LINER LTC1291DMJ8

LTC1291
Single Chip 12-Bit
Data Acquisition System
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DESCRIPTIO
FEATURES
■
■
■
■
■
■
■
Built-In Sample-and-Hold
Single Supply 5V Operation
Power Shutdown
Direct 3- or 4-Wire Interface to Most MPU Serial
Ports and All MPU Parallel Ports
Two-Channel Analog Multiplexer
Analog Inputs Common Mode to Supply Rails
8-Pin DIP Package
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KEY SPECIFICATIO S
■
■
■
Resolution: 12 Bits
Fast Conversion Time: 12µs Max Over Temp.
Low Supply Current:
6.0mA (Typ) Active Mode
10µA (Max) Shutdown Mode
The LTC1291 is a data acquisition system that contains a
serial I/O successive approximation A/D converter. It uses
LTCMOSTM switched capacitor technology to perform a
12-bit unipolar A/D conversion. The input multiplexer can
be configured for either single-ended or differential inputs. An on-chip sample-and-hold is included on the “+”
input. When the LTC1291 is idle, it can be powered down
in applications where low power consumption is desired.
An external reference is not required because the LTC1291
takes its reference from the power supply (VCC). All these
features are packaged in an 8-pin DIP.
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 or four
wires. Given the accuracy, ease of use and small package
size, this device is well suited for digitizing analog signals
in remote applications where minimum number of interconnects, small physical size, and low power consumption are important.
LTCMOS TM is a trademark of Linear Technology Corporation
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TYPICAL APPLICATI
2-Channel 12-Bit Data Acquisition System
22µF
TANTALUM
Channel-to-Channel
INL Matching
+5V
+
0.5
0.4
0.3
VCC(VREF)
DO
0.1µF
CH0
2-CHANNEL
MUX*
CLK
SCK
MC68HC11
LTC1291
CH1
DOUT
MISO
GND
DIN
MOSI
DELTA (LSB)
CS
0.2
0.1
0
–0.1
–0.2
–0.3
–0.4
1291 TA01
*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
THE OTHER CHANNEL IS OVERVOLTAGED (VIN < GND OR VIN > VCC). SEE
SECTION ON OVERVOLTAGE PROTECTION IN THE APPLICATIONS INFORMATION.
–0.5
0
512 1024 1536 2048 2560 3072 3584 4096
CODE
1291 TA02
1
LTC1291
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AXI U
RATI GS
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(Notes 1 and 2)
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ABSOLUTE
PACKAGE/ORDER I FOR ATIO
Supply Voltage (VCC) to GND .................................. 12V
Voltage
Analog 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
LTC1291BC, LTC1291CC,
LTC1291DC ............................................ 0°C to 70°C
LTC1291BI, LTC1291CI,
LTC1291DI ........................................ –40°C to 85°C
LTC1291BM, LTC1291CM,
LTC1291DM ................................... –55°C to 125°C
Storage Temperature Range ................ –65°C to 150°C
Lead Temperature (Soldering, 10 sec.)................ 300°C
ORDER PART
NUMBER
LTC1291BMJ8
LTC1291CMJ8
LTC1291DMJ8
LTC1291BIJ8
LTC1291CIJ8
LTC1291DIJ8
LTC1291BIN8
LTC1291CIN8
LTC1291DIN8
LTC1291BCN8
LTC1291CCN8
LTC1291DCN8
TOP VIEW
CS 1
8
VCC (VREF)
CH0 2
7
CLK
CH1 3
6
DOUT
GND 4
5
DIN
J8 PACKAGE
8-LEAD CERAMIC DIP
N8 PACKAGE
8-LEAD PLASTIC DIP
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CO VERTER A D ULTIPLEXER CHARACTERISTICS (Note 3)
LTC1291B
MIN
CONDITIONS
Offset Error
(Note 4)
●
±3.0
Linearity Error (INL)
(Note 4 & 5)
●
Gain Error
(Note 4)
●
●
Minimum Resolution for which No
Missing Codes are Guaranteed
TYP MAX
TYP MAX
UNITS
±3.0
±3.0
LSB
±0.5
±0.5
±0.75
LSB
±1.0
±2.0
±4.0
LSB
12
12
12
Bits
Analog Input Range
(Note 7)
On Channel Leakage Current
(Note 8)
On Channel = 5V
Off Channel = 0V
●
±1
On Channel = 0V
Off Channel = 5V
●
±1
On Channel = 5V
Off Channel = 0V
●
On Channel = 0V
Off Channel = 5V
●
Off Channel Lekage Current
(Note 8)
LTC1291D
LTC1291C
PARAMETER
MIN
TYP MAX
