The OPS can drive heavy capacitive loads very well, as shown in Figure 6-43 to Figure 6-48. All high-speed amplifiers benefit from the addition of an external series resistor to isolate the load capacitor from the feedback loop. Not using a series isolation resistor often leads to response peaking and possibly oscillation. If frequency response flatness under capacitive load is the design goal, use slightly higher R_F values at the lower gains. Target a slightly-higher feedback transimpedance to increase the nominal phase margin before the capacitive load acts to decrease it. Using a higher R_F value increases the frequency response flatness across a range of capacitive loads using lower external series resistor values. Although the suggested R_F and R_G values of Table 8-1 apply when driving a 100 Ω load, if the intended load is capacitive (for example, a passive filter with a shunt capacitor as the first element, another amplifier, or a Piezo element), use the values reported in Table 8-6 as a starting point. The values in Table 8-6 were used to generate Figure 6-43 and Figure 6-44. The results come from a nominal total feedback transimpedance target of 405 Ω (versus 351 Ω used for Table 8-3), and includes the internal 18.5 kΩ resistor in the design. Table 8-6 finds the least error to target gain in the selection of standard resistor values, and limits the minimum R_G to 20 Ω. The gains calculated here put 18.5 kΩ in parallel with the reported external standard value R_F.

As the capacitive load or amplifier gain increases, the series resistor values can be reduced to hold a flat response (see Figure 6-43). See Figure 6-44 for the measured SSBW shapes for various capacitive loads configured with the suggested series resistor from the output of the OPS and the R_F and R_G values suggested in Table 8-6 for a gain of 2.5 V/V. This measurement includes a 200 Ω shunt resistor in parallel with the capacitive load as a measurement path.

Figure 6-45 and Figure 6-46 demonstrate the OPS harmonic distortion performance when driving a range of capacitive loads. These figures show suitable performance for large-signal, piezo-driver applications. If voltage swings higher than 12 V_PP are required, consider driving the OPS output into a step-up transformer. The high peak-output current for the OPS supports very fast charging edge rates into heavy capacitive loads, as shown in the step response plots (see Figure 6-47 and Figure 6-48). This peak current occurs near the center of the transition time driving a capacitive load. Therefore, the I × R drop to the capacitive load through the series resistor is at a maximum at midtransition, and 0 V at the extremes (low dV/dT points). For even better performance driving heavy capacitive loads, consider using the THS3217, a DAC output amplifier with higher output current and slew rate.