V
REF
–2LSB
V
REF
4096
V
REF
–1LSB
V
REF
V
IN
0 0 0 0 0 0 0 0 0 0 0 1
0 0 0 0 0 0 0 0 0 0 0 0
1 1 1 1 1 1 1 1 1 1 1 1
1 1 1 1 1 1 1 1 1 1 1 0
•
•
•
1594L/98L • AI03
1LSB =
Output Code
OUTPUT CODE
•
•
•
1 1 1 1 1 1 1 1 1 1 1 1 1 1
1 1 1 1 1 1 1 1 1 1 1 1 1 0
0 0 0 0 0 0 0 0 0 0 0 0 0 1
0 0 0 0 0 0 0 0 0 0 0 0 0 0
INPUT VOLTAGE
VREF – 1LSB
VREF – 2LSB
•
•
•
1LSB
0V
INPUT VOLTAGE
(VREF = 2.500V)
2.49939V
2.49878V
•
•
•
0.00061V
0V
1594L/98L • AI04
13
LTC1594L/LTC1598L
APPLICATIONS INFORMATION
WUUU
Operation with D
IN
and D
OUT
Tied Together
The LTC1594L/LTC1598L can be operated with D
IN
and
D
OUT
tied together. This eliminates one of the lines
required to communicate to the microprocessor (MPU).
Data is transmitted in both directions on a single wire.
The processor pin connected to this data line should be
configurable as either an input or an output. The
LTC1594L/LTC1598L will take control of the data line
after CS falling and before the 6th falling CLK while the
processor takes control of the data line when CS is high
(see Figure 3). Therefore the processor port line must be
switched to an input with CS being low to avoid a conflict.
Separate Chip Selects for MUX and ADC
The LTC1594L/LTC1598L provide separate chip selects,
CSMUX and CSADC, to control MUX and ADC separately.
This feature not only provides the flexibility to select a
particular channel once for multiple conversions (see
Figure 4) but also maximizes the sample rate up to
20ksps (see Figure 5).
Figure 4. Selecting a Channel Once for Multiple Conversions
CLK
EN D1
D2
CSADC
CSMUX
B5
B6
B7
B8B9
B10B11 Hi-Z
D
OUT
CH0 TO
CH7
D
IN
t
CONV
Hi-Z
t
suCS
NULL
BIT
D0
B4 B3 B2 B1 B0
t
SMPL
t
ON
B5
B6
B7
B8B9
B10B11 Hi-Z
t
CONV
t
suCS
NULL
BIT
D0
B4 B3 B2 B1 B0
t
SMPL
ADCIN =
MUXOUT
COM = GND
1594L/98L F04
DON’T CARE DON’T CARE
12 3 456
CS
CLK
DATA (D
IN
/D
OUT
)EN D2 D1 D0 B11 B10
•••
LTC1594L/LTC1598L CONTROLS DATA LINE AND SENDS
A/D RESULT BACK TO MPU
MPU CONTROLS DATA LINE AND SENDS
MUX ADDRESS TO LTC1594L/LTC1598L
PROCESSOR MUST RELEASE DATA
LINE AFTER CS FALLING AND
BEFORE THE 6TH FALLING CLK
LTC1594L/LTC1598L TAKES CONTROL OF DATA
LINE AFTER CS FALLING AND BEFORE THE
6TH FALLING CLK
1594L/98L F03
t
suCS
Figure 3. LTC1594L/LTC1598L Operation with DIN and DOUT Tied Together
14
LTC1594L/LTC1598L
APPLICATIONS INFORMATION
WUUU
CLK
EN D1
D2
CSMUX
CSADC
B5
B6
B7
B8B9
B10B11
D
OUT
CH0 TO
CH7
D
IN
t
CONV
t
suCS
NULL
BIT
D0
B4 B3 B2 B1 B0
EN D1
t
SMPL
t
ON
t
ON
B5
B6
B7
B8B9
B10B11
t
CONV
t
suCS
NULL
BIT
D0D2
EN D1
D0D2
B4 B3 B2 B1 B0
t
SMPL
ADCIN =
MUXOUT
COM = GND
1594L/98L F05
B4 B3 B2 B1 B0
DON’T CARE DON’T CARE
MUXOUT/ADCIN Loop Economizes
Signal Conditioning
The MUXOUT and ADCIN pins of the LTC1594L/LTC1598L
form a very flexible external loop that allows Program-
mable Gain Amplifier (PGA) and/or processing analog
input signals prior to conversion. This loop is also a cost
effective way to perform the conditioning, because only
one circuit is needed instead of one for each channel.
