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Faulty Photodiode Reception

Discussion in 'General Electronics Discussion' started by biomed_dude, Feb 28, 2014.

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  1. biomed_dude

    biomed_dude

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    Nov 25, 2013
    I am designing a fiber optic blood oximeter using Red and IR LEDs. We are sending a 1KHz pulse into the both Red and IR LED. The photodiode (operated in photoconductive mode) is receiving an output that seems fairly reasonable for our input. I attached a photo of what we are receiving before it is filtered. The pulses on top are what the LED is sending and the output on bottom is from the photodiode. The problem is that even when the LED cables are not facing the photo sensor cable, we still receive an output like this. What would be the reason for this?
     

    Attached Files:

  2. Arouse1973

    Arouse1973 Adam

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    1,081
    Dec 18, 2013
    We need to see the circuit diagram if possible.
    Adam
     
  3. Laplace

    Laplace

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    184
    Apr 4, 2010
    Have you ruled out optical leakage and reflective surface spurious pickup? Are the LED drivers and photodiode amplifier on the same circuit board? Do they share the same power regulator? How is the pictured signal different from the 'normal' signal?
     
  4. biomed_dude

    biomed_dude

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    Nov 25, 2013
    Attached are the transmitting/receiving circuits. They are not on the same board but do use the same power supply.
     

    Attached Files:

  5. jpanhalt

    jpanhalt

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    Nov 12, 2013
    It's possible you have coupling through the supply. Can you try a separate supply or use decoupling caps across your IC power pins?

    John
     
  6. biomed_dude

    biomed_dude

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    0
    Nov 25, 2013
    The separate power supply seems to help. I use a band pass filter in matlab but still receive a DC offset. Will it help more to high pass filter in hardware?
     
  7. jpanhalt

    jpanhalt

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    Nov 12, 2013
    I thought the problem was the coupling from your emitters to your detector. Why is a DC offset going to hurt? That should be easy to ignore or shift.

    John
     
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