I've built an LED driver and a photodetecting circuit.
Basic forms of the driver and the detector were made.
FYI: Parts used and their datasheets
- LED: Seoul semiconductor W42182 (datasheet)
- Photodiode: Hamamatsu Si PIN photodiode S6036 (datasheet)
- Op-amp: TI TL792IP (datasheet)
In this configuration, I want to driver an LED and detect it as fast as hundreds of kHz.
But, output from the photodetector shows a sawtooth form not a square form even with a very low frequency, 10 Hz:
The blue line is the
V_OUT in the dirver circuit, the photodiode outputs,
V_OUT in the detector circuit drawn with the yellow line.
As you can see a **slew rate of the detector is not fast as the driver. So I made a list of candidates that may cause this problem.
Candidates of the slow slew rate problem
V_OUTin the driver is different from an actual voltage value across the LED. (i.e. I am probing a wrong point. And brightness of the LED may not be a square form as the blue line.)
- Reverse voltage applied on the photodiode is not sufficient. In datasheet, BW > 10 MHz when
V_R= 12 V. Thus, I need to apply 12 V as the reverse voltage across the photodiode.
R1(gain) in the driver is too large. So, I need multistage low gain op-amps.
- Photoconductive mode doesn't suit. It's better to use photovoltaic, etc. (But, I think photoconductive mode is nothing wrong.)
- (Si PIN) photodiode doesn't suit. It's better to use photoresistor, phototransistor, etc.
- (Feedback or any) capacitors have too low/high capacitance.
Which one do you think is the keypoint of the problem? If I got everything wrong, please correct me.