Application Notes
(Continued)
loop. Increased capacitive drive is possible by increasing the
value of C
F
. This in turn will slow down the pulse response.
DS100922-21
DS100922-19
FIGURE 3. Indirectly Driving A Capacitive Load with
DC Accuracy
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 100kΩ 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.
FIGURE 5. Difference Application
4.2 Instrumentation Circuits
The input impendance of the previous difference amplifier is
set by the resistor R
1
, R
2
, R
3
and R
4
. To eliminate the prob-
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
DS100922-20
FIGURE 4. Cancelling 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.
DS100922-30
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 100MΩ. The gain of this instrumen-
tation amplifier is set by the ratio of R
2
/R
1
. R
3
should equal
R
1
and R
4
equal R
2
. Matching of R
3
to R
1
and R
4
to R
2
af-
fects 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 R
4
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 am-
plifier requires precise resistor matching for good CMRR. R
4
should equal to R
1
and R
3
should equal R
2
.
9
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