Datasheet

27
LTC1290
1290fe
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SA
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L
IC
AT
ITY
P
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CA
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SNEAK-A-BIT
TM
The LTC1290’s unique ability to software select the polar-
ity of the differential inputs and the output word length is
used to achieve one more bit of resolution. Using the
circuit below with two conversions and some software, a
2’s complement 12-bit + sign word is returned to memory
inside the MPU. The MC68HC05C4 was chosen as an
example, however, any processor could be used.
Two 12-bit unipolar conversions are performed: the first
over a 0V to 5V span and the second over a 0V to –5V span
(by reversing the polarity of the inputs). The sign of the
input is determined by which of the two spans contained
it. Then the resulting number (ranging from –4095 to
+4095 decimal) is converted to 2’s complement notation
and stored in RAM.
SNEAK-A-BIT Circuit
1290 TA04
22µF
LT1021-5
MC68HC05C4
SCLK
MOSI
MISO
CO
–5V
0.1µF
2MHz
CLOCK
OTHER CHANNELS
OR SNEAK-A-BIT
INPUTS
V
IN
–5V TO 5V
LTC1290
CHO
CH1
CH2
CH3
CH4
CH5
CH6
CH7
COM
DGND
V
CC
ACLK
SCLK
D
IN
D
OUT
CS
REF
+
REF
–
V
–
AGND
9V
5V
1ST CONVERSION
(+) CH6
(–) CH7
0V 0V
1ST CONVERSION
4096 STEPS
2ND CONVERSION
4096 STEPS
–5V
2ND CONVERSION
(–) CH6
(+) CH7
0V
V
IN
V
IN
V
IN
5V
–5V
SOFTWARE
8191
STEPS
1290 TA05
SNEAK-A-BIT
SNEAK-A-BIT is a trademark of Linear Technology Corp.