Application Information (Continued)
01171317
FIGURE 4. Resistive Isolation
of a 330pF Capacitive Load
01171316
FIGURE 7. Pulse Response of
LMC6482 Circuit in Figure 6
5.0 COMPENSATING FOR INPUT CAPACITANCE
It is quite common to use large values of feedback resis-
tance with amplifiers that have ultra-low input current, like
the LMC6482. Large feedback resistors can react with small
values of input capacitance due to transducers, photo-
diodes, and circuits board parasitics to reduce phase mar-
gins.
01171318
FIGURE 5. Pulse Response of
the LMC6482 Circuit in Figure 4
Improved frequency response is achieved by indirectly driv-
ing capacitive loads, as shown in Figure 6.
01171319
FIGURE 8. Canceling the Effect of Input Capacitance
The effect of input capacitance can be compensated for by
adding a feedback capacitor. The feedback capacitor (as in
Figure 8), Cf, is first estimated by:
01171315
FIGURE 6. LMC6482 Noninverting Amplifier,
Compensated to Handle a 330pF Capacitive Load
or
R1 CIN ≤ R2 Cf
which typically provides significant overcompensation.
R1 and C1 serve to counteract the loss of phase margin by
feeding forward the high frequency component of the output
signal back to the amplifiers inverting input, thereby preserv-
ing phase margin in the overall feedback loop. The values of
R1 and C1 are experimentally determined for the desired
pulse response. The resulting pulse response can be seen in
Figure 7.
Printed circuit board stray capacitance may be larger or
smaller than that of a bread-board, so the actual optimum
value for Cf may be different. The values of Cf should be
checked on the actual circuit. (Refer to the LMC660 quad
CMOS amplifier data sheet for a more detailed discussion.)
15
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