The OPAx323 is designed for use in applications where driving a capacitive load is required. As with all operational amplifiers, there can be specific instances where the device can become unstable. The particular operational amplifier circuit configuration, layout, gain, and output loading are some of the factors to consider when establishing whether or not an amplifier is stable in operation.

An operational amplifier in the unity-gain (1V/V) buffer configuration that drives a capacitive load exhibits a greater tendency to be unstable than an amplifier operating at a higher noise gain. The capacitive load, in conjunction with the operational amplifier output resistance, creates a pole within the feedback loop that degrades the phase margin. The degradation of the phase margin increases when capacitive loading increases. When operating in the unity-gain configuration, the OPAx323 has a phase margin of 50° with 30pF of capacitive load. The device remains stable with pure capacitive loads up to approximately 75pF with acceptable phase margin of 35° and has no sustained oscillations up to 150pF. The equivalent series resistance (ESR) of some very large capacitors (greater than 1μF) is sometimes sufficient to alter the phase characteristics in the feedback loop such that the amplifier remains stable. Increasing the amplifier closed-loop gain allows the amplifier to drive increasingly larger capacitance. This increased capability is evident when measuring the overshoot response of the amplifier at higher voltage gains.

One technique for increasing the capacitive load drive capability of the amplifier operating in a unity-gain configuration is to insert a small resistor (typically 10Ω to 50Ω) in series with the output, as shown in Figure 7-3. This resistor significantly reduces the overshoot and ringing associated with large capacitive loads. This is usually the circuit configuration used in ADC driver application with C_load serving as a charge bucket for the ADC sampling capacitor.

Figure 7-3 Improving Capacitive Load Drive