Op amps with low input capacitance are required in applications such as smoke detectors, photodiode transimpedance amplifiers, medical instrumentation, industrial control systems, and the piezo-sensor interface. CMOS-input op amps, for instance, require minimal input capacitance when amplifying capacitive-sensor outputs or the small signals from high-impedance sources.
Input capacitance also affects a pole in the feedback path that can cause instability in high-gain, high-frequency applications. By minimizing this input capacitance, you may be able to increase the corresponding pole frequency until it has a negligible effect on the circuit.
Measuring the input capacitance of an op amp isn’t trivial, however; especially if the value is only a few picofarads. Such low values also present difficulties in screening the op amps during production testing. Hence, semiconductor companies often provide only typical values for this parameter, using simulation results and bench measurements on a few known good units. The following discussion can provide a sanity check in the lab by assisting the system-level designer or QA engineer to accurately determine the input capacitance for any op amp.
The direct approach of observing input capacitance on a multimeter isn’t practical below a few nanofarads. A simple yet effective alternative is to insert a large resistor in series with the op-amp input ( Figure 1 ).
Figure 1: A resistor in series with an op amp input enables measurement of the op amp’s input capacitance.
Plotting the frequency response of the resulting first-order lowpass RC filter on a network analyzer (i.e., a Bode plot) lets you calculate the op amp’s input capacitance. Sounds simple, but you must follow precautions to ensure that the measurement accuracy isn’t compromised by stray capacitance in the PC board (PCB) and the test setup.
Follow these tips to minimize stray parasitics:
- Increase the measurement resolution by using only low-capacitance FET probes (<1pF), such as the Tektronix P6245.
- If the series resistor is a surface-mount component, ensure that the board capacitance