input bias current will be reduced. The circuit in Figure 6
shows how to cancel the error caused by input bias current.
Application Notes (Continued)
In Figure 3 , the isolation resistor RISO and the load capacitor
CL form a pole to increase stability by adding more phase
margin to the overall system. The desired performance de-
pends on the value of RISO. The bigger the RISO resistor
value, the more stable VOUT will be. Figure 4 is an output
waveform of Figure 3 using 620Ω for RISO and 510pF for CL..
10006006
FIGURE 6. Cancelling the Error Caused by Input Bias
Current
4.0 TYPICAL SINGLE-SUPPLY APPLICATION CIRCUITS
4.1 Difference Amplifier
Time (2µs/div)
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.
10006099
FIGURE 4. Pulse Response of the LMV324 Circuit in
Figure 3
The circuit in Figure 5 is an improvement to the one in Figure
3 because it provides DC accuracy as well as AC stability. If
there were a load resistor in Figure 3, the output would be
voltage divided by RISO and the load resistor. Instead, in
Figure 5, RF provides the DC accuracy by using feed-
forward techniques to connect VIN to RL. Caution is needed
in choosing the value of RF due to the input bias current of
the LMV321/358/324. CF and RISO serve to counteract the
loss of phase margin by feeding the high frequency compo-
nent of the output signal back to the amplifier’s inverting
input, thereby preserving phase margin in the overall feed-
back loop. Increased capacitive drive is possible by increas-
ing the value of C . This in turn will slow down the pulse
F
10006007
response.
10006019
FIGURE 7. Difference Amplifier
4.2 Instrumentation Circuits
The input impedance of the previous difference amplifier is
set by the resistors 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.
10006005
FIGURE 5. Indirectly Driving A Capacitive Load with
DC Accuracy
3.0 INPUT BIAS CURRENT CANCELLATION
The LMV321/358/324 family has a bipolar input stage. The
typical input bias current of LMV321/358/324 is 15nA with 5V
supply. Thus a 100kΩ input resistor will cause 1.5mV of error
voltage. By balancing the resistor values at both inverting
and non-inverting inputs, the error caused by the amplifier’s
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