7.3.2 Feedback Capacitor Improves Response

For optimum settling time and stability with high-impedance feedback networks, adding a feedback capacitor across the feedback resistor, R_(FB), as shown in Figure 30 may be necessary. This capacitor compensates for the zero created by the feedback network impedance and the OPAx320-Q1 input capacitance (and any parasitic layout capacitance). The effect becomes more significant with higher impedance networks.

NOTE:

Where C_(IN) is equal to the OPAx320-Q1 input capacitance (approximately 9 pF) plus any parasitic layout capacitance.

Figure 30. Feedback Capacitor Improves Dynamic Performance

It is suggested that a variable capacitor be used for the feedback capacitor because input capacitance may vary between op amps and layout capacitance is difficult to determine. For the circuit shown in Figure 30, the value of the variable feedback capacitor should be chosen so that the input resistance times the input capacitance of the OPAx320-Q1 (9 pF, typical) plus the estimated parasitic layout capacitance equals the feedback capacitor times the feedback resistor:

The capacitor value can be adjusted until optimum performance is obtained.