Help With Photo Diode and amplifier.

Discussion in 'The Projects Forum' started by kingsna1, May 14, 2013.

  1. kingsna1

    Thread Starter New Member

    Mar 20, 2013
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    Hello attached is the circuit diagram of a simple photo diode and amplifier circuit i have built. The op amp is a single supply LTC1050 chopper stabilized op amp, and the photo-diode is a BPW24R . I am powering the op amp off a 9v Battery.

    The problem with my circuit is that it all works fine untill the power supply is turned off, when it turns back on i am getting an offset voltage Typically 9V) on the output (hooked up to an oscilloscope), the only way to rectify this problem is if i disconnect the feedback loop and reconnect again, all of a sudden the offset is gone and the circuit is working fine again. Having to put a switch in the feedback loop seems a bit of a hack.

    Also if i fully saturate the photodiode (enough so without the offset it would max out the amplifier at 9v) the problem also then goes away, has anyone any ideas as to how to rectify this. I have tried decoupling capacitors on the power supply to no avail.[​IMG]

    See link to circuit diagram https://skydrive.live.com/redir?resid=10CDA58407A779C1!16567&authkey=!AAQempVIe3j42E8
     
  2. Dodgydave

    Distinguished Member

    Jun 22, 2012
    4,980
    744
    Try putting a 10uF electrolytic from the Non inverting input (+) to ground.
     
  3. GopherT

    AAC Fanatic!

    Nov 23, 2012
    6,029
    3,790
    Not all photodiodes can operate at such low currents. Try 10k to 100k for resistors.
     
  4. kingsna1

    Thread Starter New Member

    Mar 20, 2013
    6
    0
    Thanks for the suggestions.
    Just tried using lower value resistors, 100 k ohm, to no avail. The same problem is occurring . Same for the 10 uf capacitor from the non inverting output , it only happens when first powered and the offset disappears once the feedback loop is disconnected then reconnected again. I guess i could place a switch on the feedback loop so when the power is turned on the switch can then close.
     
  5. GopherT

    AAC Fanatic!

    Nov 23, 2012
    6,029
    3,790
    In this schematic, you need the exact PIN photodiode in reverse bias as shown in the schematic. Linear's DATASHEET for this op amp shows the exact schematic but specifies the photodiode as HP 5082-4204. See page 13 of DATASHEET.

    Reverse bias means very low current and not all photodiodes will behave the same way.
     
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