LTC1287
3V Single Chip 12-Bit
Data Acquisition System
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FEATURES
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DESCRIPTIO
Single Supply 3.3V Operation
Built-In Sample-and-Hold
Direct 3-Wire Interface to Most MPU Serial Ports and
All MPU Parallel Ports
30kHz Maximum Throughput Rate
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KEY SPECIFICATIO S
■
■
■
■
The serial I/O is designed to communicate without external
hardware to most MPU serial ports and all MPU parallel
I/O ports allowing data to be transmitted and received over
three wires. The low voltage operating capability and the
low power consumption of this device make it ideally
suited for battery applications. Given the ease of use, small
package size and the minimum number of interconnects
for I/O, the LTC1287 can be used for remote sensing
applications.
Minimum Guaranteed Supply Voltage: 2.7V
Resolution: 12 Bits
Fast Conversion Time: 24µs Max Over Temp.
Low Supply Current: 1.0mA
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APPLICATIO S
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■
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The LTC®1287 is a 3V data acquisition component which
contains a serial I/O successive approximation A/D converter. The device specifications are guaranteed at a
supply voltage of 2.7V. It uses LTCMOSTM switched capacitor technology to perform a 12-bit unipolar, A/D
conversion. The differential input has an on-chip sampleand-hold on the (+) input.
Battery-Powered Instruments
Data Logger
Data Acquisition Modules
, LTC and LT are registered trademarks of Linear Technology Corporation.
LTCMOS is a trademark of Linear Technology Corporation
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TYPICAL APPLICATIO
3V Differential Input Data Acquisition System
22µF TANTALUM
+
INL with VREF = 1.2V
+
3V
LITHIUM
10k
CLK
+IN
0.5
LTC1287
–
–IN
DOUT
GND
VREF
22µF
TANTALUM
+
LT1004-1.2
ERROR (LSB)
DIFFERENTIAL INPUTS
COMMON MODE RANGE
0V TO VCC*
1.0
VCC
CS
0
–0.5
TO AND FROM MPU
* FOR OVERVOLTAGE PROTECTION, LIMIT THE INPUT CURRENT TO 15mA
PER PIN OR CLAMP THE INPUTS TO VCC AND GND WITH 1N4148 DIODES.
CONVERSION RESULTS ARE NOT VALID WHEN THE SELECTED CHANNEL OR
OTHER CHANNEL IS OVERVOLTAGED (VIN < GND OR VIN > VCC). SEE SECTION
ON OVERVOLTAGE PROTECTION IN THE APPLICATIONS INFORMATION.
–1.0
0
512
1024
1536
2048
2560
3072
3584
4096
CODE
LTC1287 TA02
1287 TA01
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LTC1287
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RATI GS
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(Notes 1 and 2)
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ABSOLUTE
PACKAGE/ORDER I FOR ATIO
ORDER PART
NUMBER
TOP VIEW
Supply Voltage (VCC) to GND .................................. 12V
Voltage
Analog and Reference Inputs .... –0.3V to VCC + 0.3V
Digital Inputs ........................................ –0.3V to 12V
Digital Outputs .......................... –0.3V to VCC + 0.3V
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
CS 1
8 VCC
+IN 2
7 CLK
–IN 3
6 DOUT
GND 4
5 VREF
LTC1287BCN8
LTC1287CCN8
N8 PACKAGE
8-LEAD PLASTIC DIP
TJMAX = 100°C, θJA = 130°C/W (N)
J8 PACKAGE
8-LEAD CERAMIC DIP
TJMAX = 150°C, θJA = 100°C/W (J)
LTC1287BCJ8
LTC1287CCJ8
OBSOLETE PACKAGE
Consider N8 Package for Alternate Source
Consult LTC Marketing for parts specified with wider operating temperature ranges.