MIN
V
– 0.05V to VCC + 0.05V
±1
±1
µA
±1
±1
µA
±1
±1
±1
µA
±1
±1
±1
µA
AC CHARACTERISTICS (Note 3)
LTC1291B/LTC1291C/LTC1291D
MIN
TYP
MAX
SYMBOL
PARAMETER
CONDITIONS
fCLK
Clock Frequency
VCC = 5V (Note 6)
tSMPL
Analog Input Sample Time
See Operating Sequence
2.5
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
2
(Note 9)
1.0
18 CLK
+ 500ns
●
UNITS
MHz
Cycles
160
300
ns
LTC1291
AC CHARACTERISTICS (Note 3)
LTC1291B/LTC1291C/LTC1291D
MIN
TYP
MAX
SYMBOL
PARAMETER
CONDITIONS
tdis
Delay Time, CS↑ to DOUT Hi-Z
See Test Circuits
●
ten
Delay Time, CLK↓ to DOUT Enabled
See Test Circuits
●
thDI
Hold Time, DIN after CLK↑
VCC = 5V (Note 6)
50
thDO
Time Output Data Remains Valid after CLK↓
tWHCLK
CLK High Time
VCC = 5V (Note 6)
300
tWLCLK
CLK Low Time
VCC = 5V (Note 6)
tf
DOUT Fall Time
See Test Circuits
●
65
130
ns
tr
DOUT Rise Time
See Test Circuits
●
25
50
ns
tsuDI
Setup Time, DIN Stable before CLK↑
VCC = 5V (Note 6)
50
ns
tsuCS
Setup Time, CS↓ before CLK↑
VCC = 5V (Note 6)
50
ns
tWHCS
CS High Time During Conversion
VCC = 5V (Note 6)
500
ns
tWLCS
CS Low Time During Data Transfer
VCC = 5V (Note 6)
18
CLK Cycles
CIN
Input Capacitance
Analog Inputs On Channel
Analog Inputs Off Channel
Digital Inputs
80
150
80
200
UNITS
ns
ns
ns
130
ns
ns
400
ns
100
5
5
pF
pF
pF
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DIGITAL A D DC ELECTRICAL CHARACTERISTICS (Note 3)
LTC1291B/LTC1291C/LTC1291D
MIN
TYP
MAX
SYMBOL
PARAMETER
CONDITIONS
VIH
High Level Input Voltage
VCC = 5.25V
●
VIL
Low Level Input Voltage
VCC = 4.75V
●
IIH
High Level Input Current
VIN = VCC
IIL
Low Level Input Current
VIN = 0V
VOH
High Level Output Voltage
VCC = 4.75V, IOUT = –10µA
VCC = 4.75V, IOUT = – 360µA
●
2.0
UNITS
V
0.8
V
●
2.5
µA
●
–2.5
µA
2.4
4.7
4.0
V
V
VOL
Low Level Output Voltage
VCC = 4.75V, IOUT = 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
ISINK
Output Sink Current
VOUT = VCC
20
ICC
Positive Supply Current
CS High
CS High
Power shutdown
CLK Off
mA
mA
●
6
12
mA
LTC1291BC, LTC1291CC, LTC1291DC
●
5
10
µA
LTC1291BI, LTC1291CI, LTC1291DI,
LTC1291BM, LTC1291CM, LTC1291DM
●
5
15
µA
The ● denotes specifications which apply over the operating temperature
range; all other limits and typicals TA = 25°C.
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, CLK = 1.0MHz unless otherwise specified.
Note 4: One LSB is equal to VCC divided by 4096. For example, when VCC =
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 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 (4.5V), as high level
analog inputs (5V) 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 ≥ 125kHz at 125°C,
fCLK ≥ 30kHz at 85°C and fCLK ≥ 3kHz at 25°C.
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LTC1291
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TYPICAL PERFOR A CE CHARACTERISTICS
10
CLK = 1MHz
TA = 25°C
CLK = 1MHz
VCC = 5V
9
SUPPLY CURRENT (mA)
8
6
4
8
7
6
5
2
4
5
4
3
–50 –30 –10 10 30 50 70 90 110 130
AMBIENT TEMPERATURE (°C)
6
SUPPLY VOLTAGE (V)
1291 G01
0.4
0.3
0.2
0.1
0
4.0
4.5
5.5
5.0
SUPPLY VOLTAGE (V)
6.0
0.1
0
–0.1
–0.2
–0.3
–0.4
–0.5
4.0
4.5
5.5
5.0
SUPPLY VOLTAGE (V)
6.0
Change in Offset vs Temperature
0.5
0.5
0.4
0.3
0.2
0.1
0
–0.1
–0.2
–0.3
–0.4
–0.5
4.0
4.5
5.5
5.0
SUPPLY VOLTAGE (V)
VCC = 5V
CLK = 1MHz
0.4
0.3
0.2
0.1
0
50
25
0
75 100
–50 –25
AMBIENT TEMPERATURE (°C)
6.0
125
1291 G06
1291 G05
Change in Linearity vs
Temperature
Minimum Clock Rate for
0.1 LSB Error
Change in Gain vs Temperature
0.5
0.5
VCC = 5V
CLK = 1MHz
MAGNITUDE OF GAIN CHANGE (LSB)
MAGNITUDE OF LINEARITY CHANGE (LSB)
0.3
0.2
Change in Gain Error vs Supply
Voltage
1291 G04
0.4
0.3
0.2
0.1
0
50
25
0
75 100
–50 –25
AMBIENT TEMPERATURE (°C)
125
1291 G07
VCC = 5V
CLK = 1MHz
VCC = 5V
0.4
0.3
0.2
0.1
0
50
25
0
75 100
–50 –25
AMBIENT TEMPERATURE (°C)
125
1291 G08
* 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.
