The OPA928 features a patented output stage capable of driving large capacitive loads, and in a unity-gain configuration, directly drives up to 1 nF of pure capacitive load. Increasing the gain enhances the ability of the amplifier to drive greater capacitive loads; see Figure 7-1. The particular op-amp circuit configuration, layout, gain, and output loading are some of the factors to consider when establishing whether an amplifier will be stable in operation.

Figure 7-1 Transient Response With a Purely Capacitive Load of 1 nF

For additional drive capability in unity-gain configurations, improve capacitive load drive by inserting a small, 10‑Ω to 20‑Ω isolation resistor (R_ISO) in series with the output; Figure 7-2 shows this resistor. This resistor significantly reduces ringing while maintaining dc performance for purely capacitive loads. However, if there is a resistive load in parallel with the capacitive load, a voltage divider is created, introducing a gain error at the output and slightly reducing the output swing. The error introduced is proportional to the ratio R_ISO / R_L, and is generally negligible at low output levels. R_ISO modifies the open-loop gain of the system for increased phase margin.

Figure 7-2 Extending Capacitive Load Drive With the OPA928