ZHCSJO8 May 2019 OPA818

ADVANCE INFORMATION for pre-production products; subject to change without notice.

With high GBWP, low input voltage and current noise, and low input capacitance, the OPA818 design is optimized for wideband, low-noise transimpedance applications. The high voltage capability allows greater flexibility of supply voltages along with wider output voltage swings. Figure 13 shows an example circuit of a typical photodiode amplifier circuit. Generally the photodiode is reverse biased in a TIA application so the photodiode current in the circuit ofFigure 13 flows into the op amp feedback loop resulting in an output voltage that reduces from V_{REF} with increasing photodiode current. In this type of configuration and depending on the application needs, V_{REF} can be biased closer to V_{S+} to achieve the desired output swing. Input common-mode range must be considered so as not to violate it when V_{REF} bias is used.

The key design elements that determine the closed-loop bandwidth, f_{–3dB}, of the circuit are below:

- The op amp GBWP
- The transimpedance gain, R
_{F}, and, - The total input capacitance, CTOT, that includes photodiode capacitance, input capacitance of the amplifier (common-mode and differential capacitance), and PCB parasitic capacitance

Equation 1 shows the relationship between the above mentioned three elements for a Butterworth response.

Equation 1.

The feedback resistance R_{F} and the total input capacitance C_{TOT} cause a zero in the noise gain that results in instability if left uncompensated. To counteract the effect of the zero, a pole is inserted in the noise gain by adding the feedback capacitor, C_{F}. . The *Transimpedance Considerations for High-Speed Amplifiers* application report discusses theories and equations that show how to compensate a transimpedance amplifier for a particular gain and input capacitance. The bandwidth and compensation equations from the application report are available in a Microsoft Excel™ calculator. *What You Need To Know About Transimpedance Amplifiers – Part 1* provides a link to the calculator.