Datasheet

31
LTC2420
TYPICAL APPLICATIO S
U
Single-Chip Instrumentation Amplifier
for the LTC2420
The circuit in Figure 37 is a simple solution for processing
differential signals in pressure transducer, weigh scale or
strain gauge applications that can operate on a supply
voltage range of ±5V to ±15V. The circuit uses an LT
®
1920
single-chip instrumentation amplifier to perform a differ-
ential to single-ended conversion. The amplifier’s output
voltage is applied to the LTC2420’s input and converted to
a digital value with an overall accuracy exceeding 17 bits
(0.0008%). Key circuit performance results are shown in
Table 5.
The practical gain range for this topology as shown is from
5 to 100 because the LTC2420’s wide dynamic range
makes gains below 5 virtually unnecessary, whereas gains
up to 100 significantly reduce the input referred noise.
The optional passive RC lowpass filter between the
amplifier’s output and the LTC2420’s input attenuates
high frequency noise and its effects. Typically, the filter
reduces the magnitude of averaged noise by 30% and
improves resolution by 0.5 bit without compromising
linearity. Resistor R2 performs two functions: it isolates
C1 from the LTC2420’s input and limits the LTC2420’s
input current should its input voltage drop below –300mV
or swing above V
CC
+ 300mV.
The LT1920 is the choice for applications where low cost
is important. For applications where more precision is
required, the LT1167 is a pin-to-pin alternative choice with
a lower offset voltage, lower input bias current and higher
gain accuracy than the LT1920. The LT1920’s maximum
total input-referred offset (V
OST
) is 135µV for a gain of
100. At the same gain, the LT1167’s V
OST
is 63µV. At gains
of 10 or 100, the LT1920’s maximum gain error is 0.3%
and its maximum gain nonlinearity is 30ppm. At the same
gains, the LT1167’s maximum gain error is 0.1% and its
maximum gain nonlinearity is 15ppm. Table 6 summa-
rizes the performance of Figure 37’s circuit using the
LT1167.
Figure 37. The LT1920 is a Simple Solution That Converts a Differential Input
to a Ground Referred Single-Ended Signal for the LTC2420
V
S
+
V
S
–
7
2
1
8
3
4
6
V
IN
+
V
IN
–
R
G
R
G
**
DIFFERENTIAL
INPUT
R
G
3
2
1
††
*OPTIONAL—SEE TEXT
**R
G
= 49.4k/(A
V
– 1): USE 5.49k FOR A
V
= 10; 499Ω FOR A
V
= 100
†
USE SHORT LEAD LENGTHS
†
48
2420 F37
5
6
7
CHIP SELECT
SERIAL DATA OUT
SERIAL CLOCK
CS
SDO
SCK
0.1µF
0.1µF5V
R1*
47Ω
C1*
1µF
SINGLE POINT
“STAR” GROUND
R2*
10k
0.1µF
V
IN
V
REF
V
REFIN
V
CC
GND
LTC2420
F
O
LT1920