8.2.1.2.1 Optimizing The Transimpedance Circuit

To achieve the best performance, components should be selected according to the following guidelines:

1. For lowest noise, select R_(FB) to create the total required gain. Using a lower value for R_(FB) and adding gain after the transimpedance amplifier generally produces poorer noise performance. The noise produced by R_(FB) increases with the square-root of R_(FB), whereas the signal increases linearly. Therefore, signal-to-noise ratio improves when all the required gain is placed in the transimpedance stage.
2. Minimize photodiode capacitance and stray capacitance at the summing junction (inverting input). This capacitance causes the voltage noise of the op amp to be amplified (increasing amplification at high frequency). Using a low-noise voltage source to reverse-bias a photodiode can significantly reduce the capacitance. Smaller photodiodes have lower capacitance. Use optics to concentrate light on a small photodiode.
3. Noise increases with increased bandwidth. Limit the circuit bandwidth to only that required. Use a capacitor across the R_(FB) to limit bandwidth, even if not required for stability.
4. Circuit board leakage can degrade the performance of an otherwise well-designed amplifier. Clean the circuit board carefully. A circuit board guard trace that encircles the summing junction and is driven at the same voltage can help control leakage.

For additional information, refer to the following documents:

• Texas Instruments, Noise Analysis of FET Transimpedance Amplifiers Application Bulletin
• Texas Instruments, Noise Analysis for High-Speed Op Amps Application Report