4
0.4
1291 G03
MAGNITUDE OF OFFSET CHANGE (LSB)
0.5
CHANGE IN GAIN ERROR (LSB = 1/4096 × VCC (VREF))
CHANGE IN LINEARITY (LSB = 1/4096 × VCC (VREF))
Change in Linearity vs Supply
Voltage
0.5
1291 G02
MINIMUM CLK FREQUENCY* (MHz)
0
CHANGE IN OFFSET (LSB = 1/4096 × VCC (VREF))
10
SUPPLY CURRENT (mA)
Change in Offset vs Supply
Voltage
Supply Current vs Temperature
Supply Current vs Supply Voltage
0.25
0.20
0.15
0.10
0.05
–50
–25
25
50
75 100
0
AMBIENT TEMPERATURE (°C)
125
1291 G09
LTC1291
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TYPICAL PERFOR A CE CHARACTERISTICS
10k
1.0
VCC = 5V
CLK = 1MHz
MAXIMUM CLK FREQUENCY* (MHz)
DOUT DELAY TIME FROM CLK↓ (ns)
VCC = 5V
MSB-FIRST DATA
150
LSB-FIRST DATA
100
50
0
–50
–25
25
50
75 100
0
AMBIENT TEMPERATURE (°C)
0.8
RSOURCE–
– –IN
+VIN
1k
RFILTER
CFILTER ≥1µF
+
–
100
10
0.2
0
100
125
+ +IN
0.4
1291 G10
1
1k
10k
RSOURCE – (Ω)
100k
1291 G11
Sample-and-Hold Acquisition
Time vs Source Resistance
10
10k
100
1k
CYCLE TIME (µs)
1291 G12
Input Channel Leakage Current
vs Temperature
100
1000
VCC = 5V
TA = 25°C
0V TO 5V INPUT STEP
VIN
10
RSOURCE+
INPUT CHANNEL LEAKAGE CURRENT (nA)
S/H AQUISITION TIME TO 0.02% (µs)
+VIN
0.6
MAXIMUM RFILTER** (Ω)
250
200
Maximum Filter Resistor vs
Cycle Time
Maximum Clock Rate vs Source
Resistance
DOUT Delay Time vs Temperature
+
–
1
100
1k
10k
RSOURCE+ (Ω)
900
GUARANTEED
800
700
600
* 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.
500
400
300
200
100
ON CHANNEL
OFF CHANNEL
0
–50 –30 –10 10 30 50 70 90 110 130
AMBIENT TEMPERATURE (°C)
1291 G13
**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.
1291 G14
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#
PIN
FUNCTION
DESCRIPTION
1
2, 3
4
5
6
7
8
CS
CH0, CH1
GND
DIN
DOUT
CLK
VCC(VREF)
Chip Select Input
Analog Inputs
Analog Ground
Digital Data Input
Digital Data Output
Shift Clock
Positive Supply and
Reference Voltage
A logic low on this input enables the LTC1291.
These inputs must be free of noise with respect to GND.
GND should be tied directly to an analog ground plane.
The multiplexer address is shifted into this input.
The A/D conversion result is shifted out of this output.
This clock synchronizes the serial data transfer.
This pin provides power and defines the span of the A/D converter. This supply must be kept free of noise and
ripple by bypassing directly to the analog ground plane.
5
LTC1291
W
BLOCK DIAGRA
7
VCC (VREF)
DIN
CH0
CH1
CLK
8
INPUT
SHIFT
REGISTER
5
OUTPUT
SHIFT
REGISTER
2
SAMPLE
AND
HOLD
ANALOG
INPUT MUX
3
6
DOUT
COMP
12-BIT
SAR
12-BIT
CAPACITIVE
DAC
CONTROL
AND
TIMING
4
GND
1
CS
1291 BD
TEST CIRCUITS
Load Circuit for tdis and ten
Load Circuit for tdDO, tr and tf
1.4V
TEST POINT
3k
3k
DOUT
TEST POINT
5V tdis WAVEFORM 2, ten
DOUT
100pF
100pF
tdis WAVEFORM 1
1291 TC02
1291 TC05
On and Off Channel Leakage Current
Voltage Waveforms for tdis
2.0V
CS
5V
ION
A
ON CHANNEL
IOFF
A
90%
tdis
OFF CHANNEL
POLARITY
DOUT
WAVEFORM 2
(SEE NOTE 2)
1291 TC06
1291 TC01
6
DOUT
WAVEFORM 1
(SEE NOTE 1)
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.
LTC1291
TEST CIRCUITS
Voltage Waveforms for DOUT Delay Time, tdDO
Voltage Waveforms for DOUT Rise and Fall Times, tr, tf
CLK
2.4V
DOUT
0.8V
tdDO
0.4V
2.4V
tr
tf
DOUT
1291 TC04
0.4V
1291 TC03
Voltage Waveforms for ten
CS
DIN
START
CLK
2
1
3
4
5
DOUT
B11
0.8V
ten
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APPLICATI
1291 TC07
S I FOR ATIO
The LTC1291 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 LTC1291 communicates with microprocessors and
other external circuitry via a synchronous, half duplex,
four-wire serial interface (see Operating Sequence). The
clock (CLK) synchronizes the data transfer with each bit
being transmitted on the falling CLK edge and captured on
the rising CLK edge in both transmitting and receiving
systems.
