loading reduces the phase margin of op-amps. The combi-

nation of the op-amp’s output impedance and the capacitive

load induces phase lag. This results in either an under-

damped pulse response or oscillation.

Capacitive load compensation can be accomplished using

resistive isolation as shown in

Figure 4. This simple tech-

nique is useful for isolating the capacitive input of multiplex-

ers and A/D converters.

Improved frequency response is achieved by indirectly driv-

ing capacitive loads as shown in

Figure 6.

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 amplifier’s 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.

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 LMC6484. Large feedback resistors can react with small

values of input capacitance due to transducers, photo-

diodes, and circuit board parasitics to reduce phase

margins.

The effect of input capacitance can be compensated for by

adding a feedback capacitor. The feedback capacitor (as in

Figure 8 ), C

or

R

which typically provides significant overcompensation.

Printed circuit board stray capacitance may be larger or

smaller than that of a breadboard, so the actual optimum

value for C

checked on the actual circuit. (Refer to the LMC660 quad

CMOS amplifier data sheet for a more detailed discussion.)

6.0 Printed-Circuit-Board Layout for High-Impedance

Work

It is generally recognized that any circuit which must operate

with less than 1000 pA of leakage current requires special

layout of the PC board. when one wishes to take advantage

DS011714-17

FIGURE 4. Resistive Isolation

of a 330 pF Capacitive Load

DS011714-18

FIGURE 5. Pulse Response of

the LMC6484 Circuit in

Figure 4

DS011714-15

FIGURE 6. LMC6484 Non-Inverting Amplifier,

Compensated to Handle a 330 pF Capacitive Load

DS011714-16

FIGURE 7. Pulse Response of

LMC6484 Circuit in

Figure 6

DS011714-19

FIGURE 8. Canceling the Effect of Input Capacitance

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