9.2.1.2.1 Optimizing The Transimpedance Circuit
To achieve the best performance, select components according to the following guidelines:
- 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 results in poorer noise performance. R(FB) produces noise that 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.
- 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 frequencies). 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.
- Noise increases with increased bandwidth. Only use the required circuit bandwidth. Use a capacitor across the R(FB) to limit bandwidth, even if a capacitor is not required for stability.
- Circuit board leakage can degrade the performance of an otherwise well-designed amplifier. Clean the circuit board carefully. Control leakage by using a circuit board guard trace that encircles the summing junction and drives at the same voltage.
For additional information, see Noise Analysis of FET Transimpedance Amplifiers and Noise Analysis for High-Speed Op Amps).