LTC1292/LTC1297
Single Chip 12-Bit
Data Acquisition Systems
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FEATURES
■
■
■
■
■
■
DESCRIPTIO
Single Supply 5V Operation
Power Shutdown After Each Conversion (LTC1297)
Built-In Sample-and-Hold
60kHz Maximum Throughput Rate (LTC1292)
Direct 3-Wire Interface to Most MPU Serial Ports and
All MPU Parallel Ports
Analog Inputs Common Mode to Supply Rails
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KEY SPECIFICATIO S
■
■
■
■
Resolution: 12 Bits
Fast Conversion Time: 12µs Max Over Temp
Low Supply Current: 6.0mA
Shutdown Supply Current: 5µA (LTC1297)
The LTC®1292/LTC1297 are data acquisition systems that
contain a 12-bit, switched-capacitor successive approximation A/D, a differential input, sample-and-hold on the
(+) input, and serial I/O. When the LTC1297 is idle between
conversions it automatically powers down reducing the
supply current to 5µA, typically. The LTC1292 is capable
of digitizing signals at a 60kHz rate and with the device’s
excellent AC characteristics, it can be used for DSP applications. All these features are packaged in an 8-pin DIP
and are made possible using LTCMOSTM switched-capacitor technology.
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 over three wires.
Because of their accuracy, ease of use and small package
size these devices are well suited for digitizing analog
signals in remote applications where minimum number of
interconnects and power consumption are important.
, LTC and LT are registered trademarks of Linear Technology Corporation.
LTCMOS is trademark of Linear Technology Corporation
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TYPICAL APPLICATIO
Power Supply Current
vs Sampling Frequency
12-Bit Differential Input Data Acquisition System
10000
DIFFERENTIAL
INPUTS
COMMON MODE
RANGE
0V TO 5V*
+
+IN
CLK
DO
1000
MC68HC11
SCK
1N4148
LTC1297
–
5V
22µF
TANTALUM
+
–IN
DOUT
GND
VREF
MISO
LT1027
+
4.7µF
TANTALUM
8V TO 40V
AVERAGE ICC (µA)
VCC
CS
100
10
1µF
1
1
*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 ANY INPUT IS OVERVOLTAGED
(VIN < GND OR VIN > VCC). SEE SECTION ON OVERVOLTAGE PROTECTION IN
THE APPLICATIONS INFORMATION.
10
100
1k
fSAMPLE (Hz)
10k
100k
LTC1297• TA02
LTC1292/7 TA01
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LTC1292/LTC1297
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ABSOLUTE
PACKAGE/ORDER I FOR ATIO
(Notes 1 and 2)
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
LTC1292/LTC1297BC, LTC1292/LTC1297CC,
LTC1292/LTC1297DC ............................ 0°C to 70°C
LTC1292BI, LTC1292CI,
LTC1292DI ......................................... –40°C to 85°C
Storage Temperature Range ................. –65°C to 150°C
Lead Temperature (Soldering, 10 sec.)................ 300°C
ORDER PART NUMBER
TOP VIEW
CS 1
8 VCC
+IN 2
7 CLK
–IN 3
6 DOUT
GND 4
5 VREF
N8 PACKAGE
8-LEAD PLASTIC DIP
TJMAX = 100°C, θJA =130°C/W (N8)
J8 PACKAGE
8-LEAD CERAMIC DIP
TJMAX = 150°C, θJA =100°C/W (J8)
LTC1292BIN8 LTC1297BCN8
LTC1292CIN8 LTC1297CCN8
LTC1292DIN8 LTC1297DCN8
LTC1292BCN8
LTC1292CCN8
LTC1292DCN8
LTC1292BCJ8 LTC1297BCJ8
LTC1292CCJ8 LTC1297CCJ8
LTC1292DCJ8 LTC1297DCJ8
OBSOLETE PACKAGE
Consider the 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)
LTC1292B
LTC1297B
MIN TYP MAX
LTC1292C
LTC1297C
MIN TYP MAX
LTC1292D
LTC1297D
MIN TYP MAX
UNITS
PARAMETER
CONDITIONS
Offset Error
(Note 4)
●
±3.0
±3.0
±3.0
LSB
Linearity Error (INL)
(Note 4 & 5)
●
±0.5
±0.5
±0.75
LSB
Gain Error
(Note 4)
●
±0.5
±1.0
±4.0
LSB
12
12
12
Bits
Minimum Resolution for Which No
Missing Codes are Guaranteed
Analog and REF Input Range
(Note 7)
●
On Channel Leakage Current
(Note 8)
On Channel = 5V
Off Channel = 0V
●
±1
±1
±1
µA
On Channel = 0V
Off Channel = 5V
●
±1
±1
±1
µA
On Channel = 5V
Off Channel = 0V
●
±1
±1
±1
µA
On Channel = 0V
Off Channel = 5V
●
±1
±1
±1
µA
Off Channel Lekage Current
(Note 8)
–0.05V to VCC + 0.05V
V
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LTC1292/LTC1297
AC CHARACTERISTICS
The ● denotes the specifications which apply over the full operating temperature range,
otherwise specifications are at TA = 25°C. (Note 3)
LTC1292B/LTC1297B
LTC1292C/LTC1297C
LTC1292D/LTC1297D
MIN
TYP
MAX
SYMBOL
PARAMETER
CONDITIONS
fCLK
Clock Frequency
VCC = 5V (Note 6)
tSMPL
Analog Input Sample Time
See Operating Sequence
tCONV
Conversion Time
See Operating Sequence
tCYC
Total Cycle Time
See Operating Sequence
(Note 6)
tdDO
Delay Time, CLK↓ to DOUT Data Valid
See Test Circuits
●
160
300
ns
tdis
Delay Time, CS↑ to DOUT Hi-Z
See Test Circuits
●
80
150
ns
ten
Delay Time, CLK↓ to DOUT Enabled
See Test Circuits
●
80
200
ns
thDO
Time Output Data Remains Valid After CLK↓
tf
DOUT Fall Time
See Test Circuits
●
65
130
ns
tr
DOUT Rise Time
See Test Circuits
●
25
50
ns
tWHCLK
CLK High Time
VCC = 5V (Note 6)
300
ns
tWLCLK
CLK Low Time
VCC = 5V (Note 6)
400
ns
tsuCS
Setup Time, CS↓ Before CLK↑
(LTC1297 Wakeup Time)
VCC = 5V (Note 6)
LTC1292
LTC1297
50
5.5
ns
µs