In the Typical Applications section, there are a few
examples illustrating how to use the MUXOUT/ADCIN loop
to form a PGA and to antialias filter several analog inputs.
ACHIEVING MICROPOWER PERFORMANCE
With typical operating currents of 160µA and automatic
shutdown between conversions, the LTC1594L/
LTC1598L achieve extremely low power consumption
over a wide range of sample rates (see Figure 6). The auto
shutdown allows the supply current to drop with reduced
sample rate. Several things must be taken into account to
achieve such a low power consumption.
Shutdown
The LTC1594L/LTC1598L are equipped with automatic
shutdown features. They draw power when the CS pin is
low. The bias circuits and comparator of the ADC powers
down and the reference input becomes high impedance at
the end of each conversion leaving the CLK running to
clock out the LSB first data or zeroes (see Figures 1 and 2).
When the CS pin is high, the ADC powers down completely
leaving the CLK running to clock the input data word into
MUX. If the CS, D
IN
and CLK are not running rail-to-rail, the
input logic buffers will draw currents. These currents may
be large compared to the typical supply current. To obtain
the lowest supply current, run the CS, D
IN
and CLK pins
rail-to-rail.
D
OUT
Loading
Capacitive loading on the digital output can increase
power consumption. A 100pF capacitor on the D
OUT
pin
can add more than 50µA to the supply current at a 200kHz
clock frequency. An extra 50µA or so of current goes into
charging and discharging the load capacitor. The same
goes for digital lines driven at a high frequency by any
logic. The (C)(V)(f) currents must be evaluated and the
troublesome ones minimized.
Figure 5. Use Separate Chip Selects to Maximize Sample Rate
Figure 6. Automatic Power Shutdown Between Conversions
Allows Power Consumption to Drop with Sample Rate
SAMPLE FREQUENCY (kHz)
0.1
1
SUPPLY CURRENT (µA)
10
100
1000
1 10 100
1594L/98L G01
T
A
= 25°C
V
CC
= 2.7V
V
REF
= 2.5V
f
CLK
= 200kHz
15
LTC1594L/LTC1598L
APPLICATIONS INFORMATION
WUUU
BOARD LAYOUT CONSIDERATIONS
Grounding and Bypassing
The LTC1594L/LTC1598L are easy to use if some care is
taken. They should be used with an analog ground plane
and single point grounding techniques. The GND pin
should be tied directly to the ground plane.
The V
CC
pin should be bypassed to the ground plane with
a 10µF tantalum capacitor with leads as short as possible.
If the power supply is clean, the LTC1594L/LTC1598L
can also operate with smaller 1µF or less surface mount
or ceramic bypass capacitors. All analog inputs should
be referenced directly to the single point ground. Digital
inputs and outputs should be shielded from and/or
routed away from the reference and analog circuitry.
SAMPLE-AND-HOLD
Both the LTC1594L/LTC1598L provide a built-in sample-
and-hold (S&H) function to acquire signals through the
selected channel, assuming the ADCIN and MUXOUT
pins are tied together. The S & H of these parts acquire
input signals through the selected channel relative to
COM input during the t
SMPL
time (see Figure 7).
Single-Ended Inputs
The sample-and-hold of the LTC1594L/LTC1598L allows
conversion of rapidly varying signals. The input voltage
is sampled during the t
SMPL
time as shown in Figure 7.
The sampling interval begins after t
ON
time once the CS
is pulled low and continues until the second falling CLK
edge after the CS is low (see Figure 7). On this falling CLK
Figure 7. LTC1594L/LTC1598L ADCIN and COM Input Settling Windows
CLK
D
IN
D
OUT
MUXOUT = ADCIN
CH0 TO CH7
SAMPLE HOLD
“ANALOG” INPUT MUST
SETTLE DURING
THIS TIME
t
SMPL
t
ON
t
CONV
CSADC = CSMUX = CS
D2 D1EN D0 DON‘T CARE
1ST BIT TEST “COM” INPUT MUST
SETTLE DURING THIS TIME
B11
COM
1594L/98L F07
16
LTC1594L/LTC1598L
APPLICATIONS INFORMATION
WUUU
edge, the S & H goes into hold mode and the conversion
begins. The voltage on the “COM” input must remain
constant and be free of noise and ripple throughout the
conversion time. Otherwise, the conversion operation
may not be performed accurately. The conversion time is
12 CLK cycles. Therefore, a change in the “COM” input
voltage during this interval can cause conversion errors.
