When using very large value feedback resistors, (usually > 500kΩ) the large feed back resistance can react with the input capacitance due to transducers, photodiodes, 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 7-2), C_f is first estimated by:

Equation 1.

or

which typically provides significant overcompensation.

Printed circuit board stray capacitance can be larger or smaller than that of a breadboard, so the actual optimum value for C_Fcan be different. Check C_F values on the actual circuit. (Refer to the LMC660 quad CMOS amplifier data sheet for a more detailed discussion.)

Figure 7-2 Canceling the Effect of Input Capacitance