100 k

VOUT

±

+

0.5 pF

SFH213

5 V

1 F

OPAx357

13.7 k

280

22

OPA357, OPA2357

SBOS235F – MARCH 2002 – REVISED APRIL 2018

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For single-supply applications, the +INx input can be biased with a positive DC voltage to allow the output to

reach true zero when the photodiode is not exposed to any light, and respond without the added delay that

results from coming out of the negative rail; Figure 42 shows this configuration. This bias voltage appears across

the photodiode, providing a reverse bias for faster operation.

Figure 42. Single-Supply Transimpedance Amplifier

For additional information, see the Compensate Transimpedance Amplifiers Intuitively application bulletin.

8.2.1.2.2.1

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.