SBOS516F September 2010 – April 2018 OPA171 , OPA2171 , OPA4171

PRODUCTION DATA.

- 1 Features
- 2 Applications
- 3 Description
- 4 Revision History
- 5 Pin Configuration and Functions
- 6 Specifications
- 7 Detailed Description
- 8 Application and Implementation
- 9 Power Supply Recommendations
- 10Layout
- 11Device and Documentation Support
- 12Mechanical, Packaging, and Orderable Information

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, R_{o}.

Equation 1.

The transfer function in Equation 1 contains a pole and a zero. The frequency of the pole (f_{p}) is determined by (R_{o} + R_{ISO}) and C_{LOAD}. Components R_{ISO} and C_{LOAD} determine the frequency of the zero (f_{z}). Select R_{ISO} such that the rate of closure (ROC) between the open-loop gain (A_{OL}) 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 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 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 |
---|---|---|

45° | 23.3% | 2.35 dB |

60° | 8.8% | 0.28 dB |