Slow photodiode circuit

Discussion in 'General Electronics Chat' started by skyspace, Jul 1, 2008.

  1. skyspace

    Thread Starter New Member

    Jul 1, 2008
    2
    0
    Hi,

    I have a photodiode circuit (attached), where a photodiode is hooked up to a current to voltage opamp connected to a voltage divider. This circuit works as intended: when there is light, Vo ~ 3.3V, and when there is no light, Vo ~ 0.3V. However, when I measured the rise time of this circuit, I found out that it takes ~130ms to switch, which is much slower than I anticipated. The photodiode's spec says that its rise and fall time is 200us, and although I am not in the same operating condition, I expect the delay to be in the same order of magnitude, give or take. Therefore, if any1 has any idea on this, please let me know.

    Thanks.

    [opamp] http://focus.ti.com/lit/ds/symlink/opa657.pdf
    [photodiode] http://catalog.osram-os.com/catalogu...00c448000100b6
     
  2. thingmaker3

    Retired Moderator

    May 16, 2005
    5,072
    6
    What is the slew rate of your op amp? (I can't read the part number for it.)
     
  3. skyspace

    Thread Starter New Member

    Jul 1, 2008
    2
    0
    Hi,

    Thank you for your reply. In my original thread I posted the link to the specs of both the opamp and the photodiode, so feel free to check those out. For your convenience, the slew rate of the opamp is 700V/us.

    Thanks.
     
  4. steveb

    Senior Member

    Jul 3, 2008
    2,433
    469
    Hi,

    Your basic approach seems correct. However, I see in your circuit that you have both sides of the photodiode at a ground potential. There is nothing wrong with this, accept this creates a slower response and your photodiode is not fast to begin with. I believe this is why the response is slower than you expected. The response should be faster if you apply a larger reverse bias voltage to the photodiode. The OPAMP data sheet shows how to do this in the schematic, and there is a spec of 6V on the maximum allowed reverse bias voltage.

    I think you know already, but please be careful and make sure you reverse bias, and not forward bias, and don't use more voltage than specified. I think you should only need a couple of volts reverse bias to see significant speed increase. Also, the OPAMP you are using is much faster than the photodiode. There is nothing wrong with this, but you could choose to use much slower OPAMP with that particular photodiode.

    Hope this helps,
    Steve
     
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