For a sinusoidal voltage on the “COM” input this error
would be:
V
ERROR(MAX)
= V
PEAK
(2π)(f)(“COM”)12/f
CLK
Where f(“COM”) is the frequency of the “COM” input
voltage, V
PEAK
is its peak amplitude and f
CLK
is the
frequency of the CLK. In most cases, V
ERROR
will not be
significant. For a 60Hz signal on the “COM” input to
generate a 0.5LSB error (305µV) with the converter
running at CLK = 200kHz, its peak value would have to be
5.266mV.
ANALOG INPUTS
Because of the capacitive redistribution A/D conversion
techniques used, the analog inputs of the LTC1594L/
LTC1598L have capacitive switching input current spikes.
These current spikes settle quickly and do not cause a
problem. However, if large source resistances are used
or if slow settling op amps drive the inputs, care must be
taken to insure that the transients caused by the current
spikes settle completely before the conversion begins.
“Analog” Input Settling
The input capacitor of the LTC1594L/LTC1598L is switched
onto the selected channel input during the t
SMPL
time (see
Figure 7) and samples the input signal within that time. The
sample phase is at least 1 1/2 CLK cycles before conver-
sion starts. The voltage on the “analog” input must settle
completely within t
SMPL
. Minimizing R
SOURCE+
and C1 will
improve the input settling time. If a large “analog” input
source resistance must be used, the sample time can be
increased by using a slower CLK frequency.
“COM” Input Settling
At the end of the t
SMPL
, the input capacitor switches to the
“COM” input and conversion starts (see Figures 1 and 7).
During the conversion, the “analog” input voltage is
effectively “held” by the sample-and-hold and will not
affect the conversion result. However, it is critical that the
“COM” input voltage settles completely during the first
CLK cycle of the conversion time and be free of noise.
Minimizing R
SOURCE–
and C2 will improve settling time.
If a large “COM” input source resistance must be used,
the time allowed for settling can be extended by using a
slower CLK frequency.
Input Op Amps
When driving the analog inputs with an op amp it is
important that the op amp settle within the allowed time
(see Figure 7). Again, the “analog” and “COM” input
sampling times can be extended as described above to
accommodate slower op amps. Most op amps, including
the LT
®
1006 and LT1413 single supply op amps, can be
made to settle well even with the minimum settling
windows of 7.5µs (“analog” input) which occur at the
maximum clock rate of 200kHz.
Source Resistance
The analog inputs of the LTC1594L/LTC1598L look like a
20pF capacitor (C
IN
) in series with a 1k resistor (R
ON
) and
a 90Ω channel resistance as shown in Figure 8. C
IN
gets
switched between the selected “analog” and “COM”
inputs once during each conversion cycle. Large external
source resistors and capacitances will slow the settling
of the inputs. It is important that the overall RC time
constants be short enough to allow the analog inputs to
completely settle within the allowed time.
Figure 8. Analog Input Equivalent Circuit
R
ON
1k
R
ON
90Ω
C
IN
20pF
LTC1594L
LTC1598L
“ANALOG”
INPUT
R
SOURCE
+
V
IN
+
C1
“COM”
INPUT
MUXOUT
MUX
ADCIN
R
SOURCE
–
V
IN
–
C2
1594L/98L F08
17
LTC1594L/LTC1598L
Input Leakage Current
Input leakage currents can also create errors if the source
resistance gets too large. For instance, the maximum
input leakage specification of 200nA (at 85°C) flowing
through a source resistance of 600Ω will cause a voltage
drop of 120µV or 0.2LSB. This error will be much
reduced at lower temperatures because leakage drops
rapidly (see typical curve Input Channel Leakage Current
vs Temperature).
