LTC1289 Linear Technology, LTC1289 Datasheet - Page 24

• Manufacturer Part Number: LTC1289
• Description: 3 Volt Single Chip 12-Bit Data Acquisition System
• Manufacturer: Linear Technology

LTC1289
SNEAK-A-BIT
The LTC1289’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 that operates at 3.3V
could be used.
SNEAK-A-BIT is a trademark of Linear Technology Corp.
24
TYPICAL
TM
A
PPLICATI
OTHER CHANNELS
OR SNEAK-A-BIT
–2.5V TO +2.5V
V
IN
2ND CONVERSION
1ST CONVERSION
INPUTS
O
V
(+) CH6
(–) CH7
(–) CH6
(+) CH7
IN
U
S
2.5V
CHO
CH1
CH2
CH3
CH4
CH5
CH6
CH7
COM
DGND
0V
LTC1289
–2.5V
0V
1ST CONVERSION
4096 STEPS
SNEAK-A-BIT Circuit
AGND
ACLK
SCLK
D
REF
REF
V
OUT
SNEAK-A-BIT
D
CS
V
CC
IN
+
–
–
2ND CONVERSION
4096 STEPS
Two 12-bit unipolar conversions are performed: the first
over a 0V to 2.5V span and the second over a 0V to –2.5V
span (by reversing the polarity of the inputs). The sign of
the input is determined by which of the two spans con-
tained it. Then the resulting number (ranging from –4095
to +4095 decimal) is converted to 2’s complement nota-
tion and stored in RAM.
0.1 F
22 F
SOFTWARE
–3.3V
10 F
LT1019
+3.3V
–2.5
–2.5V
2.5V
1k
0V
V
2MHz
ACLK
IN
SCLK
MOSI
MISO
CO
MC68HC05C4
8191 STEPS
LTC1289 TA04
LTC1289 TA03