The OPAx328 are designed for use in high-speed applications for transimpedance amplifiers (TIA) and ADC input-driving amplifiers. As with all op amps, there can be specific instances where the OPAx328 become unstable. 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. An op amp in the unity-gain (1‑V/V) buffer configuration driving a capacitive load exhibits a greater tendency to become unstable compared to an amplifier operating at a higher noise gain (see Figure 5-28). The capacitive load, in conjunction with the op-amp output impedance, creates a pole within the loop gain that degrades the phase margin. The degradation of the phase margin increases as the capacitive loading increases. When operating in the unity-gain configuration, the OPAx328 remain stable with a pure capacitive load up to 100 pF.

Figure 7-1 shows one technique to increase the capacitive load drive capability of an amplifier operating in a unity-gain configuration is to insert a small resistor (R_S), typically 10 Ω to 50 Ω, in series with the output. This resistor significantly reduces the overshoot and ringing associated with large capacitive loads.

Figure 7-1 Improving Capacitive Load Drive