REFERENCE INPUTS
The reference input of the LTC1594L/LTC1598L is effec-
tively a 50k resistor from the time CS goes low to the end
of the conversion. The reference input becomes a high
impedance node at any other time (see Figure 9). Since
the voltage on the reference input defines the voltage
span of the A/D converter, the reference input should be
driven by a reference with low R
OUT
(ex. LT1004, LT1019
and LT1021) or a voltage source with low R
OUT
.
Reduced Reference Operation
The effective resolution of the LTC1594L/LTC1598L can
be increased by reducing the input span of the convert-
ers. The LTC1594L/LTC1598L exhibit good linearity and
gain over a wide range of reference voltages (see typical
curves Change in Linearity vs Reference Voltage and
Change in Gain vs Reference Voltage). However, care
must be taken when operating at low values of V
REF
because of the reduced LSB step size and the resulting
higher accuracy requirement placed on the converters.
The following factors must be considered when operat-
ing at low V
REF
values:
1. Offset
2. Noise
3. Conversion speed (CLK frequency)
Offset with Reduced V
REF
The offset of the LTC1594L/LTC1598L has a larger effect
on the output code when the ADCs are operated with
reduced reference voltage. The offset (which is typically
a fixed voltage) becomes a larger fraction of an LSB as the
size of the LSB is reduced. The typical curve of Change in
Offset vs Reference Voltage shows how offset in LSBs is
related to reference voltage for a typical value of V
OS
. For
example, a V
OS
of 122µV which is 0.2LSB with a 2.5V
reference becomes 0.5LSB with a 1V reference and
2.5LSBs with a 0.2V reference. If this offset is unaccept-
able, it can be corrected digitally by the receiving system
or by offsetting the “COM” input of the LTC1594L/
LTC1598L.
Noise with Reduced V
REF
The total input referred noise of the LTC1594L/LTC1598L
can be reduced to approximately 400µV peak-to-peak
using a ground plane, good bypassing, good layout
techniques and minimizing noise on the reference inputs.
This noise is insignificant with a 5V reference but will
become a larger fraction of an LSB as the size of the LSB
is reduced.
For operation with a 2.5V reference, the 400µV noise is
only 0.66LSB peak-to-peak. In this case, the LTC1594L/
LTC1598L noise will contribute virtually no uncertainty to
the output code. However, for reduced references the
noise may become a significant fraction of an LSB and
cause undesirable jitter in the output code. For example,
with a 1.25V reference this same 400µV noise is 1.32LSB
peak-to-peak. This will reduce the range of input voltages
over which a stable output code can be achieved by 1LSB.
If the reference is further reduced to 1V, the 400µV noise
becomes equal to 1.65LSBs and a stable code may be
difficult to achieve. In this case, averaging multiple
readings may be necessary.
This noise data was taken in a very clean setup. Any setup
induced noise (noise or ripple on V
CC
, V
REF
or V
IN
) will
add to the internal noise. The lower the reference voltage
to be used the more critical it becomes to have a clean,
noise free setup.
LTC1594L
LTC1598L
REF
+
R
OUT
V
REF
1
4
GND
1594L/98L F09
Figure 9. Reference Input Equivalent Circuit
APPLICATIONS INFORMATION
WUUU
18
LTC1594L/LTC1598L
Conversion Speed with Reduced V
REF
With reduced reference voltages, the LSB step size is
reduced and the LTC1594L/LTC1598L internal compara-
tor overdrive is reduced. Therefore, it may be necessary
to reduce the maximum CLK frequency when low values
of V
REF
are used.
DYNAMIC PERFORMANCE
The LTC1594L/LTC1598L have exceptional sampling
capability. Fast Fourier Transform (FFT) test techniques
are used to characterize 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 con-
tent can be examined for frequencies outside the funda-
mental. Figure 10 shows a typical LTC1594L/LTC1598L
plot.
APPLICATIONS INFORMATION
WUUU
Effective Number of Bits
The Effective Number of Bits (ENOBs) is a measurement of
the resolution of an ADC and is directly related to S/(N + D)
by the equation:
ENOB = [S/(N + D) – 1.76]/6.02
where S/(N + D) is expressed in dB. At the maximum
sampling rate of 10.5kHz with a 5V supply, the LTC1594L/
LTC1598L maintain above 10.7 ENOBs at 10kHz input
frequency. Above 10kHz the ENOBs gradually decline, as
shown in Figure 11, due to increasing second harmonic
distortion. The noise floor remains low.