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CO VERTER A D ULTIPLEXER CHARACTERISTICS
The ● denotes the specifications
which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. (Note 3)
LTC1287B
PARAMETER
CONDITIONS
Offset Error
VCC = 2.7V (Note 4)
●
Linearity Error (INL)
VCC = 2.7V (Notes 4 & 5)
Gain Error
VCC = 2.7V (Note 4)
Minimum Resolution for Which No
Missing Codes are Guaranteed
MIN
TYP MAX
LTC1287C
MIN
TYP MAX
UNITS
±3.0
±3.0
LSB
●
±0.5
± 0.5
LSB
●
±0.5
±1.0
LSB
●
12
12
Bits
– 0.05V to VCC + 0.05V
Analog and REF Input Range
(Note 7)
On Channel Leakage Current (Note 8)
On Channel = 3V
Off Channel = 0V
●
±1
±1
µA
On Channel = 0V
Off Channel = 3V
●
±1
±1
µA
On Channel = 3V
Off Channel = 0V
●
±1
±1
µA
On Channel = 0V
Off Channel = 3V
●
±1
±1
µA
Off Channel Leakage Current (Note 8)
V
AC CHARACTERISTICS
The ● denotes the specifications which apply over the full operating temperature range,
otherwise specifications are at TA = 25°C. (Note 3)
LTC1287B/LTC1287C
MIN
TYP
MAX
SYMBOL
PARAMETER
CONDITIONS
fCLK
Clock Frequency
(Note 6)
tSMPL
Analog Input Sample Time
See Operating Sequence
1.5
CLK Cycles
tCONV
Conversion Time
See Operating Sequence
12
CLK Cycles
tCYC
Total Cycle Time
See Operating Sequence (Note 6)
tdDO
Delay Time, CLK↓ to DOUT Data Valid
See Test Circuits
●
tdis
Delay Time, CS↑ to DOUT Hi-Z
See Test Circuits
●
80
160
ns
ten
Delay Time, CLK↓ to DOUT Enabled
See Test Circuits
●
130
250
ns
(Note 9)
0.5
14 CLK+5.0µs
UNITS
MHz
Cycles
250
450
ns
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LTC1287
AC CHARACTERISTICS
The ● denotes the specifications which apply over the full operating temperature range,
otherwise specifications are at TA = 25°C. (Note 3)
LTC1287B/LTC1287C
MIN
TYP
MAX
SYMBOL
PARAMETER
CONDITIONS
UNITS
thDO
Time Output Data Remains Valid After CLK↓
tf
DOUT Fall Time
See Test Circuits
●
40
100
ns
tr
DOUT Rise Time
See Test Circuits
●
40
100
ns
tWHCLK
CLK High Time
VCC = 3V (Note 6)
600
ns
tWLCLK
CLK Low Time
VCC = 3V (Note 6)
800
ns
tsuCS
Setup Time, CS↓ Before CLK↑
VCC = 3V (Note 6)
100
ns
tWHCS
CS High Time Between Data Transfer Cycles
VCC = 3V (Note 6)
5.0
µs
tWLCS
CS Low Time During Data Transfer
VCC = 3V (Note 6)
14
CLK Cycles
CIN
Input Capacitance
Analog Inputs On Channel
Analog Inputs Off Channel
Digital Inputs
50
ns
100
5
5
pF
pF
pF
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DIGITAL A D DC ELECTRICAL CHARACTERISTICS
The ● denotes the specifications which
apply over the full operating temperature range, otherwise specifications are at TA = 25°C. (Note 3)
LTC1287B/LTC1287C
MIN
TYP
MAX
SYMBOL
PARAMETER
CONDITIONS
VIH
High Level Input Voltage
VCC = 3.6V
●
VIL
Low Level Input Voltage
VCC = 3.0V
●
0.45
V
IIH
High Level Input Current
VIN = VCC
●
2.5
µA
IIL
Low Level Input Current
VIN = 0V
●
–2.5
µA
VOH
High Level Output Voltage
VCC = 3.0V, IO = 20µA
IO = 400µA
●
VCC = 3.0V, IO = 20µA
IO = 400µA
●
●
●
VOL
Low Level Output Voltage
2.1
2.7
UNITS
V
2.90
2.85
0.05
0.10
V
V
0.3
V
V
3
–3
µA
µA
IOZ
High Z Output Leakage
VOUT = VCC, CS High
VOUT = 0V, CS High
ISOURCE
Output Source Current
VOUT = 0V
–10
mA
ISINK
Output Sink Current
VOUT = VCC
9
mA
ICC
Positive Supply Current
CS High
●
1.5
5
mA
IREF
Reference Current
VREF = 2.5V
●
10
50
µA
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 ground (unless otherwise
noted).
Note 3: VCC = 3V, VREF = 2.5V, CLK = 500kHz unless otherwise specified.
Note 4: One LSB is equal to VREF divided by 4096. For example, when VREF
= 2.5V, 1LSB = 2.5V/4096 = 0.61mV.
Note 5: Integral nonlinearity error 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 6: Recommended operating conditions.
Note 7: Two on-chip diodes are tied to each analog input which will
conduct for analog voltages one diode drop below GND or one diode drop
above VCC. Be careful during testing at low VCC levels, as high level analog
inputs can cause this input diode to conduct, especially at elevated
temperature, and cause errors for inputs near full scale. This spec allows
50mV forward bias of either diode. This means that as long as the analog
input does not exceed the supply voltage by more than 50mV, the output
code will be correct.
Note 8: Channel leakage current is measured after the channel selection.
Note 9: Increased leakage currents at elevated temperatures cause the
S/H to droop, therefore it is recommended that fCLK ≥ 30kHz at 85°C and
fCLK ≥ 3kHz at 25°C.
