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1
LTC1285/LTC1288
3V Micropower Sampling
12-Bit A/D Converters in
SO-8 Packages
n
12-Bit Resolution
n
8-Pin SO Plastic Package
n
Low Cost
n
Low Supply Current: 160
m
A Typ
n
Auto Shutdown to 1nA Typ
n
Guaranteed 
3/4LSB Max DNL
n
Single Supply 3V to 6V Operation
n
Differential Inputs (LTC1285)
n
2-Channel MUX (LTC1288)
n
On-Chip Sample-and-Hold
n
100
m
s Conversion Time
n
Sampling Rates:
7.5ksps (LTC1285)
6.6ksps (LTC1288)
n
I/O Compatible with SPI, Microwire, etc.
The LTC
®
1285/LTC1288 are 3V micropower, 12-bit, suc-
cessive approximation sampling A/D converters. They
typically draw only 160
m
A of supply current when con-
verting and automatically power down to a typical supply
current of 1nA whenever they are not performing conver-
sions. They are packaged in 8-pin SO packages and
operate on 3V to 6V supplies. These 12-bit, switched-
capacitor, successive approximation ADCs include
sample-and-holds. The LTC1285 has a single differential
analog input. The LTC1288 offers a software selectable
2-channel MUX.
On-chip serial ports allow efficient data transfer to a wide
range of microprocessors and microcontrollers over three
wires. This, coupled with micropower consumption, makes
remote location possible and facilitates transmitting data
through isolation barriers.
These circuits 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.
APPLICATIONS
U
SAMPLE FREQUENCY (kHz)
0.1
1
SUPPLY CURRENT (mA)
10
100
1000
1
10
100
LTC1285/88 • TA02
T
A
= 25°C
V
CC
= 2.7V
V
REF
= 2.5V
f
CLK
= 120kHz 
Supply Current vs Sample Rate
n
Pen Screen Digitizing
n
Battery-Operated Systems
n
Remote Data Acquisition
n
Isolated Data Acquisition
n
Battery Monitoring
n
Temperature Measurement
12
m
W, S0-8 Package, 12-Bit ADC
Samples at 200Hz and Runs Off a 3V Supply
3V
1
m
F
ANALOG INPUT
0V TO 3V RANGE
–IN
GND
V
CC
CLK
D
OUT
V
REF
LTC1285
MPU
(e.g., 8051)
P1.4
P1.3
P1.2
+IN
LTC1285/88 • TA01
CS/SHDN
6
5
8
7
3
4
1
2
SERIAL DATA LINK
FEATURES
DESCRIPTION
U
TYPICAL APPLICATIONN 
U
, LTC and LT are registered trademarks of Linear Technology Corporation.
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2
LTC1285/LTC1288
(Notes 1 and 2)
Power Dissipation..............................................500mW
Operating Temperature Range....................0
°
C to 70
°
C
Storage Temperature Range.................–65
°
C to 150
°
C
Lead Temperature (Soldering, 10 sec.)................300
°
C
PART MARKING
T
JMAX
= 150
°
C,  
q
JA
= 130
°
C/W
Consult factory for Industrial and Military grade parts.