Total Harmonic Distortion
Total Harmonic Distortion (THD) is the ratio of the RMS
sum of all harmonics of the input signal to the fundamen-
tal itself. The out-of-band harmonics alias into the fre-
quency band between DC and half of the sampling
frequency. THD is defined as:
THD =++++
20log VVV V
V
2
23
24
2N
2
1
...
where V
1
is the RMS amplitude of the fundamental
frequency and V
2
through V
N
are the amplitudes of the
second through the N
th
harmonics. The typical THD
Signal-to-Noise Ratio
The Signal-to-Noise plus Distortion Ratio (S/N + D) is the
ratio between the RMS amplitude of the fundamental
input frequency to the RMS amplitude of all other fre-
quency components at the ADC’s output. The output is
band limited to frequencies above DC and below one half
the sampling frequency. Figure 11 shows a typical spec-
tral content with a 10.5kHz sampling rate.
Figure 10. LTC1594L/LTC1598L Nonaveraged,
4096 Point FFT Plot
FREQUENCY (kHz)
0
–120
MAGNITUDE (dB)
–100
–80
–60
–40
1.0 2.0 3.0 4.0
1594L/98L G13
–20
0
0.5 1.5 2.5 3.5
T
A
= 25°C
V
CC
= 2.7V
V
REF
= 2.5V
f
IN
= 3.05kHz
f
CLK
= 120kHz
f
SMPL
= 7.5kHz
Figure 11. Effective Bits and S/(N + D) vs Input Frequency
INPUT FREQUENCY (kHz)
1
0
EFFECTIVE NUMBER OF BITS (ENOBs)
S/(N + D) (dB)
3
5
7
10
10 100
1594L/98L G09
1
4
6
9
12
11
8
62
56
74
68
50
2
T
A
= 25°C
V
CC
= 2.7V
f
CLK
= 200kHz
f
SMPL
= 10.5kHz
19
LTC1594L/LTC1598L
specification in the Dynamic Accuracy table includes the
2nd through 5th harmonics. With a 1kHz input signal, the
LTC1594L/LTC1598L have typical THD of 78dB with
V
CC
= 2.7V.
Intermodulation Distortion
If the ADC input signal consists of more than one
spectral component, the ADC transfer function nonlin-
earity can produce intermodulation distortion (IMD)
in addition to THD. IMD is the change in one sinusoi-
dal input caused by the presence of another sinusoidal
input at a different frequency.
If two pure sine waves of frequencies f
a
and f
b
are applied
to the ADC input, nonlinearities in the ADC transfer
function can create distortion products at sum and differ-
ence frequencies of mf
a
± nf
b
, where m and n = 0, 1, 2, 3,
etc. For example, the 2nd order IMD terms include (f
a
+
f
b
) and (f
a
– f
b
) while 3rd order IMD terms include (2f
a
+
f
b
), (2f
a
– f
b
), (f
a
+ 2f
b
), and (f
a
– 2f
b
). If the two input sine
waves are equal in magnitudes, the value (in dB) of the
2nd order IMD products can be expressed by the follow-
ing formula:
APPLICATIONS INFORMATION
WUUU
TYPICAL APPLICATIONS N
U
Microprocessor Interfaces
The LTC1594L/LTC1598L can interface directly (without
external hardware) to most popular microprocessors’
(MPU) synchronous serial formats including
MICROWIRE, SPI and QSPI. If an MPU without a dedi-
cated serial port is used, then three of the MPU’s parallel
port lines can be programmed to form the serial link to the
LTC1594L/LTC1598L. Included here is one serial interface
example.
Motorola SPI (MC68HC05)
The MC68HC05 has been chosen as an example of an MPU
with a dedicated serial port. This MPU transfers data MSB-
first and in 8-bit increments. The D
IN
word sent to the data
register starts the SPI process. With three
8-bit transfers the A/D result is read into the MPU. The
second 8-bit transfer clocks B11 through B7 of the A/D
conversion result into the processor. The third 8-bit trans-
fer clocks the remaining bits B6 through B0 into the MPU.
ANDing the second byte with 1F
HEX
clears the three most
significant bits and ANDing the third byte with FE
HEX
clears
the least significant bit. Shifting the data to the right by one
bit results in a right justified word.