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LTC1287
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TYPICAL PERFOR A CE CHARACTERISTICS
2.8
0.9
1.9
CLK = 500kHz
TA = 25°C
CLK = 500kHz
VCC = 3V
1.8
SUPPLY CURRENT (mA)
2.4
2.2
2.0
1.8
1.6
1.4
1.7
1.6
1.5
1.2
1.4
0.8
1.3
–40 –25 –10
2.7 2.8 2.9 3.0 3.1 3.2 3.3 3.4 3.5 3.6
SUPPLY VOLTAGE (V)
0.4
0.3
VOS = 0.125mV
0.2
80
0
95
MAGNITUDE OF OFFSET CHANGE (LSB)
VCC = 3V
–0.2
0.3
–0.3
0.2
–0.4
0.1
–0.5
0.5
2.5
1.0
1.5
2.0
REFERENCE VOLTAGE (V)
3.0
0
0.5
2.5
1.0
1.5
2.0
REFERENCE VOLTAGE (V)
0.4
0.3
0.2
0.1
Change in Linearity vs
Temperature
Change in Gain vs Temperature
0.5
DOUT Delay Time vs Temperature
0.5
MAGNITUDE OF GAIN CHANGE (LSB)
0.3
0.2
0.1
0
40
20
0
60
–40 –20
80
AMBIENT TEMPERATURE (°C)
100
LTC1287 G7
0.4
100
LTC1287 G6
LTC1287 G5
LTC1287 G4
VCC = 3V
VREF = 2.5V
CLK = 500kHz
VCC = 3V
VREF = 2.5V
CLK = 500kHz
0
40
20
0
60
–40 – 20
80
AMBIENT TEMPERATURE (°C)
3.0
350
VCC = 3V
VCC = 3V
VREF = 2.5V
CLK = 500kHz
DOUT DELAY TIME FROM CLK↓ (ns)
0
3.0
Change in Offset vs Temperature
–0.1
0.4
2.5
1.0
1.5
2.0
REFERENCE VOLTAGE (V)
0.5
0
0.5
VCC = 3V
0.5
LTC1287 G3
Change in Gain vs Reference
Voltage
CHANGE IN GAIN (LSB = 1/4096 × VREF)
CHANGE IN LINEARITY (LSB = 1/4096 × VREF)
0.5
LTC1287 G2
Change in Linearity vs Reference
Voltage
MAGNITUDE OF LINEARITY CHANGE (LSB)
VOS = 0.250mV
0
5 20 35 50 65
TEMPERATURE (°C)
LTC1287 G1
0.4
0.7
0.6
0.1
1.0
0
VCC = 3V
0.8
OFFSET (LSB = 1/4096 × VREF)
2.6
SUPPLY CURRENT (mA)
Unadjusted Offset Voltage vs
Reference Voltage
Supply Current vs Temperature
Supply Current vs Supply Voltage
0.3
0.2
0.1
0
40
20
0
60
–40 –20
80
AMBIENT TEMPERATURE (°C)
100
LTC1287 G8
300
MSB-FIRST DATA
250
200
LSB-FIRST DATA
150
100
50
0
–40 –20
40
80
20
60
0
AMBIENT TEMPERATURE (°C)
100
LTC1287 G9
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LTC1287
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TYPICAL PERFOR A CE CHARACTERISTICS
Maximum Clock Rate vs Source
Resistance
Maximum Filter Resistor vs Cycle
Time
Minimum Clock Rate for 0.1LSB
Error**
10k
VCC = 3V
VREF = 2.5V
CLK = 500kHz
300
200
+VIN
RSOURCE–
100
0
100
+IN
–IN
1k
10k
RSOURCE – (Ω)
VIN
0.25
MAXIMUM RFILTER*** (Ω)
400
VCC = 3V
MINIMUM CLK FREQUENCY (MHz)
0.20
0.15
0.10
–50
100k
75
0
25
50
–25
AMBIENT TEMPERATURE (°C)
INPUT CHANNEL LEAKAGE CURRENT (nA)
S & H ACQUISITION TIME TO 0.02% (µs)
VIN
RSOURCE+
+
–
1
100
1k
100
10
1
100
10
10k
RSOURCE+ (Ω)
1000
100
CYCLE TIME (µs)
900
Noise Error vs Reference Voltage
1.0
GUARANTEED
800
700
600
500
400
300
200
100
ON CHANNEL
OFF CHANNEL
0
–50 –30 –10 10 30 50 70 90 110 130
AMBIENT TEMPERATURE (°C)
LTC1287 G13
* MAXIMUM CLK FREQUENCY REPRESENTS THE CLK FREQUENCY AT WHICH A 0.1LSB
SHIFT IN THE ERROR AT ANY CODE TRANSITION FROM ITS 500kHz VALUE IS FIRST DETECTED.
10000
LTC1287 G12
1000
VREF = 2.5V
VCC = 3V
TA = 25°C
0V TO 2.5V INPUT STEP
+
–
Input Channel Leakage Current vs
Temperature
Sample-and-Hold Acquisition
Time vs Source Resistance
10
CFILTER ≥1µF
1k
LTC1287 G11
LTC G10
100
RFILTER
0.05
LTC1287 G14
PEAK-TO-PEAK NOISE ERROR (LSB)
MAXIMUM CLK FREQUENCY* (MHz)
500
LTC1287 NOISE = 200µVP-P
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
0
0.5
1.5
2.0
2.5
1.0
REFERENCE VOLTAGE (V)
3.0
LTC1287 G15
*** MAXIMUM RFILTER REPRESENTS THE FILTER RESISTOR VALUE AT WHICH A 0.1LSB
CHANGE IN FULL SCALE ERROR FROM ITS VALUE AT RFILTER = 0Ω IS FIRST DETECTED.
** AS THE CLK FREQUENCY IS DECREASED FROM 1MHz, MINIMUM CLK FREQUENCY
(∆ERROR ≤ 0.1LSB) REPRESENTS THE FREQUENCY AT WHICH A 0.1LSB SHIFT IN ANY
CODE TRANSITION FROM ITS 500kHz VALUE IS FIRST DETECTED.
