LMV321-N, LMV321-N-Q1, LMV358-N, LMV358-N-Q1
LMV324-N, LMV324-N-Q1
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SNOS012I –AUGUST 2000–REVISED FEBRUARY 2013
Figure 55. Difference Amplifier
Instrumentation Circuits
The input impedance of the previous difference amplifier is set by the resistors R1, R2, R3, and R4. To eliminate
the problems of low input impedance, one way is to use a voltage follower ahead of each input as shown in the
following two instrumentation amplifiers.
Three-Op-Amp Instrumentation Amplifier
The quad LMV324 can be used to build a three-op-amp instrumentation amplifier as shown in Figure 56.
Figure 56. Three-Op-Amp Instrumentation Amplifier
The first stage of this instrumentation amplifier is a differential-input, differential-output amplifier, with two voltage
followers. These two voltage followers assure that the input impedance is over 100 MΩ. The gain of this
instrumentation amplifier is set by the ratio of R2/R1. R3 should equal R1, and R4 equal R2. Matching of R3 to R1
and R4 to R2 affects the CMRR. For good CMRR over temperature, low drift resistors should be used. Making R4
slightly smaller than R2 and adding a trim pot equal to twice the difference between R2 and R4 will allow the
CMRR to be adjusted for optimum performance.
Two-Op-Amp Instrumentation Amplifier
A two-op-amp instrumentation amplifier can also be used to make a high-input-impedance DC differential
amplifier (Figure 57). As in the three-op-amp circuit, this instrumentation amplifier requires precise resistor
matching for good CMRR. R4 should equal R1 and, R3 should equal R2.
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