IMD f f mplitude f f
ab ab
±
()
=±
()
20log a
amplitude at f
a
Peak Harmonic or Spurious Noise
The peak harmonic or spurious noise is the largest
spectral component excluding the input signal and DC.
This value is expressed in dBs relative to the RMS value
of a full-scale input signal.
Full-Power and Full-Linear Bandwidth
The full-power bandwidth is that input frequency at
which the amplitude of the reconstructed fundamental is
reduced by 3dB for a full-scale input.
The full-linear bandwidth is the input frequency at which
the effective bits rating of the ADC falls to 11 bits. Beyond
this frequency, distortion of the sampled input signal
increases. The LTC1594L/LTC1598L have been designed
to optimize input bandwidth, allowing the ADCs to
undersample input signals with frequencies above the
converters’ Nyquist Frequency.
20
LTC1594L/LTC1598L
TYPICAL APPLICATIONS N
U
LDA #$52 Configuration data for serial peripheral
control register (Interrupts disabled, output
enabled, master, Norm = 0, Ph = 0, Clk/16)
STA $0A Load configuration data into location $0A (SPCR)
LDA #$FF Configuration data for I/O ports
(all bits are set as outputs)
STA $04 Load configuration data into Port A DDR ($04)
STA $05 Load configuration data into Port B DDR ($05)
STA $06 Load configuration data into Port C DDR ($06)
LDA #$08 Put D
IN
word for LTC1598L into Accumulator
(CH0 with respect to GND)
STA $50 Load D
IN
word into memory location $50
START BSET 0,$02 Bit 0 Port C ($02) goes high (CS goes high)
LDA $50 Load D
IN
word at $50 into Accumulator
STA $0C Load D
IN
word into SPI data register ($0C) and
start clocking data
LOOP1 TST $0B Test status of SPIF bit in SPI status register ($0B)
MC68HC05 CODE
BPL LOOP1 Loop if not done with transfer to previous instruction
BCLR 0,$02 Bit 0 Port C ($02) goes low (CS goes low)
LDA $0C Load contents of SPI data register into Accumulator
STA $0C Start next SPI cycle
LOOP2 TST $0B Test status of SPIF
BPL LOOP2 Loop if not done
LDA $0C Load contents of SPI data register into Accumulator
STA $0C Start next SPI cycle
AND #$IF Clear 3 MSBs of first D
OUT
word
STA $00 Load Port A ($00) with MSBs
LOOP3 TST $0B Test status of SPIF
BPL LOOP3 Loop if not done
LDA $0C Load contents of SPI data register into Accumulator
AND #$FE Clear LSB of second D
OUT
word
STA $01 Load Port B ($01) with LSBs
JMP START Go back to start and repeat program
1594L/98L TA04
DOUT FROM LTC1598L STORED IN MC68HC05 RAM
B1 B0 0
B2
B3
B5
B6 B4
0
0
LSB
MSB
#00
#01
0B11 B10 B9 B8 B7
CLK
D
IN
CSMUX
CSADC
ANALOG
INPUTS
C0
SCK
MC68HC05
D
OUT
MOSI
LTC1598L
BYTE 1
BYTE 2 MISO
Hardware and Software Interface to Motorola MC68HC05
Data Exchange Between LTC1598L and MC68HC05
CSMUX
= CSADC
= CS
CLK
D
OUT
MPU
RECEIVED
WORD
1594L/98L TA03
B3B7 B6 B5 B4 B2 B1 B0 B1 B2B11 B10 B9 B8
D
IN
MPU
TRANSMIT
WORD BYTE 3
BYTE 2
EN D20D1
X
D0
BYTE 1
X
XXXXX
X
000 XXX
XX
XX
X
BYTE 3
BYTE 2
BYTE 1
B10
??0B11 B9 B7
B8 B6 B5 B3
B4 B2 B1 B1
B0
DON‘T CARE
D1D2
???
?????