ORDER PART
NUMBER
LTC1285CN8
ORDER PART
NUMBER
LTC1285CS8
1285C
PART MARKING
ORDER PART
NUMBER
LTC1288CN8
ORDER PART
NUMBER
LTC1288CS8
1288C
T
JMAX
= 150
°
C,  
q
JA
= 130
°
C/W
T
JMAX
= 150
°
C,  
q
JA
= 175
°
C/W
T
JMAX
= 150
°
C,  
q
JA
= 175
°
C/W
PACKAGE/ORDER INFORMATION
W
U
U
Supply Voltage (V
CC
) to GND...................................12V
Voltage
Analog and Reference................–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 W
W
SYMBOL
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
V
CC
Supply Voltage (Note 3)
LTC1285
2.7
6
V
LTC1288
2.7
6
V
f
CLK
Clock Frequency
V
CC
= 2.7V
(Note 4)
120
kHz
t
CYC
Total Cycle Time
LTC1285, f
CLK
= 120kHz
125.0
m
s
LTC1288, f
CLK
= 120kHz
141.5
m
s
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)
LTC1285, V
CC
= 2.7V
2
m
s
LTC1288, V
CC
= 2.7V
2
m
s
t
suDI
Setup Time, D
IN
Stable Before CLK
V
CC
= 2.7V
600
ns
t
WHCLK
CLK High Time
V
CC
= 2.7V
3.5
m
s
t
WLCLK
CLK Low Time
V
CC
= 2.7V
3.5
m
s
t
WHCS
CS High Time Between Data Transfer Cycles
V
CC
= 2.7V
2
m
s
t
WLCS
CS Low Time During Data Transfer
LTC1285, f
CLK
= 120kHz
123.0
m
s
LTC1288, f
CLK
= 120kHz
139.5
m
s
RECO  ENDED OPERATING CONDITIONS
U
U
U
U
WW
1
2
3
4
8
7
6
5
TOP VIEW
V
REF
+IN
–IN
GND
V
CC  
CLK
D
OUT
N8 PACKAGE
8-LEAD PDIP
CS/SHDN
1
2
3
4
8
7
6
5
TOP VIEW
CH0
CH1
GND
V
CC 
(V
REF
)
CLK
D
OUT
D
IN
N8 PACKAGE
8-LEAD PDIP
CS/SHDN
1
2
3
4
8
7
6
5
TOP VIEW
V
CC
CLK
D
OUT
V
REF
+IN
–IN
GND
S8 PACKAGE
8-LEAD PLASTIC SO
CS/SHDN
1
2
3
4
8
7
6
5
TOP VIEW
V
CC 
(V
REF
)
CLK
D
OUT
D
IN
CH0
CH1
GND
S8 PACKAGE
8-LEAD PLASTIC SO
CS/SHDN
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3
LTC1285/LTC1288
LTC1285
LTC1288
PARAMETER
CONDITIONS
MIN
TYP
MAX
MIN
TYP
MAX
UNITS
Resolution (No Missing Codes)
l
12
12
Bits
Integral Linearity Error
(Note 6)
l
3/4
2
3/4
2
LSB
Differential Linearity Error
l
1/4
3/4
1/4
3/4
LSB
Offset Error
l
3/4
3
3/4
3
LSB
Gain Error
l
2
8
2
8
LSB
Analog Input Range
(Note 7 and 8)
l
V
REF Input Range (LTC1285)
2.7 
£
V
CC
£
6V
V
(Notes 7, 8, and 9)
V
Analog Input Leakage Current (Note 10)
l
1
1
m
A
1.5V to V
CC
+ 0.05V
–0.05V to V
CC
+ 0.05V
SYMBOL
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
S/(N +D)
Signal-to-Noise Plus Distortion Ratio
1kHz Input Signal
67
dB
THD
Total Harmonic Distortion (Up to 5th Harmonic)
1kHz Input Signal
–80
dB
SFDR
Spurious-Free Dynamic Range
1kHz Input Signal
88
dB
Peak Harmonic or Spurious Noise
1kHz Input Signal
–88
dB
DYNAMIC ACCURACY
U W
SYMBOL
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
V
IH
High Level Input Voltage
V
CC
= 3.6V
l
2
V
V
IL
Low Level Input Voltage
V
CC
= 2.7V
l
0.8
V
I
IH
High Level Input Current
V
IN
= V
CC
l
2.5
m
A
I
IL
Low Level Input Current
V
IN
= 0V
l
–2.5
m
A
V
OH
High Level Output Voltage