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CS (Pin 1): Chip Select Input. A logic low on this input
enables the LTC1287.
DOUT (Pin 6): Digital Data Output. The A/D conversion
result is shifted out of this output.
+IN, –IN (Pin 2,3): Analog Inputs. These inputs must be
free of noise with respect to GND.
CLK (Pin 7): Shift Clock. This clock synchronizes the serial
data transfer.
GND (Pin 4): Analog Ground GND should be tied directly
to an analog ground plane.
VCC (Pin 8): Positive Supply. This supply must be kept free
of noise and ripple by bypassing directly to the analog
ground plane.
VREF (Pin 5): Reference Input. The reference input defines
the span of the A/D converter and must be kept free of
noise with respect to GND.
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LTC1287
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BLOCK DIAGRA
VCC
8
7
INPUT
SHIFT
REGISTER
+IN
–IN
OUTPUT
SHIFT
REGISTER
2
ANALOG
INPUT MUX
3
SAMPLE
AND
HOLD
6
CLK
DOUT
COMP
12-BIT
SAR
12-BIT
CAPACITIVE
DAC
4
5
GND
VREF
CONTROL
AND
TIMING
1
CS
LTC1287 BD
TEST CIRCUITS
Voltage Waveforms for DOUT Delay Time, tdDO
On and Off Channel Leakage Current
3V
CLK
ION
0.45V
A
ON CHANNEL
tdDO
IOFF
2.1V
A
DOUT
OFF CHANNEL
0.6V
LTC1287 TC03
Voltage Waveforms for DOUT Rise and Fall Times, tr, tf
POLARITY
LTC1287 TC1
Load Circuit for tdis and ten
2.1V
DOUT
0.6V
TEST POINT
tr
tf
LTC1287 TC04
3V tdis WAVEFORM 2, ten
3k
DOUT
Voltage Waveforms for tdis
tdis WAVEFORM 1
100pF
Load Circuit for tdDO, tr and tf
1.5V
2.1V
CS
LTC1287 TC05
DOUT
WAVEFORM 1
(SEE NOTE 1)
90%
tdis
3k
DOUT
TEST POINT
DOUT
WAVEFORM 2
(SEE NOTE 2)
100pF
LTC1287 TC06
LTC1287 TC02
6
10%
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.
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LTC1287
TEST CIRCUITS
Voltage Waveforms for ten
CS
CLK
B11
DOUT
0.6V
ten
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APPLICATI
LTC1287 TC07
S I FOR ATIO
does not require a configuration input word and has no DIN
pin. It is permanently configured to have a single differential input and to operate in unipolar mode. A falling CS
initiates data transfer. The first CLK pulse enables DOUT.
After one null bit, the A/D conversion result is output on the
DOUT line with a MSB-first sequence followed by a LSBfirst sequence. With the half duplex serial interface the
DOUT data is from the current conversion. This provides
easy interface to MSB- or LSB-first serial ports. Bringing
CS high resets the LTC1287 for the next data exchange.
The LTC1287 is a data acquisition component which
contains the following functional blocks:
1. 12-bit successive approximation capacitive A/D
converter
2. Analog multiplexer (MUX)
3. Sample-and-hold (S/H)
4. Synchronous, half-duplex serial interface
5. Control and timing logic
DIGITAL CONSIDERATIONS
Serial Interface
The LTC1287 communicates with microprocessors and
other external circuitry via a synchronous, half-duplex,
three-wire serial interface (see Operating Sequence). The
clock (CLK) synchronizes the data transfer with each bit
being transmitted on the falling CLK edge. The LTC1287
Logic Levels
The logic level standards for this supply range have not
been well defined. What standards that do exist are not
universally accepted. The trip point on the logic inputs of
the LTC1287 is 0.28 × VCC. This makes the logic inputs
compatible with HC-type levels and processors that are
tCYC
CS
CLK
Hi-Z
DOUT
tSMPL
B11 B10
B9
B8
B7
B6
B5
B4
B3
B2
B1
B0
B1
B2
B3
B4
B5
B6
B7
B8
B9
B10
B11
tSMPL
tCONV
LTC1287 F01
Figure 1. LTC1287 Operating Sequence
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LTC1287
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APPLICATI
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specified at 3.3V. The output DOUT is also compatible with
the above standards. The following summarizes such
levels.
VCC – 0.1V
VOH (no load)
VOL (no load)
0.1V
VOH
0.9 × VCC
VOL
0.1 × VCC
VIH
0.7 × VCC
VIL
0.2 × VCC
The LTC1287 can be driven with 5V logic even when VCC
is at 3.3V. This is due to a unique input protection device
that is found on the LTC1287.
Microprocessor Interfaces
The LTC1287 can interface directly (without external hardware) to most popular microprocessor (MPU) synchronous serial formats. If an MPU without a serial interface is
used, then three of the MPU’s parallel port lines can be
programmed to form the serial link to the LTC1287. Many
of the popular MPUs can operate with 3V supplies. For
example the MC68HC11 is an MPU with a serial format
(SPI). Likewise parallel MPUs that have the 8051 type
architecture are also capable of operating at this voltage
range. The code for these processors remains the same
and can be found in the LTC1292 data sheet.