DO
EN
21
LTC1594L/LTC1598L
TYPICAL APPLICATIONS N
U
MULTICHANNEL A/D USES A SINGLE
ANTIALIASING FILTER
This circuit demonstrates how the LTC1598L’s indepen-
dent analog multiplexer can simplify design of a 12-bit
data acquisition system. All eight channels are MUXed into
a single 1kHz, 4th order Sallen-Key antialiasing filter,
which is designed for single supply operation. Since the
LTC1598L’s data converter accepts inputs from ground to
the positive supply, rail-to-rail op amps were chosen for
the filter to maximize dynamic range. The LT1368 dual rail-
to-rail op amp is designed to operate with 0.1µF load
capacitors (C1 and C2). These capacitors provide fre-
quency compensation for the amplifiers and help reduce
the amplifier’s output impedance and improve supply
rejection at high frequencies. The filter contributes less
than 1LSB of error due to offsets and bias currents. The
filter’s noise and distortion are less than –72dB for a
100Hz, 2V
P-P
offset sine input.
The combined MUX and A/D errors result in an integral
nonlinearity error of ±3LSB (maximum) and a differential
nonlinearity error of ±3/4LSB (maximum). The typical
signal-to-noise plus distortion ratio is 68dB, with approxi-
mately –78dB of total harmonic distortion. The LTC1598L
is programmed through a 4-wire serial interface that is
compatible with MICROWIRE, SPI and QSPI. Maximum
serial clock speed is 200kHz, which corresponds to a
10.5kHz sampling rate.
The complete circuit consumes approximately 600µA
from a single 3V supply.
Simple Data Acquisition System Takes Advantage of the LTC1598L’s
MUXOUT/ADCIN Pins to Filter Analog Signals Prior to A/D Conversion
1594L/98L TA05
CH0
CH1
CH2
CH3
CH4
CH5
CH6
CH7
20
21
22
23
24
1
2
3
+
–
8-CHANNEL
MUX
8 COM
GND
4, 9
10
6
5, 14
7
11
CSADC
CSMUX
CLK
D
IN
D
OUT
12
13
NC
NC
12-BIT
SAMPLING
ADC
SERIAL DATA LINK
MICROWIRE AND SPI
COMPATIBLE
ADCINMUXOUT
18
38
1
2
17 16
3.3V
3.3V
15, 19 C7
1µF
V
REF
V
CC
LTC1598L
C6
0.1µF
–
+
C2
0.015µF
C8
0.01µF
R2
7.5k R3
7.5k R4
7.5k
R1
7.5k
C4
0.03µF
C3
0.1µF
C5
0.015µF
C1
0.03µF1/2 LT1368
5
7
6
4
–
+
1/2 LT1368
22
LTC1594L/LTC1598L
TYPICAL APPLICATIONS N
U
Using MUXOUT/ADCIN Loop as PGA
This figure shows the LTC1598L’s MUXOUT/ADCIN pins
and an LT1368 being used to create a single channel PGA
with eight noninverting gains. Combined with the LTC1391,
the system can expand to eight channels and eight gains
for each channel. Using the LTC1594L, the PGA is reduced
to four gains. The output of the LT1368 drives the ADCIN
and the resistor ladder. The resistors above the selected
MUX channel form the feedback for the LT1368. The gain
for this amplifier is R
S1
/R
S2
+ 1. R
S1
is the summation of
the resistors above the selected MUX channel and R
S2
is
Using the MUXOUT/ADCIN Pins of the LTC1598L to Form a PGA.
The LTC1391 MUX Allows Eight Input Channels to be Digitized
the summation of the resistors below the selected MUX
channel. If CH0 is selected, the gain is 1 since R
S1
is 0.
Table 1 shows the gain for each MUX channel. The LT1368
dual rail-to-rail op amp is designed to operate with 0.1µF
load capacitors. These capacitors provide frequency com-
pensation for the amplifiers, help reduce the amplifiers’
output impedance and improve supply rejection at high
frequencies. Because the LT1368’s I
B
is low, the R
ON
of the
selected channel will not affect the gain given by the
formula above.
