LMV721 datasheet(10/15 Pages) NSC

10 / 15 page

Application Notes (Continued)

4.0 TYPICAL SINGLE-SUPPLY APPLICATION CIRCUITS

4.1 Difference Amplifier

The difference amplifier allows the subtraction of two volt-

ages or, as a special case, the cancellation of a signal

common to two inputs. It is useful as a computational ampli-

fier, in making a differential to single-ended conversion or in

rejecting a common mode signal.

4.2 Instrumentation Circuits

The input impendance of the previous difference amplifier is

set by the resistor R

1,R2,R3 and R4. To eliminate the

problems of low input impendance, one way is to use a

voltage follower ahead of each input as shown in the follow-

ing two instrumentation amplifiers.

4.2.1 Three-op-amp Instrumentation Amplifier

The LMV721/722 can be used to build a three-op-amp in-

strumentation amplifier as shown in Figure 6

The first stage of this instrumentation amplifier is a

differential-input, differential-output amplifier, with two volt-

age followers. These two voltage followers assure that the

input impedance is over 100M

Ω. The gain of this instrumen-

tation amplifier is set by the ratio of R

2/R1.R3 should equal

1 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 R

4 slightly smaller

than R

2 and adding a trim pot equal to twice the difference

between R

2 and R4 will allow the CMRR to be adjusted for

optimum.

4.2.2 Two-op-amp Instrumentation Amplifier

A two-op-amp instrumentation amplifier can also be used to

make a high-input impedance DC differential amplifier (Fig-

ure 7). As in the two-op-amp circuit, this instrumentation

amplifier requires precise resistor matching for good CMRR.

4 should equal to R1 and R3 should equal R2.

4.3 Single-Supply Inverting Amplifier

There may be cases where the input signal going into the

amplifier is negative. Because the amplifier is operating in

single supply voltage, a voltage divider using R

3 and R4 is

implemented to bias the amplifier so the input signal is within

the input common-common voltage range of the amplifier.

10092220

FIGURE 4. Cancelling the Error Caused by Input Bias

Current

10092221

FIGURE 5. Difference Application

10092230

FIGURE 6. Three-op-amp Instrumentation Amplifier

10092222

FIGURE 7. Two-op-amp Instrumentation Amplifier

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