V
CC
= 2.7V, I
O
= 10
m
A
l
2.4
2.64
V
V
CC
= 2.7V, I
O
= 360
m
A
l
2.1
2.30
V
V
OL
Low Level Output Voltage
V
CC
= 2.7V, I
O
= 400
m
A
l
0.4
V
I
OZ
Hi-Z Output Leakage
CS = High
l
3
m
A
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
M
W
(LTC1285)
CS = V
IL
54
k
W
I
REF
Reference Current (LTC1285)
CS = V
CC
l
0.001
2.5
m
A
t
CYC
640
m
s, f
CLK
£
25kHz
50
m
A
t
CYC
= 134
m
s, f
CLK
= 120kHz
l
50
70
m
A
I
CC
Supply Current
CS = V
CC
l
0.001
3.0
m
A
LTC1285, t
CYC
640
m
s, f
CLK
£
25kHz
150
m
A
LTC1285, t
CYC
= 134
m
s, f
CLK
= 120kHz
l
160
320
m
A
LTC1288, t
CYC
720
m
s, f
CLK
£
25kHz
200
m
A
LTC1288, t
CYC
= 150
m
s, f
CLK
= 120kHz
l
210
390
m
A
DIGITAL AND DC ELECTRICAL CHARACTERISTICS
U
CONVERTER ANMULTIPLEXER CHARACTERISTICS
U
U W
(Note 5)
(Note 5)
f
SMPL
= 7.5kHz (LTC1285), f
SMPL
= 6.6kHz (LTC1288) (Note 5)
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4
LTC1285/LTC1288
TYPICAL PERFORMANCE CHARACTERISTICS  
WU
SYMBOL
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
t
SMPL
Analog Input Sample Time
See Operating Sequence
1.5
CLK Cycles
f
SMPL(MAX)
Maximum Sampling Frequency
LTC1285
l
7.5
kHz
LTC1288
l
6.6
kHz
t
CONV
Conversion Time
See Operating Sequence
12
CLK Cycles
t
dDO
Delay Time, CLK
to D
OUT
Data Valid
See Test Circuits
l
600
1500
ns
t
dis
Delay Time, CS
to D
OUT
Hi-Z
See Test Circuits
l
220
660
ns
t
en
Delay Time, CLK
to D
OUT
Enable
See Test Circuits
l
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
l
60
180
ns
t
r
D
OUT
Rise Time
See Test Circuits
l
80
180
ns
C
IN
Input Capacitance
Analog Inputs, On Channel
20
pF
Analog Inputs, Off Channel
5
pF
Digital Input
5
pF
(Note 5)
AC CHARACTERISTICS
The 
l
denotes specifications which apply over the full operating
temperature range.
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. For 5V specified devices, see
LTC1286 and LTC1298.
Note 4: Increased leakage currents at elevated temperatures cause the
sample-and-hold to droop, therefore it is recommended that f
CLK
75kHz
at 70
°
andf
CLK
1kHz at 25
°
C.
Note 5: V
CC
= 2.7V, V
REF
= 2.5V and CLK = 120kHz unless otherwise
specified.
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
£
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 2.7V
input voltage range will therefore require a minimum supply voltage of
2.650V over initial tolerance, temperature variations and loading. For 2.7V
< V
CC
£
6V, reference and analog input range cannot exceed 6.05V. If
reference and analog input range are greater than 6.05V, the output code
will not be guaranteed to be correct.
Note 8: The supply voltage range for the LTC1285 and the LTC1288 is
from 2.7V to 6V.
Note 9: Recommended operating conditions
Note 10: Channel leakage current is measured after the channel selection.