tWHCS
CS High Time Between Data Transfer Cycles
VCC = 5V (Note 6)
LTC1292
LTC1297
2.5
0.5
µs
µs
tWLCS
CS Low Time During Data Transfer
VCC = 5V (Note 6)
LTC1292
LTC1297
14CLK
14CLK+ 5.5µs
CIN
Input Capacitance
Analog Inputs On Channel
Analog Inputs Off Channel
Digital Inputs
(Note 9)
LTC1292
LTC1297
1.0
UNITS
MHz
1.5CLK
0.5CLK+5.5µs
12
LTC1292
LTC1297
CLK
Cycles
14CLK+2.5µs
14CLK+6µs
130
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)
LTC1292B/LTC1297B
LTC1292C/LTC1297C
LTC1292D/LTC1297D
MIN
TYP
MAX
SYMBOL
PARAMETER
CONDITIONS
VIH
High Level Input Voltage
VCC = 5.25V
●
VIL
Low Level Input Voltage
VCC = 4.75V
●
0.8
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 = 4.75V, IO = –10µA
IO = 360µA
●
2.0
2.4
UNITS
V
4.7
4.0
V
V
VOL
Low Level Output Voltage
VCC = 4.75V, IO = 1.6mA
●
0.4
V
IOZ
High Z Output Leakage
VOUT = VCC, CS High
VOUT = 0V, CS High
●
●
3
–3
µA
µA
ISOURCE
Output Source Current
VOUT = 0V
–20
mA
ISINK
Output Sink Current
VOUT = VCC
20
mA
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LTC1292/LTC1297
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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)
LTC1292B/LTC1297B
LTC1292C/LTC1297C
LTC1292D/LTC1297D
SYMBOL
PARAMETER
CONDITIONS
ICC
Positive Supply Current
CS High
LTC1292
CS Low
CS High Power Shutdown CLK Off
IREF
Reference Current
MIN
TYP
MAX
●
6
12
mA
LTC1297
●
6
12
mA
LTC1297
●
5
10
µA
●
10
50
µA
CS High
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 = 5V, VREF = 5V, CLK = 1.0MHz unless otherwise specified.
Note 4: One LSB is equal to VREF divided by 4096. For example, when
VREF = 5V, 1LSB = 5V/4096 = 1.22mV.
Note 5: Linearity error is specified between the actual end points of the
A/D 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 reference and analog input
which will conduct for reference or analog input voltages one diode drop
UNITS
below GND or one diode drop above VCC. Be careful during testing at low
VCC levels (4.5V), as high level reference or analog inputs (5V) can cause
this input diode to conduct, especially at elevated temperatures, and cause
errors for inputs near full scale. This spec allows 50mV forward bias of
either diode. 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 5V input voltage range will therefore
require a minimum supply voltage of 4.950V over initial tolerance,
temperature variations and loading.
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 ≥125kHz at 125°C,
fCLK ≥ 31kHz at 85°C, and fCLK ≥ 3kHz at 25°C.
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TYPICAL PERFOR A CE CHARACTERISTICS
Supply Current vs Supply Voltage
10
SUPPLY CURRENT (mA)
6
4
CLK = 1MHz
VCC = 5V
9
8
SUPPLY CURRENT (µA)
9
8
SUPPLY CURRENT (mA)
10
10
CLK = 1MHz
TA = 25°C
8
7
6
5
7
VCC = 5V
VREF = 5V
CS HIGH
CLK OFF
6
5
4
3
2
2
4
0
LTC1297 Supply Current (Power
Shutdown) vs Temperature
Supply Current vs Temperature
4
5
6
SUPPLY VOLTAGE (V)
LTC1292/7 G01
1
3
–50 –30 –10 10 30 50 70 90 110 130
AMBIENT TEMPERATURE (°C)
LTC1292/7 G02
0
50
25
0
75 100
–50 –25
AMBIENT TEMPERATURE (°C)
125
LTC1292/7 G03
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LTC1292/LTC1297
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TYPICAL PERFOR A CE CHARACTERISTICS
LTC1297 Supply Current (Power
Shutdown) vs CLK Frequency
Unadjusted Offset Voltage vs
Reference Voltage
25
0.9
10
5
0.7
0.6
0.5
0.4
VOS = 0.250mV
0.3
0.2
0.1
200
600
800
400
CLK FREQUENCY (kHz)
1000
3
2
4
REFERENCE VOLTAGE (V)
1
–0.4
–0.6
–0.8
–1.0
–1.2
2
3
4
REFERENCE VOLTAGE (V)
3
4
2
REFERENCE VOLTAGE (V)
0.5
0.4
0.3
0.2
0.1
0
–50
5
–25
25
50
75 100
0
AMBIENT TEMPERATURE (°C)
VCC = 5V
VREF = 5V
CLK = 1MHz
0.4
0.3
0.2
0.1
0
–50
125
–25
25
50
75 100
0
AMBIENT TEMPERATURE (°C)
LTC1292/7 G08
125
LTC1292/7 G09
Minimum Clock Rate for
0.1 LSB Error*
Change in Gain vs Temperature
5
Change in Linearity vs
Temperature
VCC = 5V
VREF = 5V
CLK = 1MHz
LTC1292/7 G07
DOUT Delay Time vs Temperature
0.5
250
VCC = 5V
VREF = 5V
CLK = 1MHz
0.2
0.1
25
50
75 100
0
AMBIENT TEMPERATURE (°C)
125
LTC1292/7 G10
DOUT DELAY TIME FROM CLK↓ (ns)
0.3
–25
VCC = 5V
VCC = 5V
MINIMUM CLK FREQUENCY (MHz)
MAGNITUDE OF GAIN CHANGE (LSB)
1
0
LTC1292/7 G06
MAGNITUDE OF LINEARITY CHANGE (LSB)
MAGNITUDE OF OFFSET CHANGE (LSB)
–0.2
0
–50
0.25
0
5
0.5
VCC = 5V
0.4
0.50
Change in Offset vs Temperature
0
1
0.75
LTC1292/7 G05
Change in Gain vs
Reference Voltage
0
1.00
VOS = 0.125mV
LTC1292/7 G04
CHANGE IN GAIN (LSB = 1/4096 × VREF)
LINEARITY (LSB = 1/4096 × VREF)
15
0
VCC = 5V
0.8
OFFSET (LSB = 1/4096 × VREF)
SUPPLY CURRENT (µA)
1.25
VCC = 5V
VCC = 5V
VREF = 5V
CS HIGH
CMOS LOGIC LEVELS
20
0
Change in Linearity vs
Reference Voltage
0.25
0.20
0.15
0.10
0.05
–50
–25
25
50
75 100
0
AMBIENT TEMPERATURE (°C)
125
LTC1292/7 G11
200
150
MSB FIRST DATA
LSB FIRST DATA
100
50
0
–50
–25
25
50
75 100
0
AMBIENT TEMPERATURE (°C)
125
LTC1292/7 G12
* 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 1MHz VALUE IS FIRST DETECTED (NOTE 9).
