Datasheet

ADC121S021
IN
V
+
LM4140A
6
2
1,4,7,8
3
V
+
1 PF
+
-
0.1 PF
10 PF
V
A
V
+
+
-
+
-
-
+
12 k:
10 k:
40 k:
½
LMP2232
R+'R
R+'R
R
R
V
+
V
+
V
+
12 k:
10 k:
40 k:
1 k:
LMP2231
½
LMP2232
LMP2231
GND
EXCITATION
SOURCE
R
1
R
2
R
3
R
4
V
OUT
(a)
V
OUT
1 +
R
3
R
1
¨
¨
©
§
¨
¨
©
§
1 +
R
4
R
2
¨
¨
©
§
¨
¨
©
§
R
3
R
1
-
R
4
R
2
x V
SOURCE
=
EXCITATION
SOURCE
R + 'R
R - 'R
R - 'R R + 'R
V
OUT
(b)
V
OUT
=
'R
R
x V
SOURCE
LMP2231
www.ti.com SNOSB01E –JANUARY 2008–REVISED MARCH 2013
Figure 51 (a) shows a typical bridge sensor and Figure 51(b) shows the bridge with four sensors. R in
Figure 51(b) is the nominal value of the sense resistor and the deviations from R are proportional to the quantity
being measured.
Figure 51. Bridge Sensor
Instrumentation amplifiers are great for interfacing with bridge sensors. Bridge sensors often sense a very small
differential signal in the presence of a larger common mode voltage. Instrumentation amplifiers reject this
common mode signal.
Figure 52 shows a strain gauge bridge amplifier. In this application the LMP2231 is used to buffer the LM4140's
precision output voltage. The LM4140A is a precision voltage reference. The other three LMP2231s are used to
form an instrumentation amplifier. This instrumentation amplifier uses the LMP2231's high CMRR and low V
OS
and TCV
OS
to accurately amplify the small differential signal generated by the output of the bridge sensor. This
amplified signal is then fed into the ADC121S021 which is a 12-bit analog to digital converter. This circuit works
on a single supply voltage of 5V.
Figure 52. Strain Gauge Bridge Amplifier
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