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 510 pF for CL..
INPUT BIAS CURRENT CANCELLATION
The LMV321/LMV358/LMV324 family has a bipolar input
stage. The typical input bias current of LMV321/LMV358/
LMV324 is 15 nA with 5V supply. Thus a 100 kΩ input resistor
will cause 1.5 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 6 shows how to cancel the error caused
by input bias current.
10006006
10006099
FIGURE 6. Cancelling the Error Caused by Input Bias
Current
FIGURE 4. Pulse Response of the LMV324 Circuit in
Figure 3
TYPICAL SINGLE-SUPPLY APPLICATION CIRCUITS
Difference Amplifier
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 Fig-
ure 5, RF provides the DC accuracy by using feed-forward
techniques to connect VIN to RL. Caution is needed in choos-
ing the value of RF due to the input bias current of theLMV321/
LMV358/LMV324. CF and RISO serve to counteract the loss
of phase margin by feeding the high frequency component of
the output signal back to the amplifier's inverting input, there-
by preserving phase margin in the overall feedback loop.
Increased capacitive drive is possible by increasing the value
of CF . This in turn will slow down the pulse response.
The difference amplifier allows the subtraction of two voltages
or, as a special case, the cancellation of a signal common to
two inputs. It is useful as a computational amplifier, in making
a differential to single-ended conversion or in rejecting a com-
mon mode signal.
10006007
10006005
10006019
FIGURE 5. Indirectly Driving A Capacitive Load with DC
Accuracy
FIGURE 7. Difference Amplifier
www.national.com
14
TI [ TEXAS INSTRUMENTS ]
