Circuit rejects ambient light

Massimo Gottardi, IRST, Trento, Italy -- EDN, 6/10/1999

In some applications, you need to detect light signals in the presence of background light whose intensity can change by orders of magnitude. The circuit in Figure 1 uses an integrated photodiode/amplifier (OPT201) in conjunction with an integrator that drives two linear optocouplers (TIL300). The optocouplers subtract the background-light-generated current from the current the optical sensor produces. C₂ integrates any dc signal present at the output of IC₁. The output of IC₂ drives two optocouplers, IC₃ and IC₄. The four photodiodes in both TIL300s connect in parallel with the diode in the OPT201 to subtract the dc component from the signal path. IC₃ and IC₄ need 60-mA drive to produce rejection current as high as 1.5 mA. This design uses the optocoupler configuration instead of a simple resistor, because the optocouplers' nonlinear characteristic allows a higher current range without substantially affecting the noise gain of the transimpedance amplifier. The total output noise in the circuit is 80 µV rms. A 6.8-kW resistor can reject the same 1.5-mA dc current, but it produces 200-µV output noise. (DI #2364).

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