CS
DIN 1
DIN 2
DOUT 1
DOUT 2
SHIFT MUX 1 NULL SHIFT A/D CONVERSION
ADDRESS IN
BIT
RESULT OUT
1291 F01
Figure 1
The input data is first received and then the A/D conversion
result is transmitted (half duplex). Because of the half
duplex operation DIN and DOUT may be tied together
allowing transmission over just 3 wires: CS, CLK and
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APPLICATI
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the conversion appears MSB-first on the DOUT line. The
conversion result is output, bit by bit, as the conversion is
performed. At the end of the data exchange CS should be
brought high. This resets the LTC1291 in preparation for
the next data exchange.
DATA (DIN/DOUT). Data transfer is initiated by a falling chip
select (CS) signal. After CS falls the LTC1291 looks for a
start bit. After the start bit is received a 4-bit input word is
shifted into the DIN input which configures the LTC1291
and starts the conversion. After one null bit, the result of
Operating Sequence
(Example: Differential Inputs (CH0 +, CH1 –))
MSB-FIRST DATA (MSBF = 1)
tCYC
CS
DON'T
CARE
CLK
START
ODD/
SIGN
PS
DIN
DOUT
DON'T CARE
HI-Z
MSBF
SGL/
DIFF
B11
B1
B0
FILLED WITH ZEROES
tCONV
tSMPL
LSB-FIRST DATA (MSBF = 0)
tCYC
CS
DON’T
CARE
CLK
START
ODD/
SIGN
PS
DON'T CARE
DIN
DOUT
HI-Z
MSBF
SGL/
DIFF
B11
tSMPL
B1
B0
B11
B1
tCONV
FILLED WITH
ZEROES
1291 AI03
Power Shutdown Operating Sequence
(Example: Differential Inputs (CH0 +, CH1 –) and MSB-First Data)
CS
SHUTDOWN*
REQUEST POWER SHUTDOWN
NEW CONVERSION BEGINS
CLK
START
ODD/
SIGN
START
PS
DIN
DOUT
ODD/
SIGN
PS
DON'T CARE
HI-Z
SGL/
DIFF
MSBF
B11
DATA NOT VALID
B0
FILLED WITH
ZEROES
HI-Z
SGL/
DIFF
MSBF
1291 AI04
* STOPPING THE CLOCK WILL HELP REDUCE POWER CONSUMPTION
CS CAN BE BROUGHT HIGH ONCE DIN HAS BEEN CLOCKED IN
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LTC1291
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Input Data Word
The 4-bit data word is clocked into the DIN pin on the rising
edge of the clock after chip select goes low and the start
bit has been recognized. Further inputs on the DIN pin are
then ignored until the next CS cycle. The input word is
defined as follows:
Multiplexer Channel Selection
MUX ADDRESS
SGL/DIFF ODD/SIGN
1
0
1
1
0
0
0
1
CHANNEL #
0
1
+
+
+
–
–
+
GND
–
–
MSB-FIRST/
LSB-FIRST
START
SGL/
DIFF
ODD/
SIGN
MUX ADDRESS
MSBF
PS
POWER
SHUTDOWN
1291 F02
Figure 2. Input Data Word
Start Bit
The first “logical one” clocked into the DIN input after CS
goes low is the start bit. The start bit initiates the data
transfer and all leading zeroes which precede this logical
one will be ignored. After the start bit is received the
remaining bits of the input word will be clocked in. Further
inputs on the DIN pin are then ignored until the next CS
cycle.
MUX Address
The bits of the input word following the START BIT assign
the MUX configuration for the requested conversion. For
a given channel selection, the converter will measure the
voltage between the two channels indicated by the “+” and
“–” signs in the selected row of the following table. In
single-ended mode, all input channels are measured with
respect to GND. Only the “+” inputs have sample-andholds. Signals applied at the “–” inputs must not change
more than the required accuracy during the conversion.
MSB-First/LSB-First (MSBF)
The output data of the LTC1291 is programmed for MSBfirst or LSB-first sequence using the MSBF bit. When the
MSBF bit is a logical one, data will appear on the DOUT line
in MSB-first format. Logical zeroes will be filled in indefinitely following the last data bit to accommodate longer
word lengths required by some microprocessors. When
the MSBF bit is a logical zero, LSB-first data will follow the
normal MSB-first data on the DOUT line (see Operating
Sequence).
Power Shutdown
The power shutdown feature of the LTC1291 is activated
by making the PS bit a logical zero. If CS remains low after
the PS bit has been received, a 12-bit DOUT word with all
logical ones will be shifted out followed by logical zeroes
until CS goes high. Then the DOUT line will go into its high
impedance state. The LTC1291 will remain in the shutdown mode until the next CS cycle. There is no warm-up
or wait period required after coming out of the power
shutdown cycle so a conversion can commence after CS
goes low (see Power Shutdown Operating Sequence).