Sharing the Serial Interface
The LTC1287 can share the same two-wire serial interface
with other peripheral components or other LTC1287s
(Figure 2). In this case, the CS signals decide which
LTC1287 is being addressed by the MPU.
ANALOG CONSIDERATIONS
Grounding
The LTC1287 should be used with an analog ground plane
and single point grounding techniques. Do not use wire
wrapping techniques to breadboard and evaluate the device.
To achieve the optimum performance use a PC board. The
ground pin (Pin 4) should be tied directly to the ground
plane with minimum lead length (a low profile socket is
fine). Pin 7 (VCC) should be bypassed to the ground plane
with a 22µF (minimum value) tantalum with leads as short
as possible and as close as possible to the pin. A 0.1µF
ceramic disk also should be placed in parallel with the
22µF and again with leads as short as possible and as close
to VCC as possible. Figure 3 shows an example of an ideal
LTC1287 ground plane design for a two-sided board. Of
course this much ground plane will not always be possible,
but users should strive to get as close to this ideal as
possible.
Bypassing
For good performance, VCC must be free of noise and
ripple. Any changes in the VCC voltage with respect to
ground during a conversion cycle can induce errors or
noise in the output code. VCC noise and ripple can be kept
below 0.5mV by bypassing the VCC pin directly to the
analog plane with a minimum of 22µF tantalum capacitor
and with leads as short as possible. The lead from the
device to the VCC supply also should be kept to a minimum
and the VCC supply should have a low output impedance
VCC
0.1µF
22µF TANTALUM
2
1
0
OUTPUT PORT
SERIAL DATA
MPU
2-WIRE SERIAL
INTERFACE TO OTHER
PERIPHERALS OR LTC1287s
2
2
2
CS
LTC1287
2
CS
LTC1287
CS
LTC1287
1
8
2
7
LTC1287
3
6
4
5
LTC1287 F03
2 CHANNELS
2 CHANNELS
2 CHANNELS
LTC1287 F02
Figure 2. Several LTC1287s Sharing One 2-Wire Serial Interface
Figure 3. Example Ground Plane for the LTC1287
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LTC1287
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such as obtained from a voltage regulator (e.g., LT1117).
For high frequency bypassing a 0.1µF ceramic disk placed
in parallel with the 22µF is recommended. Again the leads
should be kept to a minimum. Using a battery to power the
LTC1287 will help reduce the amount of bypass capacitance
required on the VCC pin. A battery placed close to the
device will only require 10µF to adequately bypass the
supply pin. Figure 4 shows the effect of poor VCC bypassing.
Figure 5 shows the settling of a LT1117 low dropout
regulator with a 22µF bypass capacitor. The noise and
ripple is kept around 0.5mV. Figure 6 shows the response
of a lithium battery with a 10µF bypass capacitor. The
noise and ripple is kept below 0.5mV.
Analog Inputs
Because of the capacitive redistribution A/D conversion
techniques used, the analog inputs of the LTC1287 have
CS
5V/DIV
VCC
0.5mV/DIV
HORIZONTAL: 20µs/DIV
Figure 6. Lithium Battery with 10µF Bypassing on VCC
capacitive switching input current spikes. These current
spikes settle quickly and do not cause a problem. If large
source resistances are used or if slow settling op amps
drive the inputs, take care to insure the transients caused
by the current spikes settle completely before the
conversion begins.
VERTICAL: 0.5mV/DIV
Source Resistance
The analog inputs of the LTC1287 look like a 100pF
capacitor (CIN) in series with a 1.5k resistor (RON). This
value for RON is for VCC = 2.7V. With larger supply voltages
RON will be reduced. For example, with VCC = 2.7V and V –
= – 2.7V, RON becomes 500Ω. CIN gets switched between
(+) and (–) 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 constant is short enough to allow the analog inputs
to settle completely within the allowed time.
HORIZONTAL: 10µs/DIV
Figure 4. Poor VCC Bypassing. Noise and
Ripple Can Cause A/D Errors
RSOURCE +
“+”
INPUT
LTC1287
VIN +
C1
CS
5V/DIV
RSOURCE –
“–”
INPUT
VIN –
VCC
0.5mV/DIV
CS↑
RON = 1.5k
tWHCS +
1/2 CLK
CIN =
100pF
C2
LTC1287 F07
Figure 7. Analog Input Equivalent Circuit
HORIZONTAL: 20µs/DIV
Figure 5. LT1117 Regulator with 22µF Bypassing on VCC
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“+” Input Settling
The input capacitor is switched onto the “+” input during
the sample phase (tSMPL, see Figures 8a, 8b and 8c). The
sample period can be as short as tWHCS + 0.5 CLK cycle or
as long as tWHCS + 1.5 CLK cycles before a conversion
starts. This variability depends on where CS falls relative
to CLK. The voltage on the “+” input must settle completely
within the sample period. Minimizing RSOURCE+ and C1
will improve the settling time. If large “+” input source
resistance must be used, the sample time can be increased
by using a slower CLK frequency. With the minimum
possible sample time of 6.0µs, RSOURCE+ < 4.0k and C1
< 20pF will provide adequate settle time.