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LTC1292/LTC1297
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TYPICAL PERFOR A CE CHARACTERISTICS
Maximum Filter Resistor vs
Cycle Time
Maximum Clock Rate vs
Source Resistance
10k
0.8
+VIN
0.6
RSOURCE
+ +IN
– –IN
0.4
0.2
0
100
1k
10k
RSOURCE– (Ω)
100
RFILTER
+VIN
CFILTER ≥1µF
1k
+
–
100
10
1
100k
100
1k
CYCLE TIME (µs)
10
LTC1292/7G13
10k
LTC1292/7 G14
Input Channel Leakage Current vs
Temperature
GUARANTEED
800
700
600
500
400
300
200
ON CHANNEL
OFF CHANNEL
100
0
–50 –30 –10 10 30 50 70 90 110 130
AMBIENT TEMPERATURE (°C)
10
+VIN
RSOURCE
+
–
1
100
1000
RSOURCE+ (Ω)
10000
Noise Error vs Reference Voltage
PEAK-TO-PEAK NOISE ERROR (LSB)
900
VREF = 5V
VCC = 5V
TA = 25°C
0V TO 5V INPUT STEP
LTC1292/7 G15
2.25
1000
INPUT CHANNEL LEAKAGE CURRENT (nA)
S & H AQUISITION TIME TO 0.02% (µs)
VCC = 5V
VREF = 5V
CLK = 1MHz
MAXIMUM RFILTER** (Ω)
MAXIMUM CLK FREQUENCY* (MHz)
1.0
Sample-and-Hold Acquisition
Time vs Source Resistance
* MAXIMUM CLK FREQUENCY REPRESENTS THE
CLK FREQUENCY AT WHICH A 0.1LSB SHIFT IN
THE ERROR AT ANY CODE TRANSITION FROM ITS
1MHz VALUE IS FIRST DETECTED.
LTC1292/LTC1297
NOISE = 200µVP-P
2.00
1.75
1.50
** 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.
1.25
1.00
0.75
0.50
0.25
0
0
1
4
3
2
REFERENCE VOLTAGE (V)
LTC1292/7 G16
5
LTC1292/7 G17
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PI FU CTIO S
CS (Pin 1): Chip Select Input. A logic low on this input
enables the LTC1292/LTC1297. Power shutdown is activated on the LTC1297 when CS is brought high.
+IN, –IN (Pin 2, 3): Analog Inputs. These inputs must be
free of noise with respect to GND.
GND (Pin 4): Analog Ground. GND should be tied directly
to an analog ground plane.
DOUT (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.
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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LTC1292/LTC1297
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BLOCK DIAGRA
VCC
8
7
INPUT
SHIFT
REGISTER
OUTPUT
SHIFT
REGISTER
2
+IN
SAMPLE
AND
HOLD
ANALOG
INPUT MUX
3
–IN
6
CLK
DOUT
COMP
12-BIT
SAR
12-BIT
CAPACITIVE
DAC
4
5
GND
VREF
CONTROL
AND
TIMING
1
CS
LTC1292/7 BD
TEST CIRCUITS
Voltage Waveforms for DOUT Delay Time, tdDO
On and Off Channel Leakage Current
5V
CLK
0.8V
ION
tdDO
ON CHANNEL
A
2.4V
IOFF
A
DOUT
OFF CHANNEL
0.4V
LTC1292/7 TC04
Voltage Waveforms for DOUT Rise and Fall Times, tr, tf
POLARITY
LTC1292/7 TC01
2.4V
DOUT
Load Circuit for tdis and ten
0.4V
TEST POINT
tr
tf
LTC1292/7 TC05
5V tdis WAVEFORM 2, ten
3k
DOUT
Voltage Waveforms for tdis
tdis WAVEFORM 1
100pF
2.0V
CS
LTC1292/7 TC02
Load Circuit for tdDO, tr and tf
1.4V
DOUT
WAVEFORM 1
(SEE NOTE 1)
90%
tdis
3kΩ
DOUT
TEST POINT
DOUT
WAVEFORM 2
(SEE NOTE 2)
100pF
LTC1292/7 TC06
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.
LTC1292/7 TC03
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LTC1292/LTC1297
TEST CIRCUITS
Voltage Waveforms for ten
CS
CLK
B11
DOUT
0.8V
ten
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APPLICATI
LTC1292/7 TC07
S I FOR ATIO
The LTC1292/LTC1297 are data acquisition components
which contain the following functional blocks:
1. 12-Bit Succesive Approximation Capacitive A/D
Converter
2. Differential Input
3. Sample-and-Hold (S/H)
4. Synchronous, Half-Duplex Serial Interface
5. Control and Timing Logic
DIGITAL CONSIDERATIONS
Serial Interface
The LTC1292/LTC1297 communicate with microprocessors and other external circuitry via a synchronous, halfduplex, 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 LTC1292/LTC1297 do not require a configuration
input word and have no DIN pin. They are permanently
configured to have a single differential input and to perform a unipolar conversion. A falling CS initiates data
transfer. To allow the LTC1297 to recover from the power
shutdown mode, tsuCS has to be met. Then 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 LSB-first sequence. With the half-duplex
serial interface the DOUT data is from the current conversion. This provides easy interface to MSB-first or LSB-first
serial ports. Bringing CS high resets the LTC1292/LTC1297
for the next data exchange and puts the LTC1297 into its
power shutdown mode.
Table 1. Microprocessor with Hardware Serial Interfaces
Compatible with the LTC1292/LTC1297**
PART NUMBER
Motorola
MC6805S2, S3
MC68HC11
MC68HC05
RCA
CDP68HC05
Hitachi
HD6305
HD6301
HD63701
HD6303
HD64180
National Semiconductor
COP400 Family
COP800 Family
NS8050U
HPC16000 Family
Texas Instruments
TMS7002
TMS7042
TMS70C02
TMS70C42
TMS32011*
TMS32020*
TMS370C050
TYPE OF INTERFACE
SPI
SPI
SPI
SPI
SCI Synchronous
SCI Synchronous
SCI Synchronous
SCI Synchronous
SCI Synchronous
MICROWIRE†
MCROWIRE/PLUS†
MICROWIRE/PLUS
MICROWIRE/PLUS
Serial Port
Serial Port
Serial Port
Serial Port
Serial Port
Serial Port
SPI
* Requires external hardware
** Contact factory for interface information for processors not on this list
† MICROWIRE and MICROWIRE/PLUS are trademarks of National
Semiconductor Corp.
