loop. Increased capacitive drive is possible by increasing the

value of C

3.0 Input Bias Current Cancellation

The LMV721/722 family has a bipolar input stage. The typi-

cal input bias current of LMV721/722 is 260nA with 5V sup-

ply. Thus a 100k

Ω input resistor will cause 26mV 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.

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 com-

mon to two inputs. It is useful as a computational amplifier, in

making a differential to single-ended conversion or in reject-

ing a common mode signal.

4.2 Instrumentation Circuits

The input impendance of the previous difference amplifier is

set by the resistor R

lems of low input impendance, one way is to use a voltage

follower ahead of each input as shown in the following 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

R

fects the CMRR. For good CMRR over temperature, low drift

resistors should be used. Making R

and adding a trim pot equal to twice the difference between

R

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 am-

plifier requires precise resistor matching for good CMRR. R

4

should equal to R

DS100922-19

FIGURE 3. Indirectly Driving A Capacitive Load with

DC Accuracy

DS100922-20

FIGURE 4. Cancelling the Error Caused by Input Bias

Current

DS100922-21

FIGURE 5. Difference Application

DS100922-30

FIGURE 6. Three-op-amp Instrumentation Amplifier

www.national.com

9