LPV321 Datasheet, PDF(13/22 Page) National Semiconductor (TI) – General Purpose, Low Voltage, Low Power, Rail-to-Rail Output Operational Amplifiers

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LPV321 Datasheet, PDF (13/22 Pages) National Semiconductor (TI) – General Purpose, Low Voltage, Low Power, Rail-to-Rail Output Operational Amplifiers

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Application Information (Continued)

10092005

FIGURE 3. Indirectly Driving A Capacitive Load with

DC Accuracy

INPUT BIAS CURRENT CANCELLATION

The LPV321/358/324 family has a bipolar input stage. The

typical input bias current of LPV321/358/324 is 1.5 nA with

5V supply. Thus a 100 kâ¦ input resistor will cause 0.15 mV

of error voltage. By balancing the resistor values at both

inverting and non-inverting inputs, the error caused by the

amplifierâs input bias current will be reduced. The circuit in

Figure 4 shows how to cancel the error caused by input bias

current.

10092007

FIGURE 5. Difference Amplifier

Instrumentation Circuits

The input impedance of the previous difference amplifier is

set by the resistor 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 follow-

ing two instrumentation amplifiers.

Three-op-amp Instrumentation Amplifier

The quad LPV324 can be used to build a three-op-amp

instrumentation amplifier as shown in Figure 6

10092006

FIGURE 4. Cancelling the Error Caused by Input Bias

Current

TYPICAL SINGLE-SUPPLY APPLICATION CIRCUITS

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.

10092085

FIGURE 6. Three-op-amp Instrumentation Amplifier

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 100 Mâ¦. The gain of this instrumen-

tation 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 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.

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