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LTC1292/LTC1297
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APPLICATI
S I FOR ATIO
LTC1292 Operating Sequence
tCYC
CS
CLK
Hi-Z
DOUT
B11 B10
B9
B7
B8
tSMPL
B6
B4
B5
B2
B3
B1
B0
B1
B3
B2
B4
B5
B6
B8
B7
B9
B10
B11
tSMPL
tCONV
LTC1292/7 AI01
LTC1297 Operating Sequence
tCYC
CS
tsuCS
POWER SHUTDOWN MODE
CLK
Hi-Z
B11 B10
DOUT
B9
B8
tSMPL
B7
B6
B5
B4
B3
B2
B1
B0
B1
B2
B3
B4
B5
B6
B7
B8
B9
B10
B11
LTC1292/7 AI02
tCONV
Microprocessor Interfaces
The LTC1292/LTC1297 can interface directly (without
external hardware) to most popular microprocessors’
(MPU) synchronous serial formats (see Table 1). If an
MPU without a dedicated serial port is used, then three of
the MPU’s parallel port lines can be programmed to form
the serial link to the LTC1292/LTC1297. Included here are
one serial interface example and one example showing a
parallel port programmed to form the serial interface.
Motorola SPI (MC68HC11)
The MC68HC11 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. A dummy DIN word sent to the
data register starts the SPI process. With two 8-bit transfers,
the A/D result is read into the MPU (Figure 1). For the
LTC1292 the first 8-bit transfer clocks B11 through B8 of
the A/D conversion result into the processor. The second
8-bit transfer clocks the remaining bits B7 through B0 into
CS
CLK
DOUT
B11
B10
B9
B8
B10
B9
B8
B7
B6
B5
B4
B6
B5
B4
?
?
?
O
B11
B2
B1
B0
B2
B1
B0
B1
BYTE 2
BYTE 1
MPU
RECEIVED WORD
B3
1ST TRANSFER
B7
B3
2ND TRANSFER
LTC1292/7 F01
Figure 1. Data Exchange Between LTC1292 and MC68HC11
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LTC1292
CS
MSB
DO
LOCATION #61
O
O
O
O
B11 B10
B9
B8
LOCATION #62
B7
B6
B5
B4
B3
B1
B0
BYTE 1
SCK
CLK
ANALOG
INPUTS
DOUT FROM LTC1292 STORED ON MC68HC11 RAM
MC68HC11
DOUT
MISO
B2
BYTE 2
LTC1292/7 F02
Figure 2. Hardware and Software Interface to Motorola MC68HC11 Microcontroller
MC68HC11 CODE for LTC1292 Interface
LABEL MNEMONIC
LDAA
STAA
LDAA
STAA
LDAA
LOOP
OPERAND
#$50
$1028
#$1B
$1009
#$00
STAA
LDX
$50
#$1000
LDAB
LDAA
#$00
$50
STAA
$102A
NOP
COMMENTS
CONFIGURATION DATA FOR SPCR
LOAD DATA INTO SPCR ($1028)
CONFIG. DATA FOR PORT D DDR
LOAD DATA INTO PORT D DDR
LOAD DUMMY DIN WORD INTO
ACC A
LOAD DUMMY DIN DATA INTO $50
LOAD INDEX REGISTER X WITH
$1000
LOAD ACC B WITH $00
LOAD DUMMY DIN INTO ACC A
FROM $50
LOAD DUMMY DIN INTO SPI,
START SCK
DELAY CS FALL TIME TO RIGHT
JUSTIFY DATA
LABEL MNEMONIC
STAB
OPERAND
$08, X
NOP
6 NOPS FOR TIMING
LDAA
LDAA
STAA
STAA
$1029
$102A
$61
$102A
CHECK SPI STATUS REG
LOAD LTC1292 MSBs INTO ACC A
STORE MSBs IN $61
LOAD DUMMY DIN INTO SPI,
START SCK
NOPS
the MPU. The data is right-justified in the two memory
locations (Figure 2). This was made possible by delaying
the falling edge of CS till after the second CLK. ANDing the
first byte with 0FHEX clears the four most significant bits.
This operation was not included in the code. It can be
inserted in the data gathering loop or outside the loop
when the data is processed.
COMMENTS
D0 GOES LOW (CS GOES LOW)
6 NOPS FOR TIMING
BSET
LDAA
LDAA
STAA
$08,X,$01
$1029
$102A
$62
D0 GOES HIGH (CS GOES HIGH)
CHECK SPI STATUS REGISTER
LOAD LTC1292 LSBs IN ACC
STORE LSBs IN $62
JMP
LOOP
START NEXT CONVERSION
For the LTC1297 (Figure 3) a delay must be introduced to
accommodate the setup time, tsuCS, before the dummy
DIN word is sent to the data register. The first 8-bit transfer
clocks B11 through B6 of the A/D conversion result into
the processor. The second 8-bit transfer clocks the remaining bits B5 through B0 into the MPU. Note B1 and B2
from the LSB-first data word have also been clocked in.
CS
CLK
DOUT
B11 B10
B9
B8
B7
B6
B8
B7
B6
B5
B4
B3
B2
B4
B3
B2
?
0
B11
B10
B9
1ST TRANSFER
B0
B1
B0
B1
B2
B3
BYTE 2
BYTE 1
MPU
RECEIVED WORD
B1
B5
B1
2ND TRANSFER
B2
LTC1292/7 F03
Figure 3. Data Exchange Between LTC1297 and MC68HC11
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LTC1297
ANALOG
INPUTS
DOUT FROM LTC1297 STORED ON MC68HC11 RAM
MC68HC11
CS
DO
CLK
SCK
DOUT
MISO
MSB
LOCATION #61
O
O
O
O
B11 B10
B9
B8
LOCATION #62
B7
B6
B5
B4
B3
B1
B0
B2
BYTE 1
BYTE 2
LTC1292/7 F04
Figure 4. Hardware and Software Interface to Motorola MC68HC11 Microcontroller
MC68HC11 CODE for LTC1297 Interface
LABEL MNEMONIC
LDAA
STAA
LDAA
STAA
LDAA
LOOP
OPERAND
#$50
$1028
#$1B
$1009
#$00
STAA
LDX
$50
#$1000
LDAB
LDAA
BCLR
NOP
NOP
NOP
STAA
#$00
$50
$08,X,$01
$102A
COMMENTS
CONFIGURATION DATA FOR SPCR
LOAD DATA INTO SPCR ($1028)
CONFIG. DATA FOR PORT D DDR
LOAD DATA INTO PORT D DDR
LOAD DUMMY DIN WORD INTO
ACC A
LOAD DUMMY DIN DATA INTO $0
LOAD INDEX REGISTER X WITH
$1000
LOAD ACC B WITH $00
LOAD DIN INTO ACC FROM $50
D0 GOES LOW (CS GOES LOW)
3 NOP FOR tsuCS TIMING
LOAD DUMMY DIN INTO SPI,
START CLK
The data is right- justified in the two memory locations by
rotating right twice (Figure 4). ANDing the first byte with
0FHEX clears the four most significant bits. This operation
was not included in the code. It can be inserted in the data
gathering loop or outside the loop when the data is
processed.
