SBOS618I December 2013 – May 2018 OPA172 , OPA2172 , OPA4172

PRODUCTION DATA.

- 1 Features
- 2 Applications
- 3 Description
- 4 Revision History
- 5 Device Comparison
- 6 Pin Configuration and Functions
- 7 Specifications
- 8 Detailed Description
- 9 Applications and Implementation
- 10Power-Supply Recommendations
- 11Layout
- 12Device and Documentation Support
- 13Mechanical, Packaging, and Orderable Information

Figure 47 depicts a unity-gain buffer driving a capacitive load. Equation 1 shows the transfer function for the circuit in Figure 47. Not depicted in Figure 47 is the open-loop output resistance of the op amp, 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}. Components R_{ISO} and C_{LOAD} determine the frequency of the zero (f_{z}). A stable system is obtained by selecting R_{ISO} such that the rate of closure (ROC) between the open-loop gain (A_{OL}) and 1 / β is 20 dB per decade. Figure 48 shows the concept. Note that 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 4 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 OPA172, refer to the precision design, *Capacitive Load Drive Solution using an Isolation Resistor* (TIPD128).

PHASE MARGIN | OVERSHOOT | AC GAIN PEAKING |
---|---|---|

45° | 23.3% | 2.35 dB |

60° | 8.8% | 0.28 dB |