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LTC1291
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Output Code
The LTC1291 performs a unipolar conversion. The following shows the output code and transfer curve:
Unipolar Transfer Curve
Unipolar Output Code
111111111111
•
•
•
000000000001
000000000000
1291 AI05a
Microprocessor Interfaces
The LTC1291 can interface directly (without external hardware) to most popular microprocessors’s (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 LTC1291. 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. The DIN word sent to the data
register starts the SPI process. With three 8-bit transfers,
the A/D result is read into the MPU. The second 8-bit
transfer clocks B11 through B8 of the A/D conversion
result into the processor. The third 8-bit transfer clocks the
remaining bits, B7 through B0, into the MPU. The data is
right justified in the two memory locations. ANDing the
second byte with 0DHEX 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.
10
VIN
VREF
4.9988V
4.9976V
•
•
•
0.0012V
0V
VREF–1LSB
VREF – 1LSB
VREF – 2LSB
•
•
•
1LSB
0V
VREF–2LSB
111111111111
111111111110
•
•
•
000000000001
000000000000
111111111110
1LSB
INPUT VOLTAGE
0V
OUTPUT CODE
INPUT VOLTAGE
(VREF = 5V)
1291 AI05b
Table 1. Microprocessor with Hardware Serial Interfaces
Compatible with the LTC1291**
PART NUMBER
TYPE OF INTERFACE
Motorola
MC6805S2, S3
SPI
MC68HC11
SPI
MC68HC05
SPI
RCA
CDP68HC05
SPI
Hitachi
HD6305
SCI Synchronous
HD6301
SCI Synchronous
HD63701
SCI Synchronous
HD6303
SCI Synchronous
HD64180
SCI Synchronous
National Semiconductor
COP400 Family
MICROWIRE†
COP800 Family
MCROWIRE/PLUS†
NS8050U
MICROWIRE/PLUS
HPC16000 Family
MICROWIRE/PLUS
Texas Instruments
TMS7002
Serial Port
TMS7042
Serial Port
TMS70C02
Serial Port
TMS70C42
Serial Port
TMS32011*
Serial Port
TMS32020*
Serial Port
TMS370C050
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.
LTC1291
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Timing Diagram for Interface to the MC68HC11
CS
CLK
SGL/
START DIFF
DIN
ODD/
EVEN
MSBF
DON'T CARE
PS
DOUT
MPU
TRANSMIT
WORD
0
0
0
0
0
ODD/
EVEN MSBF
SGL/
DIFF
1
0
PS
?
?
?
?
B10
B9
B8
X
X
X
X
X
B7
X
B6
B5
X
X
BYTE 2
BYTE 1
MPU
RECEIVED
WORD
B11
?
?
?
?
?
?
0
?
B3
B2
B1
B0
X
X
X
X
X
B2
B1
B0
BYTE 3 (DUMMY)
B11
B10
B9
B8
B7
B6
BYTE 2
BYTE 1
B4
B5
B4
B3
BYTE 3
LTC1291 AI06
Hardware and Software Interface to Motorola MC68HC11
DOUT FROM LTC1291 STORED IN MC68HC11 RAM
MSB
#62
0
0
0
0
B11
B10
B9
B8
ANALOG
INPUTS
LSB
#63
B7
B6
B5
B4
CH0
BYTE 1
B3
B2
B1
B0
CS
D0
CLK
SCK
LTC1291
DOUT
BYTE 2
CH1
DIN
MC68HC11
MISO
MOSI
LTC1291 AI07
MC68HC11 CODE
In this example the DIN word configures the input MUX for
a single-ended input to be applied to CH0. The conversion
result is output MSB-first.
LABEL MNEMONIC
LDAA
STAA
LDAA
STAA
LDAA
STAA
LDAA
STAA
OPERAND
#$50
$1028
#$1B
$1009
#$03
$50
#$60
$51
COMMENTS
CONFIGURATION DATA FOR SPCR
LOAD DATA INTO SPCR ($1028)
CONFIG. DATA FOR PORT D DDR
LOAD DATA INTO PORT D DDR
LOAD DIN WORD INTO ACC A
LOAD DIN DATA INTO $50
LOAD DIN WORD INTO ACC A
LOAD DIN DATA INTO $51
LABEL MNEMONIC
LDAA
STAA
LDX
LOOP
BCLR
LDAA
STAA
OPERAND
#$00
COMMENTS
LOAD DUMMY DIN WORD INTO
ACC A
$52
LOAD DUMMY DIN DATA INTO $52
#$1000
LOAD INDEX REGISTER X WITH
$1000
$08,X,#$01 D0 GOES LOW (CS GOES LOW)
$50
LOAD DIN INTO ACC A FROM $50
$102A
LOAD DIN INTO SPI, START SCK
11
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LABEL MNEMONIC
LDAA
WAIT1 BPL
LDAA
STAA
WAIT2 LDAA
BPL
LDAA
STAA
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OPERAND
$1029
WAIT1
$51
$102A
$1029
WAIT2
$102A
$62
$52
COMMENTS
CHECK SPI STATUS REG
CHECK IF TRANSFER IS DONE
LOAD DIN INTO ACC A FROM $51
LOAD DIN INTO SPI, START SCK
CHECK SPI STATUS REG
CHECK IF TRANSFER IS DONE
LOAD LTC1291 MSBs INTO ACC A
STORE MSBs IN $62
LOAD DUMMY DIN INTO ACC A
FROM $52
Interfacing to the Parallel Port of the Intel 8051 Family
The Intel 8051 has been chosen to show the interface
between the LTC1291 and parallel port microprocessors.
