SBOS557E August 2011 – April 2018 OPA170 , OPA2170 , OPA4170

PRODUCTION DATA.

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

- D|14

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}. 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/decade. Figure 42 depicts 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 OPA170, see the Precision Design, *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 |