Circuit rejects ambient light
Massimo Gottardi, IRST, Trento, Italy -- EDN, June 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).