1594L/98L TA06
CH0
CH1
CH2
CH3
CH4
CH5
CH6
CH7
20
21
22
23
24
1
2
3
64R
32R
16R
8R
4R
2R
R
R
+
–
8 COM
18 MUXOUT
= DAISY CHAIN CONFIGURATION FOR THE LTC1391 AND THE LTC1598L
GND
4, 9
10
6
5, 14
11
7
CSADC
CSMUX
CLK
D
OUT
D
IN
12
13
NC
NC
12-BIT
SAMPLING
ADC
8-CHANNEL
MUX
1
2
3
4
5
6
7
8
16
15
14
13
12
11
10
9
LTC1391
3V
1µF
ADCIN
17 16 15, 19 1µF
0.1µF
3V
1µF
3V
V
REF
V
CC
–
+
CH0
CH1
CH2
CH3
CH4
CH5
CH6
CH7
V+
D
V–
D
OUT
D
IN
CS
CLK
GND
1/2 LT1368
LTC1598L
µP/µC
3(5)
2(6)
1(7)
4
8
23
LTC1594L/LTC1598L
G Package
24-Lead Plastic SSOP (0.209)
(LTC DWG # 05-08-1640)
Dimensions in inches (millimeters) unless otherwise noted.
PACKAGE DESCRIPTION
U
S Package
16-Lead Plastic Small Outline (Narrow 0.150)
(LTC DWG # 05-08-1610)
Information furnished by Linear Technology Corporation is believed to be accurate and reliable.
However, no responsibility is assumed for its use. Linear Technology Corporation makes no represen-
tation that the interconnection of its circuits as described herein will not infringe on existing patent rights.
G24 SSOP 1098
0.13 – 0.22
(0.005 – 0.009)
0° – 8°
0.55 – 0.95
(0.022 – 0.037)
5.20 – 5.38**
(0.205 – 0.212)
7.65 – 7.90
(0.301 – 0.311)
12345678 9 10 11 12
8.07 – 8.33*
(0.318 – 0.328)
2122 18 17 16 15 14 13
19202324
1.73 – 1.99
(0.068 – 0.078)
0.05 – 0.21
(0.002 – 0.008)
0.65
(0.0256)
BSC 0.25 – 0.38
(0.010 – 0.015)
NOTE: DIMENSIONS ARE IN MILLIMETERS
DIMENSIONS DO NOT INCLUDE MOLD FLASH. MOLD FLASH
SHALL NOT EXCEED 0.152mm (0.006") PER SIDE
DIMENSIONS DO NOT INCLUDE INTERLEAD FLASH. INTERLEAD
FLASH SHALL NOT EXCEED 0.254mm (0.010") PER SIDE
*
**
0.016 – 0.050
(0.406 – 1.270)
0.010 – 0.020
(0.254 – 0.508)× 45°
0° – 8° TYP
0.008 – 0.010
(0.203 – 0.254)
12345678
0.150 – 0.157**
(3.810 – 3.988)
16 15 14 13
0.386 – 0.394*
(9.804 – 10.008)
0.228 – 0.244
(5.791 – 6.197)
12 11 10 9
S16 1098
0.053 – 0.069
(1.346 – 1.752)
0.014 – 0.019
(0.355 – 0.483)
TYP
0.004 – 0.010
(0.101 – 0.254)
0.050
(1.270)
BSC
DIMENSION DOES NOT INCLUDE MOLD FLASH. MOLD FLASH
SHALL NOT EXCEED 0.006" (0.152mm) PER SIDE
DIMENSION DOES NOT INCLUDE INTERLEAD FLASH. INTERLEAD
FLASH SHALL NOT EXCEED 0.010" (0.254mm) PER SIDE
*
**
24
LTC1594L/LTC1598L
LINEAR TECHNOLOGY CORPORATION 1997
15948lfa LT/TP 0500 2K
REV A
• PRINTED IN USA
TYPICAL APPLICATION
U
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1594L/98L TA07
20
21
22
23
24
1
2
3
8 COM GND
4, 9
10
6
5, 14
7
11
CSADC
CSMUX
CLK
D
IN
D
OUT
12
13
NC
NC
ADCINMUXOUT
18 17 16 15, 19 1µF
1µF
3V
V
REF
V
CC
1
2
3
4
5
6
7
8
16
15
14
13
12
11
10
9
LTC1391
CH0
CH7
D
IN
CLK
CS
D
OUT
3V
LTC1598L
= DAISY CHAIN CONFIGURATION FOR THE LTC1391 AND THE LTC1598L
CH0
CH1
CH2
CH3
CH4
CH5
CH6
CH7
+
–
12-BIT
SAMPLING
ADC
8-CHANNEL
MUX
CH0
CH1
CH2
CH3
CH4
CH5
CH6
CH7
V+
D
V–
D
OUT
D
IN
CS
CLK
GND
Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900
●
FAX: (408) 434-0507
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