The OPAx197-Q1 feature a patented output stage capable of driving large capacitive loads, and in a unity-gain configuration, directly drive up to 1 nF of pure capacitive load. Increasing the gain enhances the ability of these amplifiers to drive greater capacitive loads; see Figure 7-7 and Figure 7-8. The particular op-amp circuit configuration, layout, gain, and output loading are some of the factors to consider when establishing whether an amplifier is stable in operation.

Figure 7-7 Small-Signal Overshoot vs Capacitive Load (100-mV Output Step)

Figure 7-8 Small-Signal Overshoot vs Capacitive Load (100-mV Output Step)

For additional drive capability in unity-gain configurations, improve capacitive load drive by inserting a small (10-Ω to 20-Ω) resistor, R_ISO, in series with the output, as shown in Figure 7-9. This resistor significantly reduces ringing and maintains dc performance for purely capacitive loads. However, if a resistive load is in parallel with the capacitive load, then a voltage divider is created, thus 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. A high capacitive load drive makes the OPAx197-Q1 a great choice for applications such as reference buffers, MOSFET gate drives, and cable-shield drives. The circuit shown in Figure 7-9 uses an isolation resistor, R_ISO, to stabilize the output of an op amp. R_ISO modifies the open-loop gain of the system for increased phase margin, and results using the OPAx197-Q1 are summarized in Table 7-2. For additional information on techniques to optimize and design using this circuit, TI Precision Design TIPD128 details complete design goals, simulation, and test results.

Figure 7-9 Extending Capacitive Load Drive With the OPAx197-Q1