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 k

Ω 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

amplifier’s input bias current will be reduced. The circuit in

Figure 4 shows how to cancel 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.

Instrumentation Circuits

The input impedance of the previous difference amplifier is

set by the resistor R

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

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 M

Ω. The gain of this instrumen-

tation amplifier is set by the ratio of R

R

affects the CMRR. For good CMRR over temperature, low

drift resistors should be used. Making R

than R

between R

optimum.

10092005

FIGURE 3. Indirectly Driving A Capacitive Load with

DC Accuracy

10092006

FIGURE 4. Cancelling the Error Caused by Input Bias

Current

10092007

FIGURE 5. Difference Amplifier

10092085

FIGURE 6. Three-op-amp Instrumentation Amplifier

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