“–” Input Settling
At the end of the sample phase the input capacitor switches
to the “–” input and the conversion starts (see Figures 8a,
8b and 8c). During the conversion, the “+” input voltage is
“+” and “–” Input Settling Windows
tWHCS
CS
tSUCS
CLK
tSMPL
(+) INPUT MUST SETTLE DURING THIS TIME
DOUT
B11
HI-Z
B10
B9
1ST BIT TEST (–) INPUT MUST
SETTLE DURING THIS TIME
(+) INPUT
(–) INPUT
LTC1287 F8a
Figure 8a. Setup Time (tSUCS) is Met
tWHCS
CS
CLK
tSMPL
(+) INPUT MUST SETTLE DURING THIS TIME
DOUT
B11
B10
B9
HI-Z
1ST BIT TEST (–) INPUT MUST
SETTLE DURING THIS TIME
(+) INPUT
(–) INPUT
Figure 8b. Setup Time (tSUCS) is Met
LTC1287 F8b
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tWHCS
CS
CLK
tSMPL
(+) INPUT MUST SETTLE DURING THIS TIME
B11
DOUT
B10
HI-Z
1ST BIT TEST (–) INPUT MUST
SETTLE DURING THIS TIME
(+) INPUT
(–) INPUT
LTC1287 F8c
Figure 8c. Setup Time (tSUCS) is Not Met
effectively “held” by the sample and hold and will not affect
the conversion result. It is critical that the “–” input voltage
be free of noise and settle completely during the first CLK
cycle of the conversion. Minimizing RSOURCE – and C2 will
improve settling time. If large “–” input source resistance
must be used the time can be extended by using a slower
CLK frequency. At the maximum CLK frequency of 500kHz,
RSOURCE – < 200Ω and C2 < 20pF will provide adequate
settling.
VERTICAL: 5mV/DIV
VERTICAL: 5mV/DIV
Input Op Amps
When driving the analog inputs with an op amp it is
important that the op amp settles within the allowed time
(see Figures 8a, 8b and 8c). Again the “+” and “–” input
sampling times can be extended as described above to
accommodate slower op amps. For single supply low
voltage application the LT1797 and LT1677 can be made
to settle well even with the minimum settling windows of
6µs (“+” input) and 2µs (“–” input) which occur at the
maximum clock rates (CLK = 500kHz). Figures 9 and 10
show examples of adequate and poor op amp settling. The
LT1077, LT1078 or LT1079 can be used here to reduce
power consumption. Placing an RC network at the output
of the op amps will inprove the settling response and also
reduce the broadband noise.
HORIZONTAL: 500ns/DIV
Figure 9. Adequate Settling of Op Amp Driving Analog Input
HORIZONTAL: 20µs/DIV
Figure 10. Poor Op Amp Settling Can Cause A/D Errors
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RC Input filtering
It is possible to filter the inputs with an RC network as
shown in Figure 11. For large values of CF (e.g., 1µF) the
capacitive input switching currents are averaged into a net
DC current. A filter should be chosen with a small resistor
and large capacitor to prevent DC drops across the resistor.
The magnitude of the DC current is approximately IDC =
100pF × VIN/tCYC and is roughly proportional to VIN. When
running at the minimum cycle time of 33µs, the input
current equals 7.6µA at VIN = 2.5V. Here a filter resistor of
8Ω will cause 0.1LSB of full-scale error. If a large filter
resistor must be used, errors can be reduced by increasing
the cycle time as shown in the Typical Performance
Characteristics curve Maximum Filter Resistor vs Cycle
Time.
RFILTER
IDC
VIN –
“+”
CFILTER
Acquisition Time vs Source Resistance). The input voltage
is sampled during the tSMPL time as shown in Figure 8. The
sampling interval begins at rising edge of CS and continues
until the falling edge of the CLK before the conversion
begins. On this falling edge the S&H goes into the hold
mode and the conversion begins.
Differential Input
With a differential input the A/D no longer converts a single
voltage but converts the difference between two voltages.
The voltage on the +IN pin is sampled and held and can be
rapidly time varying. The voltage on the –IN pin must
remain constant and be free of noise and ripple throughout
the conversion time. Otherwise the differencing operation
will not be done accurately. The conversion time is 12 CLK
cycles. Therefore a change in the –IN input voltage during
this interval can cause conversion errors. For a sinusoidal
voltage on the –IN input this error would be:
LTC1287
“–”
LTC1287 F11
Figure 11. RC Input Filtering
Input Leakage Current
Input leakage currents also can create errors if the source
resistance gets too large. For example, the maximum input
leakage specification of 1µA (at 85°C) flowing through a
source resistance of 1k will cause a voltage drop of 1mV
or 1.6LSB with VREF = 2.5V. This error will be much
reduced at lower temperatures because leakage drops
rapidly (see Typical Performance Characteristics curve
Input Channel Leakage Current vs Temperature).
SAMPLE-AND-HOLD
Single-Ended Input
The LTC1287 provides a built-in sample and hold (S&H)
function on the +IN input for signals acquired in the single
ended mode (–IN pin grounded). The sample and hold
allows the LTC1287 to convert rapidly varying signals (see
Typical Performance Characteristics curve of S&H
Where f(–IN) is the frequency of the –IN input voltage,
VPEAK is its peak amplitude and fCLK is the frequency of the
CLK. Usually VERROR will not be significant. For a 60Hz
signal on the –IN input to generate a 0.25LSB error
(150µV) with the converter running at CLK = 500kHz, its
peak value would have to be 16mV. Rearranging the above
equation, the maximum sinusoidal signal that can be
digitized to a given accuracy is given as:
For 0.25LSB error (150µV) the maximum input sinusoid
with a 2.5V peak amplitude that can be digitized is 0.4Hz.