Interfacing to the Parallel Port of the Intel 8051 Family
The Intel 8051 has been chosen to show the interface
between the LTC1292/LTC1297 and parallel port
microprocessors. The signals CS and CLK are generated
LABEL MNEMONIC
LOOP1 LDAA
BPL
LDAA
STAA
STAA
OPERAND
$1029
LOOP1
$102A
$61
$102A
LOOP2 LDAA
BPL
BSET
LDAA
STAA
ROR
ROR
ROR
ROR
JMP
$1029
LOOP2
$08X,$01
$102A
$62
$61
$62
$61
$62
LOOP
COMMENTS
CHECK SPI STATUS REG
CHECK IF TRANSFER IS DONE
LOAD LTC1297 MSBs INTO ACC A
STORE MSBs IN $61
LOAD DUMMY DIN INTO SPI,
START SCK
CHECK SPI STATUS RES
CHECK IF TRANSFER IS DONE
D0 GOES HIGH (CS GOES HIGH)
LOAD LTC1297 LSBs INTO ACC A
STORE LSBs IN $62
ROTATE RIGHT WITH CARRY
NEEDED TO RIGHT JUSTIFY
THE DATA IN $61 AND $62
START NEXT CONVERSION
on two port lines and the DOUT signal is read on a third port
line. After a falling CLK edge each data bit is loaded into the
carry bit and then rotated into the accumulator. Once the
first 8 MSBs have been shifted into the accumulator they
are loaded into register R2. The last four bits are shifted in
the same way and loaded into register R3. The output data
is left-justified in registers R2 and R3 (Figure 5).
For the LTC1297 four NOPs need to be inserted in the 8051
code after CS goes low to allow the LTC1297 to wake up
from power shutdown (tsuCS).
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LTC1292
LTC1297
ANALOG
INPUTS
DOUT FROM LTC1292/LTC1297 STORED IN 8051 RAM
8051
CS
MSB
P1.4
CLK
P1.3
DOUT
P1.1
R2
B11 B10
B9
B8
B7
B6
B5
B4
R3
B3
B2
B1
B0
O
O
O
O
B5
B4
B3
B1
B0
CS
CLK
DOUT
B11
B10
B9
B8
B7
B6
B2
LTC1292/7 F05
Figure 5. Hardware and Software Interface to Intel 8051 Processor
8051 CODE
LABEL MNEMONIC
MOV
CLR
SETB
CONT CLR
NOP
NOP
NOP
NOP
SETB
CLR
SETB
CLR
MOV
LOOP MOV
RLC
SETB
CLR
DJNZ
MOV
MOV
OPERAND
P1,#02h
P1.3
P1.4
P1.4
COMMENTS
BIT 1 PORT 1 SET AS INPUT
CLK GOES LOW
CS GOES HIGH
CS GOES LO
4 NOP FOR LTC1297 tsuCS (Wakeup
Time) (Not Needed for LTC1292)
P1.3
P1.3
P1.3
P1.3
R4,#08H
C,P1.1
A
P1.3
P1.3
R4,LOOP
R2,A
C,P1.1
CLK GOES HIGH
CLK GOES LOW
CLK GOES HIGH
CLK GOES LOW
LOAD COUNTER
READ DATA BIT INTO CARRY
ROTATE DATA BIT INTO ACC
CLK GOES HIGH
CLK GOES LOW
NEXT BIT
STORE MSBs IN R2
READ DATA BIT INTO CARRY
Sharing the Serial Interface
The LTC1292/LTC1297 can share the same two-wire
serial interface with other peripheral components or other
LTC1292/LTC1297s (Figure 6). In this case, the CS signals
decide which LTC1292 is being addressed by the MPU.
LABEL MNEMONIC
CLR
RLC
CLR
MOV
RLC
SETB
CLR
MOV
RLC
SETB
CLR
MOV
SETB
RRC
RRC
RRC
RRC
MOV
AJMP
OPERAND
A
A
SETB
P1.3
C,P1.1
A
P1.3
P1.3
C,P1.1
A
P1.3
P1.3
C,P1.1
P1.4
A
A
A
A
R3,A
CONT
COMMENTS
CLEAR ACC
ROTATE DATA BIT (B3) INTO ACC
P1.3
CLK GOES HIGH
CLK GOES LOW
READ DATA BIT INTO CARRY
ROTATE DATA BIT (B2) INTO ACC
CLK GOES HIGH
CLK GOES LOW
READ DATA BIT INTO CARRY
ROTATE DATA BIT (B1) INTO ACC
CLK GOES HIGH
CLK GOES LOW
READ DATA BIT INTO CARRY
CS GOES HIGH
ROTATE DATA BIT (B0) INTO ACC
ROTATE RIGHT INTO ACC
ROTATE RIGHT INTO ACC
ROTATE RIGHT INTO ACC
STORE LSBs IN R3
START NEXT CONVERSION
ANALOG CONSIDERATIONS
Grounding
The LTC1292/LTC1297 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
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0
OUTPUT PORT
SERIAL DATA
MPU
VCC
22µF
TANTALUM
2-WIRE SERIAL
INTERFACE TO OTHER
PERIPHERALS OR
LTC1292/LTC1297s
2
2
2
2
CS
LTC1292
LTC1297
CS
LTC1292
LTC1297
CS
LTC1292
LTC1297
+
+
+
–
–
1
0.1µF
8
2 LTC1292 7
3 LTC1297 6
–
4
5
LTC1292/7 F06
Figure 6. Several LTC1292/LTC1297s Sharing One 2-Wire Serial Interface
LTC1292/7 F07
Figure 7. Example Ground Plane
for the LTC1292/LTC1297
minimum and the VCC supply should have a low output
impedance such as obtained from a voltage regulator
(e.g., LT323A). 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. Figures 8 and 9 show the effects of good and
poor VCC bypassing.
