9.2.2.1 Optimizing the Transimpedance Circuit

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

1. For lowest noise, select R_(F) to create the total required gain. Using a lower value for R_(F) and adding gain after the transimpedance amplifier generally produces poorer noise performance. The noise produced by R_(F) increases with the square-root of R_(F), 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 amplify (increasing amplification at high frequency). Using a low-noise voltage source to reverse-bias a photodiode 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 the required bandwidth. Use a capacitor across the R_(F) to limit bandwidth, even if a capacitor not required for stability.

4. Circuit board leakage degrades 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 helps control leakage.