The OPA838 can drive small capacitive loads directly without oscillation (less than 6@pF). When driving capacitive loads greater than 6 pF, Figure 6-49 illustrates the recommended R_OUT vs capacitor load parametric on gains. At higher gains, the amplifier starts with greater phase margin into a resistive load and can operate with lower R_OUT for a given capacitive load. Without R_OUT, output capacitance interacts with the output impedance of the amplifier, which causes phase shift in the loop gain of the amplifier that reduces the phase margin. This causes peaking in the frequency response with overshoot and ringing in the pulse response. Figure 6-49 targets a 30° phase margin for the OPA838. A 30° phase margin produces a 5.7‑dB peaking in the frequency response at the amplifier output pin that is rolled off by the output RC pole; see Figure 7-7. This peaking can cause clipping for large signals driving a capacitive load. Increasing the R_OUT value can reduce the peaking at the cost of a more band-limited overall response.

Figure 7-6 R_OUT versus C_L Test Circuit

Figure 7-7 Frequency Response to Output Pin and Capacitive Load