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 ground 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
Analog Inputs
Because of the capacitive redistribution A/D conversion
techniques used, the analog inputs of the LTC1292/
LTC1297 have 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 that
the transients caused by the current spikes settle completely
before the conversion begins.
VERTICAL: 0.5mV/DIV
VERTICAL: 0.5mV/DIV
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). Figure 7 shows an example of
an ideal LTC1292/LTC1297 ground plane design for a twosided 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.
CS
VCC
HORIZONTAL: 10µs/DIV
Figure 8. Poor VCC Bypassing. Noise and
Ripple Can Cause A/D Errors
HORIZONTAL: 10µs/DIV
Figure 9. Good VCC Bypassing Keeps
Noise and Ripple on VCC Below 1mV
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Source Resistance
The analog inputs of the LTC1292/LTC1297 look like a
100pF capacitor (CIN) in series with a 500Ω resistor (RON)
(Figures 10a and 10b). 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.
“+” Input Settling
The input capacitor for the LTC1292 is switched onto the
“+” input during the sample phase (tSMPL, see Figures 11a,
11b and 11c). The sample period can be as short as tWHCS
+ 1/2 CLK cycle or as long as tWHCS + 1 1/2 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 3.0µs, RSOURCE+ < 2.0k
and C1 < 20pF will provide adequate settling time.
The sample period for the LTC1297 starts on the falling
edge of CS and ends on the falling edge of the first CLK
RSOURCE +
“+”
INPUT
VIN +
CS↑
C1
RSOURCE –
“–”
INPUT
VIN –
LTC1292
RON
500Ω
CIN
100pF
tWHCS
+ 0.5 CLK
C2
LTC1292/7 F10a
Figure 10a. Analog Input Equivalent Circuit for the LTC1292
RSOURCE +
“+”
INPUT
LTC1297
VIN +
CS↓
C1
RSOURCE –
“–”
INPUT
VIN –
RON
500Ω
CIN
100pF
tsuCS
+ 0.5 CLK
C2
LTC1292/7 F10b
Figure 10b. Analog Input Equivalent Circuit for the LTC1297
(Figure 12). The length of the sample period is tsuCS +0.5
CLK cycles. Again, the voltage on the “+” input must settle
completely within the sample period. If large “+” input
source resistance must be used, the sample time can be
increased by using a slower CLK frequency or by increasing
“+” 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
LTC1292/7 F11a
Figure 11a. Setup Time (tsuCS) Is Met for the LTC1292
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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
LTC1292/7 F11b
Figure 11b. Setup Time (tsuCS) Is Met for the LTC1292
tWHCS
CS
CLK
tSMPL
(+) INPUT MUST SETTLE DURING THIS TIME
DOUT
B11
B10
HI-Z
1ST BIT TEST (–) INPUT MUST
SETTLE DURING THIS TIME
(+) INPUT
(–) INPUT
LTC1292/7 F11c
Figure 11c. Setup Time (tsuCS) Is Not Met for the LTC1292
tsuCS. With the minimum possible sample time of 6µs,
RSOURCE+ < 5k and C1 < 20pF will provide adequate
settling time. In general for both the LTC1292 and LTC1297
keep the product of the total resistance and the total
capacitance less than tSMPL/9. If this condition can not be
met, then make C1 > 0.47µF (see RC Input Filtering
section).
“–” Input Settling
At the end of the sample phase the input capacitor switches
to the “–” input and the conversion starts (see Figures 11a,
11b, 11c and 12). During the conversion, the “+” input
voltage is effectively “held” by the sample-and-hold and
will not affect the conversion result. It is critical that the
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tWHCS
tsuCS
CS
CLK
DOUT
tSMPL
(+) INPUT MUST SETTLE
DURING THIS TIME
B11
B10
HI-Z
1ST BIT TEST (–) INPUT MUST
SETTLE DURING THIS TIME
(+) INPUT
(–) INPUT
LTC1292/7 F12
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 11a, 11b, 11c and 12). Again the “+” and “–
” input sampling times can be extended as described
above to accommodate slower op amps. Most op amps
including the LT1797 and LT1677 single supply op amps
can be made to settle well even with the minimum settling
windows of 3.0µs for the LTC1292 or 6.0µs for the
LTC1297 (“+” input) and 1µs (“–” input) that occurs at the
maximum clock rate of 1MHz. Figures 13 and 14 show
examples of both adequate and poor op amp settling.
HORIZONTAL: 500ns/DIV
Figure 13. Adequate Settling of Op Amp Driving Analog Input
VERTICAL: 5mV/DIV
“–” 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 1MHz, RSOURCE – < 250Ω and C2 < 20pF
will provide adequate settling.
VERTICAL: 5mV/DIV
Figure 12. “+” and “–” Input Settling Windows for the LTC1297
HORIZONTAL: 20µs/DIV
Figure 14. 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 15. 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 the LTC1292(LTC1297) at the minimum cycle
time of 16.5µs (20µs), the input current equals 30µA
(25µA) at VIN = 5V. Here a filter resistor of 4Ω (5Ω) 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
LTC1292
LTC1297
curve of S&H Acquisition Time vs Source Resistance). The
input voltage is sampled during the tSMPL time as shown
in Figure 11. The sampling interval begins at the rising
edge of CS for the LTC1292, and at the falling edge of CS
for the LTC1297, 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 as in single-ended mode. 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:
“–”
LTC1292/7 F15
Figure 15. 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 flowing through a source
resistance of 1k will cause a voltage drop of 1mV or
0.8LSB. 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 LTC1292/LTC1297 provide a built-in sample-andhold (S&H) function on the +IN input for signals acquired
in the single-ended mode (–IN pin grounded). The sampleand-hold allows the LTC1292/LTC1297 to convert rapidly
varying signals (see typical performance characteristics
12
VERROR(MAX) = 2πf(–IN)VPEAK
fCLK
(
)
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
(300µV) with the converter running at CLK = 1MHz, its
peak value would have to be 66mV. Rearranging the above
equation the maximum sinusoidal signal that can be
digitized to a given accuracy is given as:
VERROR(MAX)
f(–IN) MAX =
2πVPEAK
fCLK
12
For 0.25LSB error (300µV) the maximum input sinusoid
with a 5V peak amplitude that can be digitized is 0.8Hz.
