Figure 42 shows a unity-gain buffer driving a capacitive load. Equation 1 shows the transfer function for the circuit in Figure 42. Not shown in Figure 42 is the open-loop output resistance of the operational amplifier, Ro.
The transfer function in Equation 1 contains a pole and a zero. The frequency of the pole (fp) is determined by (Ro + RISO) and CLOAD. Components RISO and CLOAD determine the frequency of the zero (fz). Select RISO such that the rate of closure (ROC) between the open-loop gain (AOL) and 1/β is 20 dB/decade to obtain a stable system. Figure 42 shows the concept. The 1/β curve for a unity-gain buffer is 0 dB.
ROC stability analysis is typically simulated. The validity of the analysis depends on multiple factors, especially the accurate modeling of Ro. In addition to simulating the ROC, a robust stability analysis includes a measurement of overshoot percentage and AC gain peaking of the circuit using a function generator, oscilloscope, and gain and phase analyzer. Phase margin is then calculated from these measurements. Table 3 shows the overshoot percentage and AC gain peaking that correspond to phase margins of 45° and 60°. For more details on this design and other alternative devices that can be used in place of the OPAx171, see Capacitive Load Drive Solution using an Isolation Resistor.
|PHASE MARGIN||OVERSHOOT||AC GAIN PEAKING|