SAMPLE RATE (kHz)
0.1
1
SUPPLY CURRENT (mA)
10
1000
100
1
10
LTC1285/88 • TPC01
LTC1288
LTC1285
T
A
= 25°C
V
CC
= 2.7V
V
REF
= 2.5V
f
CLK
= 120kHz
Supply Current vs Sample Rate
FREQUENCY (kHz)
1
0
SUPPLY CURRENT (mA)
1
2
3
4
5
6
20
40
60
80
LTC1285/88 • TPC03
100
7
8
9
0.002
120
T
A
= 25°C
V
CC
= 2.7V
V
REF
= 2.5V
CS = 0
(AFTER CONVERSION)
CS = V
CC
Shutdown Supply Current vs Clock
Rate with CS High and CS Low
TEMPERATURE (°C)
–55
SUPPLY CURRENT (mA)
150
200
250
105
LTC1285/88 • TPC02
100
50
0
–15
25
65
–35
125
5
45
85
V
CC
= 2.7V
V
REF
= 2.5V
f
CLK
= 120kHz
LTC1285
f
SMPL
= 7.5kHz
LTC1288
f
SMPL
= 6.6kHz
Supply Current vs Temperature
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5
LTC1285/LTC1288
TEMPERATURE (°C)
0
CHANGE IN OFFSET (LSB)
0.15
30
LTC1285/88 • TPC07
0
–0.10
10
20
40
–0.15
–0.20
0.20
0.10
0.05
–0.05
50
60
70
V
CC
= 2.7V
V
REF
= 2.5V
f
CLK
= 120kHz
f
SMPL
= f
SMPL(MAX)
Change in Offset vs Temperature
Reference Current vs
Sample Rate (LTC1285)
SAMPLE RATE (kHz)
0
0
REFERENCE CURRENT (mA)
5
15
20
25
50
35
2
4
5
LTC1285/88 • TPC04
10
40
45
30
1
3
6
7
8
T
A
= 25°C
V
CC
= 2.7V
V
REF
= 2.5V
f
CLK
= 120kHz
TYPICAL PERFORMANCE CHARACTERISTICS  
WU
REFERENCE VOLTAGE (V)
1.0
0
CHANGE IN GAIN (LSB)
–1
–3
–4
–5
–10
–7
1.4
1.8 2.0
2.8
LTC1285/88 • TPC09
–2
–8
–9
–6
1.2
1.6
2.2 2.4 2.6
T
A
= 25°C
V
CC
= 2.7V
f
CLK
= 120kHz
f
SMPL
= 7.5kHz
Change in Gain
vs Reference Voltage
Change in Offset
vs Reference Voltage
TEMPERATURE (°C)
–55
43
REFERENCE CURRENT (mA)
44
46
47
48
53
50
–15
25 45
125
LTC1285/88 • TPC05
45
51
52
49
–35
5
65 85 5 105
V
CC
= 2.7V
V
REF
= 2.5V
f
CLK
= 120kHz
f
SMPL
= 7.5kHz
Reference Current vs Temperature
Change in Linearity
vs Reference Voltage
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
LTC1285/88 • TPC08
0.10
0.40
0.45
0.30
1.2
1.6
2.2 2.4 4 2.6
T
A
= 25°C
V
CC
= 2.7V
f
CLK
= 120kHz
f
SMPL
= 7.5kHz
REFERENCE VOLTAGE (V)
0.5
0
CHANGE IN OFFSET (LSB = 1/4096 · VREF)
0.5
1.0
1.5
2.0
2.5
3.0
1.0
1.5
2.0
2.5
LTC1285/88 • TPC06
3.0
T
A
= 25°C
V
CC
= 2.7V
f
CLK
= 120kHz
f
SMPL
= 7.5kHz
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
LTC1285/88 • TPC12
1
4
6
9
12
11
8
62
56
74
68
50
2
T
A
= 25°C
V
CC
= 2.7V
f
CLK
= 120kHz
Differential Nonlinearity vs Code
CODE
0
–1
DIFFERENTIAL NONLINEARITY ERROR (LSB)
–0.5
0
0.5
1
512 10241536 6 2048
LTC1285/88 • TPC11
2560307235844096
T
A
= 25°C
V
CC
= 2.7V
V
REF
= 2.5V
f
CLK
= 120kHz
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6
LTC1285/LTC1288
TYPICAL PERFORMANCE CHARACTERISTICS  
WU
INPUT FREQUENCY (Hz)
80
ATTENUATION (%)
60
40
50
20
0
90
70
30
10
1k
100k
1M
10M
LTC1285/86 • TPC15
100
10k
T
A
= 25°C
V
CC
= 2.7V
V
REF
= 2.5V
f
SMPL
= f
SMPL(MAX)
Attenuation vs Input Frequency
Power Supply Feedthrough
vs Ripple Frequency
Intermodulation Distortion
Maximum Clock Frequency
vs Supply Voltage
FREQUENCY (kHz)
0
–120
MAGNITUDE (dB)
–100
–80
–60
–40
1.0
2.0
3.0
4.0
LTC1285/88 • TPC17
–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