Reference Input
The voltage on the reference input of the LTC1292/
LTC1297 determine the voltage span of the A/D converter. The reference input has transient capacitive
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switching currents due to the switched-capacitor conversion technique (see Figure 16). During each bit test
of the 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
EVERY
CLK CYCLE
ROUT
VREF
REF –
13
RON
LTC1292
LTC1297
8pF TO 40pF
LTC1292/7 F16
VERTICAL: 0.5mV/DIV
Figure 16. Reference Input Equivalent Circuit
HORIZONTAL: 1µs/DIV
VERTICAL: 0.5mV/DIV
Figure 17. Adequate Reference Settling (LT1027)
HORIZONTAL: 10µs/DIV
Figure 18. Poor Reference Settling Can Cause A/D Errors
Figures 17 and 18 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 1MHz most references and op amps can be
made to settle within the 1µs bit time. For example the
LT1790 will settle adequately.
Reduced Reference Operation
The effective resolution of the LTC1292/LTC1297 can
be increased by reducing the input span of the converter. The LTC1292/LTC1297 exhibit good linearity
over a range of reference voltages (see typical 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. The internal reference for VREF has been tied to
the GND pin. Any voltage drop from the GND pin to the
ground plane will cause a gain error.
Offset with Reduced VREF
The offset of the LTC1292/LTC1297 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 0.1LSB
with a 5V reference becomes 0.4LSB with a 1.25V
reference. If this offset is unacceptable, it can be
corrected digitally by the receiving system or by offsetting the –IN input to the LTC1292/LTC1297.
Noise with Reduced VREF
The total input referred noise of the LTC1292/LTC1297
can be reduced to approximately 200µVP-P using a
ground plane, good bypassing, good layout techniques
and minimizing noise on the reference inputs. This
noise is insignificant with a 5V reference input but will
become a larger fraction of an LSB as the size of the LSB
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18
LTC1292/LTC1297
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is reduced. The typical performance characteristics
curve of Noise Error vs Reference Voltage shows the
LSB contribution of this 200µV of noise.
For operation with a 5V reference, the 200µV noise is
only 0.16LSB peak-to-peak. Here the LTC1292/LTC1297
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 peakto-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.
Gain Error Due to Reduced VREF
The gain error of the LTC1292/LTC1297 is very good
over a wide range of reference voltages. The error
component that is seen in the typical performance
characteristics curve Change in Gain Error vs Reference Voltage is due to the voltage drop on the GND pin
from the device to the ground plane. To minimize this
error the LTC1292/LTC1297 should be soldered directly onto the PC board. The internal reference point
for VREF is tied to GND. Any voltage drop in the GND pin
will make the reference voltage, internal to the device,
less than what is applied externally (Figure 19). This
drop is typically 420µV due to the product of the pin
resistance (RPIN) and the LTC1292/LTC1297 supply
LTC1292
LTC1297
DAC
REF– REF+
GND
ICC
RPIN
VREF
±
REFERENCE
VOLTAGE
LTC1292/7 F19
Figure 19. Parasitic Resistance in GND Pin
current. For example, with VREF = 1.25V this will result
in a gain error change of –1.0LSB from the gain error
measured with VREF = 5V.
LTC1292 AC Characteristics
Two commonly used figures of merit for specifying the
dynamic performance of the A/Ds in digital signal
processing applications are the Signal-to-Noise Ratio
(SNR) and the “Effective Number of Bits (ENOB).” SNR
is the ratio of the RMS magnitude of the fundamental to
the RMS magnitude of all the non-fundamental signals
up to the Nyquist frequency (half the sampling frequency). The theoretical maximum SNR for a sine wave
input is given by:
SNR = (6.02N + 1.76dB)
where N is the number of bits. Thus the SNR depends
on the resolution of the A/D. For an ideal 12-bit A/D the
SNR is equal to 74dB. Fast Fourier Transform (FFT)
plots of the output spectrum of the LTC1292 are shown
in Figures 20a and 20b. The input (fIN) frequencies are
1kHz and 28kHz with the sampling frequency (fS) at
58.8 kHz. The SNRs obtained from the plots are 73.0dB
and 61.5dB.
By rewriting the SNR expression it is possible to obtain
the equivalent resolution based on the SNR measurement.
SNR – 1.76dB
N=
6.02
This is the effective number of bits (ENOB). For the
example shown in Figures 20a and 20b, N = 11.8 bits
and 9.9 bits, respectively. Figure 21 shows a plot of
ENOB as a function of input frequency. The 2nd harmonic distortion term accounts for the degradation of
the ENOB as fIN approaches fS/2.
Figure 22 shows an FFT plot of the output spectrum for
two tones applied to the input of the A/D. Nonlinearities
in the A/D will cause distortion products at the sum and
difference frequencies of the fundamentals and products of the fundamentals. This is classically referred to
as intermodulation distortion (IMD).
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0
0
–20
–20
–40
–40
MAGNITUDE (dB)
MAGNITUDE (dB)
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–60
–80
–60
–80
–100
–100
–120
–120
–140
–140
0
20
15
10
FREQUENCY (kHz)
5
30
25
LTC1292/7 F20a
Figure 20a. fIN = 1kHz, fS = 58.8kHz, SNR = 73.0dB
0
–20
MAGNITUDE (dB)
–40
–60
–80
–100
–120
–140
0
20
15
10
FREQUENCY (kHz)
5
30
25
LTC1292/7 F20b
Figure 20b. fIN = 28kHz, fS = 58.8kHz, SNR = 61.5dB
12.0
fS = 58.8kHz
EFFECTIVE NUMBER OF BITS
11.5
11.0
10.5
10.0
9.5
9.0
8.5
8.0
0
20
40
60
FREQUENCY (kHz)
80
100
LT1292/7 F21
Figure 21. LTC1292 ENOB vs Input Frequency
0
5
20
15
10
FREQUENCY (kHz)
25
30
LTC1292/7 F22
Figure 22. fIN1 = 5.1kHz, fIN2 = 5.6kHz, fS = 58.8kHz
Overvoltage Protection
Applying signals to the LTC1292/LTC1297’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 devices. For example this condition
would occur if a signal is applied to the analog inputs
before power is applied to the LTC1292/LTC1297. Another example is the input source is operating from
different supplies of larger value than the LTC1292/
LTC1297. 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 23
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
250Ω when clocked at its maximum clock frequency of
1MHz. If the LTC1292/LTC1297 are clocked at the
maximum clock frequency and 250Ω is not enough to
current limit the input source, then the clamp diodes are
recommended (Figures 24a and 24b). The reason for
the limit on the resistor value is that 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 Maximum CLK Frequency
vs Source Resistance).
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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 25).
