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Topic: modulated photodetection circuit (Read 701 times) previous topic - next topic


Hi all,

I'm wanting to build a circuit that measures light absorption through a material.  A LED is modulated in the kHz range (via a digital out line from the UNO) and a photodiode circuit measures the signal on the other side of the material i'm measuring (using a LED in the red).  I then want to read in the signal with a 16 bit ADC controlled via a SPI interface.  Ideally I'd like to measure both the AC (modulated) and DC components into the ADC (though recognise I might need two channels into the ADC for this)

The ambient light intensity is likely to vary quite significantly during measurement, and I'm curious the best way to tackle the circuit design.  I'll need pretty decent gain because the absorption signal isn't going to be large, but at the same time I don't want a large ambient signal to saturate the photodiode. 

I'd be hugely appreciative if anyone can through a few suggestions my way.  Obviously I'm interested in a low noise solution as to take as much of an advantage of the full resolution of the ADC.  I'm guessing a demodulating circuit (coupling the 0-5V square wave signal from the Arduino pin with the output from a photodiode/transimpedance amp stage) could be a good place to start?  Or can what I'm trying to achieve be without the need for a synchronous circuit?

Thanks in advance!

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