8.2.2 Detailed Design Procedure

Figure 41 shows a unity-gain buffer driving a capacitive load. Equation 1 shows the transfer function for the circuit in Figure 41. Not shown in Figure 41 is the open-loop output resistance of the operational amplifier, R_O.

Equation 1.

The transfer function in Equation 1 has a pole and a zero. The frequency of the pole (f_p) is determined by (R_O + R_ISO) and C_LOAD. R_ISO and C_LOAD determine the frequency of the zero (f_z). A stable system is obtained by selecting R_ISO, so the rate of closure (ROC) between the open-loop gain (A_OL) and 1/β is 20 dB / decade. Figure 42 depicts the concept. The 1/β curve for a unity-gain buffer is 0 dB.

Figure 42. Unity-Gain Amplifier With R_ISO Compensation

ROC stability analysis is typically simulated. The validity of the analysis depends on multiple factors, especially the accurate modeling of R _O . 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 6 shows the overshoot percentage and ac gain peaking that correspond to 45° and 60° phase margins. For more details on this design and other alternative devices that can be used in place of the OPAx170-Q1 family, see Capacitive Load Drive Solution Using an Isolation Resistor.