For an amplifier directly driving a capacitive load, the amplifier is prone to oscillations as a result of the additional phase shift introduced in the loop-gain expression by the amplifier open-loop output impedance and the capacitive load. For a current-feedback amplifier such as the THS3491, the open-loop output impedance and the capacitive load introduce a pole in the open-loop transimpedance gain response. If the pole is at a frequency lower than the non-dominant pole of the amplifier, then the transimpedance loop-gain is reduced and the phase margin is reduced. To counteract the effect of this pole, a series isolation resistor (R_S) is used between the device output and the capacitive load that introduces a zero in the response. TI recommends placing R_S close to the device output to avoid the printed circuit board (PCB) trace parasitic affecting the frequency response of the amplifier.

Depending upon the capacitive load, and as shown in Figure 2-7, R_S must be adjusted for a flat frequency response. The inclusion of R_S can result in an increased voltage drop across the series resistor at higher output currents and limits the available output voltage swing at the capacitive load. However, when selecting the series isolation resistance, stability must be of greater concern than output voltage drop.