LMV321M7SLASHNOPB datasheet(17/44 Pages) TI1

17 / 44 page

LMV321-N, LMV321-N-Q1, LMV358-N, LMV358-N-Q1

LMV324-N, LMV324-N-Q1

www.ti.com

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 M

Ω. 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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Product Folder Links: LMV321-N LMV321-N-Q1 LMV358-N LMV358-N-Q1 LMV324-N LMV324-N-Q1

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