RIPPLE FREQUENCY (Hz)
–80
FEEDTHROUGH (dB)
–60
–40
–50
–20
0
–90
–70
–30
–10
1k
100k
1M
10M
LTC1285/86 • TPC18
–100
10k
T
A
= 25°C
V
CC
= 2.7V (V
RIPPLE
= 1mV)
V
REF
= 2.5V
f
CLK
= 120kHZ
SUPPLY VOLTAGE (V)
2.5
100
CLOCK FREQUENCY (kHz)
120
160
180
200
300
240
3.5
4.5
5.0
LTC1285/88 • TPC21
140
260
280
220
3.0
4.0
5.5
6.0
T
A
= 25°C
V
REF
= 2.5V
4096 Point FFT Plot
FREQUENCY (kHz)
0
–120
MAGNITUDE (dB)
–100
–80
–60
–40
1.0
2.0
3.0
4.0
LTC1285/88 • TPC16
–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
Spurious-Free Dynamic Range
vs Input Frequency
INPUT FREQUENCY (kHz)
1
0
SPURIOUS-FREE DYNAMIC RANGE (dB)
20
40
60
80
10
100
LTC1285/88 • G13
100
10
30
50
70
90
T
A
= 25°C
V
CC
= 2.7V
V
REF
= 2.5V
f
SMPL
= f
SMPL(MAX)
Maximum Clock Frequency
vs Source Resistance
SOURCE RESISTANCE (k
W
)
0.1
0
CLOCK FREQUENCY (kHz)
40
80
120
160
1
10
LTC1285/88 • G19
200
20
60
100
140
180
T
A
= 25°C
V
CC
= 2.7V
V
REF
= 2.5V
–INPUT
+INPUT
V
IN
R
SOURCE
S/(N + D) vs Input Level
INPUT LEVEL (dB)
–45
SIGNAL-TO-NOISE PLUS DISTORTION (dB)
40
50
60
–5
LTC1285/88 • TPC14
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)
Sample-and-Hold Acquisition
Time vs Source Resistance
SOURCE RESISTANCE (
W
)
 1
100
S & H ACQUISITION TIME (ns)
1000
10000
100
1000
10
10000
LTC1285/88 • TPC20
T
A
= 25°C
V
CC
= 2.7V
V
REF
= 2.5V
–INPUT
+INPUT
V
IN
R
SOURCE
+
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LTC1285/LTC1288
LTC1285
V
REF
(Pin 1): Reference Input. The reference input defines
the span of the A/D converter.
IN
+
(Pin 2): Positive Analog Input.
IN
(Pin 3): Negative Analog Input.
GND (Pin 4): Analog Ground. GND should be tied directly
to an analog ground plane.
CS/SHDN (Pin 5): Chip Select Input. A logic low on this
input enables the LTC1285. A logic high on this input
disables and powers down the LTC1285.
D
OUT
(Pin 6): Digital Data Output. The A/D conversion
result is shifted out of this output.
CLK (Pin 7): Shift Clock. This clock synchronizes the serial
data transfer and determines conversion speed.
V
CC
(Pin 8): Power Supply Voltage. This pin provides
power to the A/D converter. It must be kept free of noise
and ripple by bypassing directly to the analog ground
plane.
TYPICAL PERFORMANCE CHARACTERISTICS  
WU
Minimum Clock Frequency
for 0.1 LSB Error vs Temperature
TEMPERATURE (°C)
0
CLOCK FREQUENCY (kHz)
80
100
120
40
LTC1285/88 • TPC22
60
40
0
10
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
LTC1285/88 • TPC24
1
0.1
0.01
–15
25
65
–35
125
5
45
85
V
CC
= 2.7V
V
REF
= 2.5V
ON CHANNEL
OFF CHANNEL
Input Channel Leakage Current
vs Temperature
Digital Input Logic Threshold
vs Supply Voltage
SUPPLY VOLTAGE (V)
2.5
DIGITAL INPUT LOGIC THRESHOLD VOLTAGE (V)
2.0
2.5
3.0
4.0
5.0
LTC1285/88 • TPC23
1.5
1.0
3.0
3.5
4.5
5.5
6.0
0.5
0
T
A
= 25°C
PIN FUNCTIONS
U
U
U
LTC1288
CS/SHDN (Pin 1): Chip Select Input. A logic low on this
input enables the LTC1288. A logic high on this input
disables and powers down the LTC1288.
