Photodiode Circuit Help

Discussion in 'The Projects Forum' started by MrL, Apr 26, 2012.

  1. MrL

    Thread Starter Member

    Oct 21, 2009
    46
    0
    Hi,

    I'm current working on a photodiode circuit. My schematic for the circuit is below:

    http://img829.imageshack.us/img829/33/transimpedanceamplifier.png

    Below are the datasheets for the op-amp and photodiode used:

    http://www.ti.com/lit/ds/symlink/lmv793.pdf
    http://pdf1.alldatasheet.com/datasheet-pdf/view/45619/SIEMENS/SFH206K.html

    The issue i have at the moment is the output reading doesn't seem to be what i'd expect. I've tried using different resistor values, and when i decrease the resistor value, the output voltage reading increases, which shouldn't be the case given that for this transimpedance layout, Vout/Iin = -Rf.

    Also, when i place my laser diode (655nm wavelength) onto the sensitive area of the photodiode, the output voltage doesn't change.

    Here are some photos of the circuit:

    http://img812.imageshack.us/img812/9346/img0417ylf.jpg
    http://img841.imageshack.us/img841/1977/img0414wk.jpg

    Please note, i rotated the op-amp the other way round for convenience in the circuit layout. Also, the reason i'm using a surface mount component like this is because the other components are through hole.

    Any help would be appreciated.

    Thanks.
     
  2. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    Your circuit will work properly if you add a negative supply to the opamp.

    A photo-diode conducts a little with dark current and conducts more when light shines on it. Then the opamp will try to make its output negative but it can't because it is missing a negative supply so its output stays at 0V. If the opamp has a dual polarity supply then its output will go negative when light shines on the photo-diode.
     
  3. MrL

    Thread Starter Member

    Oct 21, 2009
    46
    0
    Ok thanks, i'll give it a try and post my outcome in here.
     
  4. MrL

    Thread Starter Member

    Oct 21, 2009
    46
    0
    Hi. I've tried making the changes you suggested by supplying -2.5V (to V-) and +2.5V (to V+), along with 2.5V bias on the photodiode, but i'm still getting no alteration in the voltage level when placing the laser diode near the photodiode. The voltage output is always 0V, and i've tried various resistor values. Could this indicate a faulty/broken photodiode?

    Thanks.
     
    Last edited: Apr 29, 2012
  5. CDRIVE

    Senior Member

    Jul 1, 2008
    2,223
    99
    Try it like this. If you want the OpAmp output to go negative then invert the Photo Diode (fd) so the Anode is connected to R4 and Cathode to GND. ;)
     
  6. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    A photodiode can be a little solar cell where it generates a voltage when exposed to light. Then it has no bias voltage added.

    Or a photodiode can have a reverse bias voltage which reduces its capacitance so it can switch faster. Then it leaks current when exposed to light.
     
  7. Adjuster

    Well-Known Member

    Dec 26, 2010
    2,147
    300
    There may be more than one possible issue to be investigated here, beginning with whether you have any optical output from the laser, and whether this is being effectively coupled to the photodiode.

    That said, if you have an output always very close to zero, with no sign of offsets changing with resistor values, perhaps a detector circuit issue is likely.

    Bear in mind that all the circuit must be suspect for errors, or faulty parts, not just the photodiode. Check over the circuitvmethodically, do not just assume things are correct.

    On a reverse-biased photodiode circuit, a suitable high-value resistor could be temporarily put in place of the photodiode to imitate the DC photocurrent. If that does not give what you expect, you would need to find out why.
    Good luck.
     
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