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Photodiode Amplifier

Discussion in 'Electronic Design' started by QQ, Mar 30, 2006.

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  1. QQ

    QQ Guest

    Hello Folks

    In a photodiode transimpedence amplifier if I want to connect the
    photodiode in the zero bias configuration is it okay if i connect anode
    to the feedback resistor (and -ve input of opamp) and cathode to the
    ground instead of the other way round (which is the standard way in
    most books).

    My thinking is that since the voltage across the photodiode is going to
    have zero bias across it in either case, it doesnt really matter. Is
    there any other consideration which would affect the operation in this

  2. Guest

    The critical point is to mimimise the stray capacitance between the
    virtual ground summing point and real ground.

    Minimising this capacitance minimises the broad-band noise on the ouput
    of the transimpedance amplifier, and maximises the bandwidth you can
    get. The Burr-Brown (now Texas Instruments) data sheet for their OPA655
    FET-input op amp

    goes through the analysis (which starts on page 8 of the data sheet and
    fills most of sheet 9).

    In general, you should aim to tie the case of your photo-diode to real
    ground. This is usually tied to the anode of your photodiode.
  3. What would you gain by connecting it the other way ?

  4. QQ wrote...
    Some photodiodes have metal shield cases, and sometimes they have
    only two terminals, with one tied to the shield. In that case
    you'd want to be selective about which one you ground.
  5. QQ

    QQ Guest

    Hi Rene,

    The application is a monolithic one. I have another diode whose anode
    is fixed connected to p-substrate (which is the chip ground point). The
    current flowing in the two diodes need to be subtracted. Hence this
    diode has to have its cathode connected to psubstrate (im just going to
    use two antiparallel diodes to do the subtraction..).

  6. Ken Smith

    Ken Smith Guest

    No, it often isn't. One surface of the photo diode is facing towards the
    light source through a clear (nonconductive) window. The other surface is
    facing towards the mounting hardware. If there is much electrostatic
    noise in the environment, you are better off to ground the face of the
    photodiode and take the extra capacitance. This assumes the choice wasn't
    made for you by the mechanical design.
  7. Boris Mohar

    Boris Mohar Guest

    You can get away without using ground by going differential. See my schematic
    in ABS.


    Boris Mohar

    Got Knock? - see:
    Viatrack Printed Circuit Designs (among other things)

    void _-void-_ in the obvious place
  8. Tim Wescott

    Tim Wescott Guest

    Your output voltage will want to range from 0 to negative voltages, if I
    have my anodes and cathodes straight.


    Tim Wescott
    Wescott Design Services

    Posting from Google? See
  9. w2aew

    w2aew Guest

    Yes, it seems your output polarity will be reversed. Can your opamp
    swing below ground? The other consideration, in either zero-bias
    configuration, is the fact that you're stuck with the zero-bias
    capacitance of the diode, which is typically much higher than if you
    reverse bias the photodiode. This will severely limit your BW and will
    require more compensation of the feedback resistor to prevent
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