Usually the signals CS, DIN and CLK are generated on three
port lines and the DOUT signal is read on a fourth port line.
LABEL MNEMONIC
STAA
OPERAND
$102A
WAIT3 LDAA
BPL
BSET
LDAA
STAA
$1029
WAIT3
$08,X#$01
$102A
$63
COMMENTS
LOAD DUMMY DIN INTO SPI,
START SCK
CHECK SPI STATUS REG
CHECK IF TRANSFER IS DONE
D0 GOES HIGH (CS GOES HIGH)
LOAD LTC1291 LSBs IN ACC
STORE LSBs IN $63
LOOP
START NEXT CONVERSION
JMP
This works very well. One can save a line by tying the DIN
and DOUT lines together. The 8051 first sends the start bit
and MUX Address to the LTC1291 over the line connected
to P1.2. Then P1.2 is reconfigured as an input and the 8051
reads back the 12-bit A/D result over the same data line.
Timing Diagram for Interface to Intel 8051
PS BIT LATCHED
INTO LTC1291
CS
1
3
2
4
5
CLK
SGL/
DIFF MSBF
ODD/
START SIGN
DATA
(DIN/DOUT)
B11 B10 B9
PS
B8
B7 B6
B5
B4
B3 B2
B1
LTC1291 SENDS A/D RESULT
BACK TO 8051 P1.2
8051 P1.2 OUTPUT DATA
TO LTC1291
B0
LTC1291 AI08
LTC1291 TAKES CONTROL OF DATA
LINE ON 5TH FALLING CLK
8051 P1.2 RECONFIGURED
AS INPUT AFTER THE 5TH RISING
CLK BEFORE THE 5TH FALLING CLK
Hardware and Software Interface to Intel 8051
DOUT FROM LTC1291 STORED IN 8051 RAM
MSB
R2
B11
CH0
B10
B9
B8
B7
B6
B5
B4
ANALOG
INPUTS
LSB
R1
B3
B2
B1
B0
0
0
0
CS
P1.4
CLK
P1.3
DOUT
P1.2
LTC1291
0
CH1
8051
DIN
MUX ADDRESS
A/D RESULT
12
LTC1291 AI09
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8051 Code
In this example the input MUX is configured to accept a
differential input between CH0 and CH1. The result from
the conversion is clocked out MSB-first.
LABEL MNEMONIC
SETB
CONT MOV
CLR
MOV
LOOP1 RLC
CLR
MOV
SETB
DJNZ
MOV
CLR
MOV
LOOP MOV
RLC
SETB
CLR
DJNZ
MOV
MOV
SETB
OPERAND
P1.4
A,#98H
P1.4
R4,#05H
A
P1.3
P1.2,C
P1.3
R4,LOOP1
P1,#04H
P1.3
R4,#09H
C,P1.2
A
P1.3
P1.3
R4,LOOP
R2,A
C,P1.2
P1.3
COMMENTS
CS GOES HIGH
DIN WORD FOR LTC1291
CS GOES LOW
LOAD COUNTER
ROTATE DIN BIT INTO CARRY
CLK GOES LOW
OUTPUT DIN BIT TO LTC1291
CLK GOES HIGH
NEXT DIN BIT
P1.2 BECOMES AN INPUT
CLK GOES LOW
LOAD COUNTER
READ DATA BIT INTO CARRY
ROTATE DATA BIT (B3) INTO ACC
CLK GOES HIGH
CLK GOES LOW
NEXT DOUT BIT
STORE MSBS IN R2
READ DATA BIT INTO CARRY
CLK GOES HIGH
LABEL MNEMONIC
CLR
CLR
RLC
MOV
RLC
SETB
CLR
MOV
RLC
SETB
CLR
MOV
SETB
RRC
RRC
RRC
RRC
MOV
AJMP
Sharing the Serial Interface
The LTC1291 can share the same 3-wire serial interface
with other peripheral components or other LTC1291s
2
1
OPERAND
P1.3
A
A
C,P1.2
A
P1.3
P1.3
C,P1.2
A
P1.3
P1.3
C,P1.2
P1.4
A
A
A
A
R3,A
CONT
COMMENTS
CLK GOES LOW
CLEAR ACC
ROTATE DATA BIT (B3) INTO ACC
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
ROTAGE RIGHT INTO ACC
ROTAGE RIGHT INTO ACC
ROTAGE RIGHT INTO ACC
STORE LSBs IN R3
START NEXT CONVERSION
(Figure 3). The CS signals decide which LTC1291 is being
addressed by MPU.