Reference Input
The voltage on the reference input of the LTC1287
determines the voltage span of the A/D converter. The
reference input has transient capacitive switching currents due to the switched capacitor conversion technique (see Figure 12). During each bit test of the
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conversion (every CLK cycle) a capacitive current spike
will be generated on the reference pin by the A/D. These
current spikes settle quickly and do not cause a problem. If slow settling circuitry is used to drive the
reference input, take care to insure that transients
caused by these current spikes settle completely during
each bit test of the conversion.
REF+
14
ROUT
VREF
LTC1287
EVERY CLK CYCLE
RON
8pF – 40pF
GND
13
LTC 1287 F12
VERTICAL: 0.5mV/DIV
Figure 12. Reference Input Equivalent Circuit
HORIZONTAL: 1µs/DIV
VERTICAL: 0.5mV/DIV
Figure 13. Adequate Reference Settling
HORIZONTAL: 10µs/DIV
Figures 13 and 14 show examples of both adequate and
poor settling. Using a slower CLK will allow more time
for the reference to settle. Even at the maximum CLK
rate of 500kHz most references and op amps can be
made to settle within the 2µs bit time. For example an
LT1790 with a 4.7µF bypass capacitor will settle
adequately.
Reduced Reference Operation
The effective resolution of the LTC1287 can be increased by reducing the input span of the converter.
The LTC1287 exhibits good linearity over a range of
reference voltages (seeTypical Performance Characteristics curves of Change in Linearity vs Reference
Voltage). Care must be taken when operating at low
values of VREF because of the reduced LSB step size and
the resulting higher accuracy requirement placed on
the converter. Offset and Noise are factors that must be
considered when operating at low VREF values.
Offset with Reduced VREF
The offset of the LTC1287 has a larger effect on the
output code when the A/D is operated with a 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 Performance Characteristics curve of Unadjusted Offset Error vs Reference
Voltage shows how offset in LSBs is related to reference
voltage for a typical value of VOS. For example a VOS of
0.1mV, which is 0.2LSB with a 2.5V reference becomes
0.4LSB with a 1.25 reference. If this offset is unacceptable, it can be corrected digitally by the receiving system
or by offsetting the –IN input to the LTC1287.
Noise with Reduced VREF
The total input referred noise of the LTC1287 can be
reduced to approximately 200µV 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 2.5V reference input but will
become a larger fraction of an LSB as the size of the LSB
is reduced. The Typical Performance Characteristics
Figure 14. Poor Reference Settling Can Cause A/D Errors
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curve of Noise Error vs Reference Voltage shows the
LSB contribution of this 200µV of noise.
For operation with a 2.5V reference, the 200µV noise is
only 0.32LSB peak-to-peak. Here the LTC1287 noise
will contribute virtually no uncertainty to the output
code. 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 200µV noise is 0.64LSB peak-to-peak.
This will reduce the range of input voltages over which
a stable output code can be achieved by 0.64LSB. Now
averaging readings may be necessary.
This noise data was taken in a very clean test fixture.
Any setup induced noise (noise or ripple on VCC, VREF
or VIN) will add to the internal noise. The lower the
reference voltage used, the more critical it becomes to
have a noise-free setup.
Overvoltage Protection
Applying signals to the LTC1287’s analog inputs that
exceed the positive supply or that go below ground will
degrade the accuracy of the A/D and possibly damage
the device. For example this condition would occur if a
signal is applied to the analog inputs before power is
applied to the LTC1287. Another example is the input
source operating from different supplies of larger value
than the LTC1287. These conditions should be prevented either with proper supply sequencing or by use
of external circuitry to clamp or current limit the input
source. There are two ways to protect the inputs. In
Figure 15 diode clamps from the inputs to VCC and GND
are used. The second method is to put resistors in
series with the analog inputs for current limiting. Limit
the current to 15mA per channel. The +IN input can
accept a resistor value of 1k but the –IN input cannot
accept more than 200Ω when clocked at its maximum
clock frequency of 500kHz. If the LTC1287 is clocked at
the maximum clock frequency and 200Ω is not enough
to current limit the input source then the clamp diodes
are recommended (Figures 16 and 17). The reason for
the limit on the resistor value is the MSB bit test is
affected by the value of the resistor placed at the –IN
input (see discussion on Analog Inputs and the Typical
Performance Characteristics curve of Maximum CLK
Frequency vs Source Resistance).
If VCC and VREF are not tied together, then VCC should
be turned on first, then VREF. If this sequence cannot be
met, connecting a diode from VREF to VCC is recommended (see Figure 18).
Because a unique input protection structure is used on
the digital input pins, the signal levels on these pins can
exceed the device VCC without damaging the device.
