Datasheet
23
LTC1090
1090fc
SNEAK-A-BIT
TM
The LTC1090’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 10-bit + sign word is returned to memory
inside the MPU. The MC68HC05C4 was chosen as an
example; however, any processor could be used.
Two 10-bit unipolar conversions are performed: the first
over a 0 to 5V span and the second over a 0 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 –1023 to 1023
decimal) is converted to 2’s complement notation and
stored in RAM.
A “Quick Look” Circuit for the LTC1090
Scope Trace of LTC1090 “Quick Look” Circuit
Showing A/D Output of 0101010101 (155
HEX
)
A “Quick Look” Circuit for the LTC1090
Users can get a quick look at the function and timing of the
LTC1090 by using the following simple circuit. REF
+
and
D
IN
are tied to V
CC
selecting a 5V input span, CH7 as a
single ended input, unipolar mode, MSB first format and
16-bit word length. ACLK and SCLK are tied together and
driven by an external clock. CS is driven at 1/64 the clock
rate by the CD4520 and D
OUT
outputs the data. All other
pins are tied to a ground plane. The output data from the
D
OUT
pin can be viewed on an oscilloscope which is set up
to trigger on the falling edge of CS.
U
TYPICAL APPLICATIO
SNEAK-A-BIT Circuit
SNEAK-A-BIT is a trademark of Linear Technology Corp.
LTC1090 • TA04
10µF
9V
V
IN
–5V TO 5V
OTHER CHANNELS
OR SNEAK-A-BIT
INPUTS
CH0
CH1
CH2
CH3
CH4
CH5
SCK
MOSI
MISO
CO
CH6
CH7
0.1µF
–5V
COM
DGND
V
CC
ACLK
SCLK
D
IN
D
OUT
REF
+
REF
–
V
–
AGND
LTC1090
MC68HC05C4
2MHz
CLOCK
CS
LT1021-5
CS
D
OUT
DEGLITCHER
TIME
MSB
(B9)
LSB
(B0)
FILLS
ZERO
LTC1090 • TA03
CH0
CH1
CH2
CH3
CH4
CH5
CH6
CH7
COM
DGND
V
CC
ACLK
SCLK
D
IN
D
OUT
REF
+
REF
–
V
–
AGND
LTC1090
CS
CLK
EN
f/64
5V
4.7µF
f
Q1
Q2
Q3
Q4
RESET
0.1
RESET
Q4
Q3
V
SS
V
IN
V
DD
Q1
EN
CLK
LTC1090
Q2
CLOCK IN
1MHz MAX
TO OSCILLOSCOPE