0
OUTPUT PORT
SERIAL DATA
MPU
3-WIRE SERIAL
INTERFACE TO OTHER
PERIPHERALS OR LTC1291s
3
3
3
3
CS
LTC1291
CS
LTC1291
2 CHANNELS
2 CHANNELS
CS
LTC1291
2 CHANNELS
LTC1291 F03
Figure 3. Several LTC1291s Sharing One 3-Wire Serial Interface
13
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Grounding
The LTC1291 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. Figure 4 shows an
example of an ideal LTC1291 ground plane 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.
22µF
TANTALUM
ANALOG GROUND
PLANE
Figure 5. Poor VCC Bypassing. Noise and
Ripple Can Cause A/D Errors
8
2
7
3
6
4
5
CS
VERTICAL: 0.5mV/DIV
1
VCC
HORIZONTAL: 10µs/DIV
LTC1291 F04
Figure 4. Example Ground Plane for the LTC1291
Bypassing
For good performance, VCC must be free of noise and
ripple. Any changes in the VCC voltage with respect to
ground during the conversion cycle can induce error 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. A 0.1µF
ceramic disk capacitor should also be placed directly
across VCC (Pin 8) and GND (Pin 4) as close to the pins as
possible. The VCC supply should have a low output
impedance such as that obtained from a voltage regulator
(e.g., LT323A). Figures 5 and 6 show the effects of good
and poor VCC bypassing.
14
HORIZONTAL: 10µs/DIV
VCC
0.1µF
LTC1291
VERTICAL: 0.5mV/DIV
ANALOG CONSIDERATIONS
Figure 6. Good VCC Bypassing Keeps
Noise and Ripple on VCC Below 1mV
Analog Inputs
Because of the capacitive redistribution A/D conversion
techniques used, the analog inputs of the LTC1291 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 the transients caused
by the current spikes settle completely before the
conversion begins.
Minimizing Gain and Offset Error
Because the LTC1291’s reference is taken from the power
supply pin (VCC) proper PC board layout and supply
bypassing is important for attaining the best performance
from the A/D converter. Any parasitic resistance in the VCC
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or GND lead will cause gain errors and offset errors (Figure
7). For the best performance the LTC1291 should be
soldered directly to the PC board. If the source can not be
placed next to the pin and the gain parameter is important
the pin should be Kelvin-sensed to eliminate parasitic
resistances due to long PC traces. For example, 0.1Ω of
resistance in the VCC lead can typically cause 0.5LSB
(ICC × 0.1Ω/VCC) of gain error for VCC = 5V.
When the input MUX is selected for single-ended input the
minus terminal is connected to GND internally on the die.
Any parasitic resistance from the GND pin to the ground
plane will lead to an offset voltage (ICC × RP2).
VCC RP1
LTC1291
D/A
5V
REF +
REF –
–
+
RP2
GND
LTC1291 F07
Figure 7. Parasitic Resistance in the VCC and GND Leads
Source Resistance
The analog inputs of the LTC1291 look like a 100pF
capacitor (CIN) in series with a 500Ω resistor (RON). CIN
gets switched between “+” and “–” inputs once during
each conversion cycle. Large external source resistors
RSOURCE +
“+”
INPUT
C1
LTC1291
3RD CLK↑
RON = 500Ω
“–”
INPUT
5TH CLK↓
VIN +
RSOURCE –
CIN =
100pF
VIN –
C2
LTC1291 F08
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 is switched onto the “+” input during
the sample phase (tSMPL, see Figure 9). The sample period
is 2.5 CLK cycles before a conversion starts. 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 2.5µs, RSOURCE+ < 1.0k 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 Figure 9).
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 “–” 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.
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 Figure 9). Again the “+” and “–” input sampling times
can be extended as described above to accommodate
slower op amps. Most op amps including the LT1006 and
LT1013 single supply op amps can be made to settle well
even with the minimum settling windows of 2.5µs (“+”
input) and 1µs (“–” input) that occurs at the maximum
clock rate of 1MHz. Figures 10 and 11 show examples
adequate and poor op amp settling.
Figure 8. Analog Input Equivalent Circuit
15
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HOLD
SAMPLE
CS
CLK
DIN
SGL/
DIFF
START
ODD/
SIGN
MSBF
PS
tSMPL
“+” INPUT MUST SETTLE DURING THIS TIME
DOUT
B11
HI-Z
1ST BIT TEST “–” INPUT MUST
SETTLE DURING THIS TIME
(+) INPUT
(–) INPUT
LTC1291 F09
VERTICAL: 5mV/DIV
VERTICAL: 5mV/DIV
Figure 9. “+” and “–” Input Settling Windows
HORIZONTAL: 500ns/DIV
Figure 10. Adequate Settling of Op Amp Driving Analog Input
16
HORIZONTAL: 20µs/DIV
Figure 11. Poor Op Amp Settling Can Cause A/D Errors
(Note Horizontal Scale)
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RC Input Filtering
It is possible to filter the inputs with an RC network as
shown in Figure 12. 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 a 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 18.5µs, the
input current equals 27µA at VIN = 5V. Here a filter resistor
of 4.5Ω 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
LTC1291
“–”
LTC1291 F12
Figure 12. 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 125°C) 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 LTC1291 provides a built-in sample-and-hold (S/H)
function on the +IN input for signals acquired in the singleended mode (–IN pin grounded). The sample-and-hold
allows the LTC1291 to convert rapidly varying signals (see
typical performance characteristics curve of S/H Acquisition
Time vs Source Resistance). The input voltage is sampled
during the tSMPL time as shown in Figure 9. The sampling
interval begins as the bit preceding the MSBF bit is shifted
in and continues until the falling edge of the PS bit is
received. 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:


VERROR(MAX) = 2πf(−IN)VPEAK  12 
 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) = 

 2πVPEAK 
 fCLK 
 12 


For 0.25LSB error (300µV) the maximum input sinusoid
with a 5V peak amplitude that can be digitized is 0.8Hz.
