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:
½
LMP2232
GND
½
LMP2232
½
LMP2232
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
LMP2232
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SNOSB02C –JANUARY 2008–REVISED MARCH 2013
Figure 50(a) shows a typical bridge sensor and Figure 50(b) shows the bridge with four sensors. R in
Figure 50(b) is the nominal value of the sense resistor and the deviations from R are proportional to the quantity
being measured.
Figure 50. 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 51 shows a strain gauge bridge amplifier. In this application one of the LMP2232 amplifiers is used to
buffer the LM4140A's precision output voltage. The LM4140A is a precision voltage reference. The other three
amplifiers in the LMP2232 are used to form an instrumentation amplifier. This instrumentation amplifier uses the
LMP2232'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 51. Strain Gage Bridge Amplifier
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