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
1N4148 DIODES
CS
CS
5V
VCC
5V
VCC
1k
+IN
–IN
CLK
+IN
DOUT
–IN
LTC1292
LTC1297
GND
CLK
LTC1292
LTC1297
DOUT
GND
VREF
VREF
LTC1292/7 F24
LTC1292/7 F23
Figure 23. Overvoltage Protection with Clamp Diodes
CS
VCC
+IN
CLK
Figure 24b. Overvoltage Protection with
Diode Clamps and Current Limiting Resistor
5V
CS
1k
250Ω
–IN
LTC1292
LTC1297
+IN
DOUT
–IN
GND
5V
VCC
CLK
LTC1292
LTC1297
1N4148
DOUT
VREF
GND
LTC1292/7 F24a
5V
VREF
LTC1292/7 F25
Figure 24a. Overvoltage Protection with
Current Limiting Resistors
Figure 25. Separate VCC and VREF Supplies
+5V
f/32
22µF
CS
VCC
VDD
CLK
VIN
+IN
CLK
LTC1292
–IN
DOUT
EN
RESET
Q1
Q4
GND
Q3
Q2
Q3
VREF
CD4520
Q2
Q4
Q1
RESET
EN
VSS
TO OSCILLOSCOPE
0.1µF
CLOCK IN
1MHz
CLK
LTC1292/7 F26
Figure 26. “Quick Look” Circuit for the LTC1292
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LTC1292/LTC1297
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A “Quick Look” Circuit for the LTC1292
Users can get a quick look at the function and timing of
the LTC1292 by using the “Quick Look” circuit in Figure
26. 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 CD4520 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 27). Note the LSB data is
partially clocked out before CS goes high.
A “Quick Look” Circuit for the LTC1297
A circuit similar to the one used for the LTC1292 can be
used for the LTC1297(Figure 28). A one shot has been
generated with NAND gates, a resistor and capacitor to
satisfy the setup time tsuCS. This can be eliminated if a
slower clock is used. When CS goes low the one shot is
triggered. This turns off the clock to the LTC1297 for a
fixed time to meet tsuCS. Once the clock starts DOUT is
shifted out one bit at a time. CS is driven at 1/64 the
clock rate by the 74HC393. The output data from the
DOUT pin can be viewed on an oscilloscope that is set to
trigger on the falling edge of CS. See Figure 29.
CLK
CS
CLK
DOUT
CS
NULL
BIT
MSB
LSB
(B11)
(B0)
VERTICAL: 5V/DIV
HORIZONTAL: 2µs/DIV
DOUT
LSB-FIRST DATA
(B1)
NULL MSB LSB LSB-FIRST DATA
(B1)
BIT (B11) (B0)
VERTICAL: 5V/DIV
HORIZONTAL: 5µs/DIV
Figure 27. Scope Trace of the LTC1292 “Quick Look”
Circuit Showing A/D Output 101010101010 (AAAHEX)
Figure 29. Scope Trace of the LTC1297 “Quick Look”
Circuit Showing A/D Output 101010101010 (AAAHEX)
22µF
TANTALUM
+
5V
f/64
CS
f
VCC
A1
VCC
CLR1
VIN
+IN
LTC1297
–IN
1QB
DOUT
GND
A2
1QA
CLK
1QC
VREF
0.1µF
CLR2
74HC393
2QA
2QB
1QD
2QC
GND
2QD
TO OSCILLOSCOPE
340Ω
0.02µF
CLOCK IN 1MHz
LTC1292/7 F28
Figure 28. “Quick Look” Circuit for the LTC1297
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Opto-Isolated Temperature Monitor
Amplification of sensor outputs is often required to
generate a signal large enough to be properly digitized.
For example, a J-type thermocouple provides only
52µV/°C. The 5µV offset of the LTC1050 chopper op
amp generates less than 0.1°C error (Figure 31). Cold
junction compensation is provided by the LT1025A.
(For more detail see LTC Design Note 5).
In the opto-isolated interface two signals are generated
from one. This allows a two-wire interface to the
LTC1292. A long high signal (>1ms) on the CLK IN input
allows the 0.1µF capacitor to discharge taking CS high.
This resets the A/D for the next conversion. When CLK
IN starts toggling, CS goes low and stays there until the
next extended CLK IN high time. See Figure 30.
5V/DIV
CLK IN
A
CS
DATA OUT
20µs/DIV
Figure 30. Opto-Isolated Temperature
Monitor Digital Waveforms
ISOLATED
5V
+
2k
0.1%
22 µ F
3.4k
0.1%
LT1019-2.5
A
+
0.33 µ F
2
2
VIN
7
J
H
–
+
+
R
5
+
1
6
LTC1050
3
LT1025A
GND
4
1N4148
47Ω
2
3
4
1 µF
4
1 µF
+
3
1N4148
10k
+
LTC1292
–
4N28s
1N4148
178k
0.1%
CS
+IN
–IN
GND
VCC
100k
8
2
4
5V
1k
CLK IN
6
0.1 µ F
500k
CLK 7
6
DOUT
5
VREF
4.7 µ F
1
5k
+
5V
74C14
1k
TYPE J
5k
1
3Ω
3
DATA
OUT
0°C – 500°C TEMPERATURE RANGE
4
2
6
500k
LTC1292/7 F31
Figure 31. Opto-Isolated Temperature Monitor
12927fb
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 its circuits as described herein will not infringe on existing patent rights.
23
LTC1292/LTC1297
U
PACKAGE DESCRIPTIO
J8 Package
8-Lead CERDIP (Narrow .300 Inch, Hermetic)
(Reference LTC DWG # 05-08-1110)
0.300 BSC
(0.762 BSC)
0.200
(5.080)
MAX
CORNER LEADS OPTION
(4 PLCS)
0.008 – 0.018
(0.203 – 0.457)
0° – 15°
0.023 – 0.045
(0.584 – 1.143)
HALF LEAD
OPTION
0.045 – 0.068
(1.143 – 1.727)
FULL LEAD
OPTION
0.015 – 0.060
(0.381 – 1.524)
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)
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
0.100
(2.54)
BSC
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
LTC1286
12-Bit, Micropower ADC in SO-8
12.5ksps, 1.3mW
LTC1402
12-Bit, 2.2Msps Serial ADC
Unipolar (5V) or Bipolar (±5V), 90mW, 16-Pin SSOP Package
LTC1404
12-Bit, 600ksps Serial ADC in SO-8
Unipolar (5V) or Bipolar (±5V), 75mW
LTC1860
12-Bit, 250ksps Serial ADC in MSOP
4.25mW, Auto Shutdown-10µW at 1ksps
LTC1864
16-Bit, 250ksps Serial ADC in MSOP
4.25mW, Auto Shutdown-10µW at 1ksps
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24
Linear Technology Corporation
LT/CPI 0202 1.5K REV B • PRINTED IN USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900 ● FAX: (408) 434-0507
●
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