17
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Overvoltage Protection
Applying signals to the LTC1291’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 LTC1291. It can also happen if the input source is
operating from supplies of larger value than the LTC1291
supply. 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 13 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 LTC1291
is clocked at the maximum clock frequency and 250Ω is
not enough to current limit the input source then the clamp
diodes are recommended (Figures 14 and 15). 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 Maximum CLK Frequency vs Source
Resistance).
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
5V
VCC (VREF)
CH0
CLK
LTC1291
CH1
DOUT
GND
DIN
LTC1291 F13
Figure 13. Overvoltage Protection for Inputs
CS
VCC (VREF)
5V
1k
CH0
250Ω
CLK
LTC1291
CH1
DOUT
GND
DIN
LTC1291 F14
Figure 14. Overvoltage Protection for Inputs
1N4148 DIODES
CS
VCC (VREF)
5V
1k
CH0
CLK
LTC1291
CH1
DOUT
GND
DIN
LTC1291 F15
Figure 15. Overvoltage Protection for Inputs
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A “Quick Look” Circuit for the LTC1291
Users can get a quick look at the function and timing of
the LTC1291 by using the following simple circuit
(Figure 16). DIN is tied to VCC. This requires VIN be
applied to CH1 with respect to the ground plane. The
data is output MSB-first. CS is driven at 1/64 the clock
frequency by the 74HC393 and DOUT outputs the data.
The output data from the DOUT pin can be viewed on a
oscilloscope that is set up to trigger on the falling edge
of CS (Figure 17).
22µF TANTALUM
5V
+
f/64
CS
0.1µF
CH0
VIN
CLK
LTC1291
CH1
DOUT
GND
f
DIN
CLOCK IN 1MHz
TO OSCILLOSCOPE
0.1µF
A1
VCC
CLR1
A2
1QA
CLR2
1QB 74HC393 2QA
1QC
2QB
1QD
2QC
GND
2QD
VCC (VREF)
LTC1291 F16
Figure 16. “Quick Look” Circuit for the LTC1291
CLK
CS
DOUT
NULL
BIT
MSB
(B11)
LSB
(B0)
FILLS WITH
ZEROES
VERTICAL: 5V/DIV
HORIZONTAL: 5µs/DIV
Figure 17. Scope Trace of the LTC1291 "Quick Look"
Circuit Showing Output 101010101010 (AAAHEX)
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.
19
LTC1291
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PACKAGE DESCRIPTIO
Dimensions in inches (millimeters) unless otherwise noted.
J8 Package
8-Lead Ceramic DIP
0.005
(0.127)
MIN
0.290 – 0.320
(7.37 – 8.13)
0.200
(5.080)
MAX
0.025
(0.635)
RAD TIP
0.405
(10.287)
MAX
8
7
6
5
0.015 – 0.060
(0.381 – 1.524)
0.008 – 0.018
(0.203 – 0.460)
0.220 – 0.310
(5.588 – 7.874)
0° – 15°
1
0.385 ± 0.025
(9.779 ± 0.635)
0.014 – 0.026
(0.360 – 0.660)
0.125
3.175
MIN
0.038 – 0.068
(0.965 – 1.727)
2
3
4
0.055
(1.397)
MAX
0.100 ± 0.010
(2.540 ± 0.254)
TJMAX
θJA
150°C
100°C/W
N8 Package
8-Lead Plastic DIP
0.300 – 0.320
(7.620 – 8.128)
0.045 – 0.065
(1.143 – 1.651)
0.130 ± 0.005
(3.302 ± 0.127)
0.065
(1.651)
TYP
0.400
(10.160)
MAX
0.020
(0.508)
MIN
8
7
6
5
0.250 ± 0.010
(6.350 ± 0.254)
0.009 – 0.015
(0.229 – 0.381)
+0.025
0.325 –0.015
(8.255 +0.635
–0.381)
20
0.125
(3.175)
MIN
0.045 ± 0.015
(1.143 ± 0.381)
0.100 ± 0.010
(2.540 ± 0.254)
Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7487
(408) 432-1900 ● FAX: (408) 434-0507 ● TELEX: 499-3977
1
2
3
4
0.018 ± 0.003
(0.457 ± 0.076)
TJMAX
θJA
100°C
130°C/W
LT/GP 0892 10K REV 0
 LINEAR TECHNOLOGY CORPORATION 1992