CH0 (Pin 2): Analog Input.
CH1 (Pin 3): Analog Input.
GND (Pin 4): Analog Ground. GND should be tied directly
to an analog ground plane.
D
IN
(Pin 5): Digital Data Input. The multiplexer address is
shifted into this input.
D
OUT
(Pin 6): Digital Data Output. The A/D conversion
result is shifted out of this output.
CLK (Pin 7): Shift Clock. This clock synchronizes the
serial data transfer and determines conversion speed.
V
CC
/V
REF
(Pin 8): Power Supply and Reference Voltage.
This pin provides power and defines the span of the A/D
converter. It must be kept free of noise and ripple by
bypassing directly to the analog ground plane.
10
LTC1285/LTC1288
APPLICATION INFORMATION
W
U
U
U
OVERVIEW
The LTC1285 and LTC1288 are 3V micropower, 12-bit,
successive approximation sampling A/D converters. The
LTC1285 typically draws 160
m
A of supply current when
sampling at 7.5kHz while the LTC1288 nominally con-
sumes 210
m
A of supply current when sampling at 6.6 kHz.
The extra 50
m
A of supply current on the LTC1288 comes
from the reference input which is intentionally tied to the
supply. Supply current drops linearly as the sample rate is
reduced (see Supply Current vs Sample Rate). The ADCs
automatically power down when not performing conver-
sions, drawing only leakage current. They are packaged in
8-pin SO and DIP packages. The LTC1285 and LTC1288
operate on a single supply from 2.7V to 6V.
Both the LTC1285 and the LTC1288 contain a 12-bit,
switched-capacitor ADC, a sample-and-hold, and a serial
port (see Block Diagram). Although they share the same
basic design, the LTC1285 and LTC1288 differ in some
respects. The LTC1285 has a differential input and has an
external reference input pin. It can measure signals float-
ing on a DC common-mode voltage and can operate with
reduced spans to 1.5V. Reducing the spans allows it to
achieve 366
m
V resolution. The LTC1288 has a two-chan-
nel input multiplexer and can convert either channel with
respect to ground or the difference between the two. The
reference input is tied to the supply pin.
SERIAL INTERFACE
The 2-channel LTC1288 communicates with micropro-
cessors and other external circuitry via a synchronous,
half duplex, 4-wire serial interface. The single channel
LTC1285 uses a 3-wire interface (see Operating Sequence
in Figures 1 and 2).
Figure 1. LTC1285 Operating Sequence
CLK
CS
t
CYC
B11
B5
B6
B7
B8
B9
B10
B11
HI-Z
D
OUT
t
CONV
t
DATA
HI-Z
t
suCS
NULL 
BIT
B4 B3
B2
B1
POWER
DOWN
POWER DOWN
B0*
NULL 
BIT
B10 B9
B8
t
SMPL
(MSB)
(MSB)
CLK
CS
t
CYC
B11*
B5
B6
B7
B8
B9
B10
B11
HI-Z
D
OUT
t
CONV
t
DATA
HI-Z
t
suCS
NULL
BIT
LTC1285/88 • F01
B4
B3
B3
B4 B5
B6
B7
B2
B2
B1
B0 B1
B10
B9
B8
t
SMPL
*AFTER COMPLETING THE DATA TRANSFER, IF FURTHER CLOCKS ARE APPLIED WITH CS LOW, 
THE ADC WILL OUTPUT ZEROS INDEFINITELY.
*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.
t
DATA
: DURING THIS TIME, THE BIAS CIRCUIT AND THE COMPARATOR POWER DOWN AND THE REFERENCE INPUT
             BECOMES A HIGH IMPEDANCE NODE, LEAVING THE CLK RUNNING TO CLOCK OUT LSB-FIRST DATA OR ZEROES.
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