7.3.6 Driving Capacitive Loads

The OPAx837 can drive a parasitic load capacitance up through 4 pF on the output with no special considerations. When driving capacitive loads greater than 4 pF, TI recommends using a small resistor (R_O) in series with the output as close to the device as possible. Without R_O, output capacitance interacts with the output impedance of the amplifier causing phase shift in the loop gain of the amplifier that reduces the phase margin. This reduction causes peaking in the frequency response and overshoot and ringing in the pulse response. Inserting R_O isolates the phase shift from the loop-gain path and restores the phase margin; however R_O can also limit bandwidth to the capacitive load.

Figure 66 shows the test and Figure 49 illustrates the recommended values of R_O versus capacitive loads, C_L using a 30° phase margin target for the op amp. See Figure 50 for the frequency responses with various values of C_L and R_OUT parametric on gain.

Figure 66. R_OUT versus C_L Test Circuit