1N4148 DIODES
CS
VCC
+IN
CLK
+3V
LTC1287
–IN
DOUT
GND
VREF
LTC1287 F15
Figure 15. Overvoltage Protection for Inputs
CS
VCC
+IN
CLK
+3V
1k
200Ω
LTC1287
–IN
DOUT
GND
VREF
LTC1287 F16
Figure 16. Overvoltage Protection for Inputs
1N4148 DIODES
CS
VCC
+IN
CLK
+3V
1k
LTC1287
–IN
DOUT
GND
VREF
LTC1287 F17
Figure 17. Overvoltage Protection for Inputs
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CS
VCC
+IN
CLK
1N4148
LTC1287
–IN
DOUT
GND
A “Quick Look” Circuit for the LTC1287
Users can get a quick look at the function and timing of
the LTC1287 by using the following simple circuit
(Figure 19). VREF is tied to VCC. VIN is applied to the +IN
input and the –IN input is tied to the ground plane. CS
is driven at 1/32 the clock rate by the 74HC393 and DOUT
outputs the data. The output data from the DOUT pin can
be viewed on an oscilloscope that is set up to trigger on
the falling edge of CS (Figure 20). Note the LSB data is
partially clocked out before CS goes high.
+3V
+2.5V
VREF
LTC1287 F18
Figure 18
22µF TANTALUM
+
+3V
f/32
VIN
CS
VCC
+IN
CLK
–IN
LTC1287
DOUT
GND
VREF
TO OSCILLOSCOPE
A1
f
VCC
CLR1
0.1µF
A2
1QA
CLR2
1QB
74HC393 2QA
1QC
2QB
1QD
2QC
GND
2QD
CLOCK IN
500kHz
LTC1287 F19
Figure 19. "Quick Look" Circuit for the LTC1287
CLK
CS
DOUT
NULL MSB
LSB LSB DATA
BIT (B11)
(B1)
(B0)
VERTICAL: 5V/DIV
HORIZONTAL: 5µs/DIV
Figure 20. Scope Trace of the LTC1287 “Quick Look” Circuit
Showing A/D Output 1010101010 (AAAHEX)
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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 representation that the
interconnection of circuits as described herein will not infringe on existing patent rights.
15
LTC1287
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PACKAGE DESCRIPTIO
J8 Package
8-Lead CERDIP (Narrow .300 Inch, Hermetic)
(Reference LTC DWG # 05-08-1110)
0.300 BSC
(0.762 BSC)
CORNER LEADS OPTION
(4 PLCS)
0.008 – 0.018
(0.203 – 0.457)
0° – 15°
0.015 – 0.060
(0.381 – 1.524)
0.023 – 0.045
(0.584 – 1.143)
HALF LEAD
OPTION
0.045 – 0.068
(1.143 – 1.727)
FULL LEAD
OPTION
0.405
(10.287)
MAX
0.005
(0.127)
MIN
8
NOTE: LEAD DIMENSIONS APPLY TO SOLDER
DIP/PLATE OR TIN PLATE LEADS
0.014 – 0.026
(0.360 – 0.660)
0.100
(2.54)
BSC
5
0.025
(0.635)
RAD TYP
0.220 – 0.310
(5.588 – 7.874)
1
0.045 – 0.065
(1.143 – 1.651)
6
7
2
3
4
0.125
3.175
MIN
0.200
(5.080)
MAX
J8 1298
OBSOLETE PACKAGE
N8 Package
8-Lead PDIP (Narrow .300 Inch)
(Reference LTC DWG # 05-08-1510)
0.300 – 0.325
(7.620 – 8.255)
0.009 – 0.015
(0.229 – 0.381)
(
0.045 – 0.065
(1.143 – 1.651)
0.400*
(10.160)
MAX
0.130 ± 0.005
(3.302 ± 0.127)
0.065
(1.651)
TYP
8
7
6
5
1
2
3
4
0.255 ± 0.015*
(6.477 ± 0.381)
+0.035
0.325 –0.015
+0.889
8.255
–0.381
)
0.100
(2.54)
BSC
0.125
(3.175) 0.020
MIN (0.508)
MIN
0.018 ± 0.003
(0.457 ± 0.076)
N8 1098
*THESE DIMENSIONS DO NOT INCLUDE MOLD FLASH OR PROTRUSIONS.
MOLD FLASH OR PROTRUSIONS SHALL NOT EXCEED 0.010 INCH (0.254mm)
RELATED PARTS
PART NUMBER
DESCRIPTION
COMMENTS
LTC1289
12-Bit, 8-Channel Serial ADC
3V or ±3V Supply, Programmable MUX, 25ksps
LTC1401
12-Bit, 200ksps Serial ADC in SO-8
3V Supply, 15mW, Internal References
LTC1594L/LTC1598L
12-Bit, 4-/8-Channel Serial ADC
3V, Micropower, Auto Shutdown, 10ksps
LTC1852/LTC1853
10-Bit/12-Bit, 8-Channel, 400ksps Parallel ADC
3V to 5V Supply, Programmable MUX and Sequencer
LTC1860/LTC1861
12-Bit, 1-/2-Channel Serial ADCs
5V, Micropower, 250ksps, MSOP Package
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Linear Technology Corporation
LT/TP 0102 1.5K REV A • PRINTED IN USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900 ● FAX: (408) 434-0507
●
www.linear.com
LINEAR TECHNOLOGY CORPORATION 1992
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