To achieve the best performance, select components
according to the following guidelines:
- For the
lowest noise, select RF to create the
total required gain. A lower value for RF
and adding gain after the transimpedance amplifier
generally produces poorer noise performance. The
noise produced by RF increases with the
square-root of RF; 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 op-amp voltage noise to be amplified
(increased amplification at high frequency). Use a
low-noise voltage source to reverse-bias a
photodiode to significantly reduce capacitance.
Smaller photodiodes have lower capacitance. Use
optics to concentrate light on a small
photodiode.
- Noise
increases with increased bandwidth. Limit the
circuit bandwidth to only that required. Use a
capacitor across the RF to limit
bandwidth, even if not required for stability.
- 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 helps to control leakage.
For more information, see the Noise Analysis of FET Transimpedance
Amplifiers and the Noise Analysis for High-Speed Op Amps
application reports.