LTC1290BI LINER [Linear Technology], LTC1290BI Datasheet - Page 25

• Manufacturer Part Number: LTC1290BI
• Description: Single Chip 12-Bit Data Acquisition System
• Manufacturer: LINER [Linear Technology]

SNEAK-A-BIT
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.
TYPICAL
OTHER CHANNELS
OR SNEAK-A-BIT
–5V TO 5V
INPUTS
V
IN
TM
A
PPLICATI
9V
SNEAK-A-BIT Circuit
CHO
CH1
CH2
CH3
CH4
CH5
CH6
CH7
COM
DGND
LT1021-5
LTC1290
AGND
ACLK
SCLK
D
REF
REF
V
OUT
D
O
CS
V
CC
IN
+
–
–
ACLK/
SCLK
U
D
CS
OUT
S
Scope Trace of LTC1290 “Quick Look” Circuit
Showing A/D Output of 010101010101 (555
DEGLITCHER
CLOCK
2MHz
0.1 F
22 F
TIME
–5V
SCLK
MOSI
MISO
CO
MC68HC05C4
1290 TA04
VERTICAL: 5V/DIV
HORIZONTAL: 1 s/DIV
(B11)
MSB
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 is a trademark of Linear Technology Corp.
V
V
IN
IN
2ND CONVERSION
1ST CONVERSION
(B0)
LSB
(+) CH6
(–) CH7
(–) CH6
(+) CH7
ZEROS
HEX
FILLS
5V
0V
)
–5V
0V
1ST CONVERSION
4096 STEPS
SNEAK-A-BIT
2ND CONVERSION
4096 STEPS
SOFTWARE
LTC1290
–5V
0V
5V
V
IN
25
1290 TA05
8191
STEPS