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photodiode dark current at zero bias

Discussion in 'Electronic Design' started by Adam S, Sep 28, 2005.

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  1. Adam S

    Adam S Guest

    I'm trying to detect very low levels of light using a PN photodiode. The
    photodiode is connected to input of a CMOS op-amp and the bias voltage
    is less than +-0.3mV. When I remove all light by placing the detector
    in a sealed box, there still remains about 200pA of output current.

    Disconnecting the photodiode showed the circuit stray leakage current to
    be less than 5pA, so that cannot be blamed.

    My question is does dark current supposed to exist at zero bias ? If not
    then is the current caused by IR radiation from small temperature
    differences between the photodiode junction and surrounding surfaces ?

    Adam
     
  2. John Larkin

    John Larkin Guest

    Might be DC offset of the opamp driving a little leakage in the pd.
    Add an offset trimpot maybe?

    You might have thermocouple effects, driven by opamp self-heating, but
    I'm guessing that's unlikely. A common PN photodiode will be blind to
    longwave (thermal) IR.

    John
     
  3. Robert Baer

    Robert Baer Guest

    I believe the sensitivity of a silicon junction has a peak in the
    (high) IR region...
     
  4. Johnson

    Johnson Guest

    and most have to be used with a blue filter which reduces transmission ...

    have you also checked to see whether the opamp itself is oscillating ? that
    is, this in itself can cause offset problems -- the photodiode's junction
    capacitance is related to the surface area so the opamps get a little
    troublesome to compensate. check out Burr Brown (TI's) website for ideas on
    compensating photodiode amplifiers.
     
  5. redbelly

    redbelly Guest

    Silicon photodiodes are not sensitive above about 1100 nm. The peak
    (in amps/watt) is typically around 950 nm, which could be shifted if
    filters are used but it can't go above 1100 nm (photon energy becomes
    less than band gap energy).

    This is all way below room-temperature thermal IR.

    Mark
     
  6. redbelly

    redbelly Guest

    I've also had problems connecting a photodiode directly to an op-amp.
    Mind you, these were just cheap 741's. But I did find that putting a
    small resistor between the photodiode and opamp helped (I think I used
    100 ohm, might have been 10 ohm). I assumed the problem was with the
    input offset voltage, but didn't analyze it in detail once I got the
    thing to work.

    Try a resistor in series with the photodiode.

    Mark
     
  7. Phil Hobbs

    Phil Hobbs Guest

    Hmm. No, that doesn't seem right at first blush. How about the following?

    1. (Tire-kicking)
    I assume you're using the usual op amp TIA, with the + input and the PD
    anode grounded, the - input connected to the PD cathode, and a big
    feedback resistor connected between the - input and output of the op
    amp. How big is the feedback resistor, and how are you measuring the
    output voltage? Scope? DVM? ADC card and Labview? (involuntary
    shudder) Are you using split supplies, or an op amp with common-mode
    range including the negative rail? How sure are you that the input
    offset voltage is < 300 uV at your actual bias point? (You probably
    didn't learn that by hanging a DVM off the summing junction.)

    2. (PD details)
    If you're trying to detect very low levels, you're probably using a huge
    PD, right? Diodes at zero bias have a large but finite resistance.
    300 uV/200 pA = 1.5 Mohm, which is certainly on the low side even for a
    big PD. What exactly are you using for a PD, and does it have a spec
    for zero-bias resistance? (A solar-cell type diode might very well have
    a zero-bias resistance of this order.) A definitive test for PD
    leakage would be to build a voltage divider, using a 10k resistor to a
    variable *negative* supply voltage, and a 10-ohm resistor to ground;
    connect the PD to the tap and see if you can make the output move around
    by changing the PD bias by a few millivolts.

    Does warming the PD very slightly make it noticeably worse? Besides the
    zero-bias resistance, the thermocouple potential between silicon and any
    metal is gigantic--400 uV/K or thereabouts, which under some
    circumstances might dwarf the 300 uV input offset voltage.

    3. (Light leaks)
    Or you might be using a plastic cover that looks black but lets 800-1100
    nm light pass through--the window filter of an IR remote control is like
    that, and it's not unknown in other plastics--some very early plastic IC
    packages had that problem, in fact. If your box is plastic, try
    wrapping it in aluminum foil. In fact, try wrapping it in aluminum foil
    anyway, to make sure it isn't light leaks that you're seeing.

    4. (Instability)
    Make sure you have a capacitor in parallel with your feedback resistor,
    to reduce the noise BW and eliminate the instability that the other guys
    are warning about--though I'd expect an oscillation to result in at
    least hundreds of millivolts of dc shift, even if you're measuring the
    output with a DVM.

    Cheers,

    Phil Hobbs
     
  8. John Larkin

    John Larkin Guest

    And if you could put a pv device inside a box, and have it generate
    voltage from the IR emitted thermally by the walls of said box, it
    would violate conservation of energy and a law or two of
    thermodynamics.

    John
     
  9. Adam S

    Adam S Guest



    Thanks Phil, for your 4 points. I just logged on to reply with the
    problem I found causing the small DC shift of ~20mV. Instability.

    After attaching a CRO to the output of the op-amp I noticed an amazingly
    high frequency 20mVpk-pk signal, almost in the VHF range. Who would
    think a slow GBW 1.5MHz LMC6482 could cause such a thing ?
    Actually I think the oscillation was due to the 2nd op-amp in that
    package which was driving a light feedback stabilized LED circuit.
    Adding some bypass capacitors fixed the problem.

    The output voltage was measured with DVM, the op-amp feedback was 20Mohm
    in parallel with 47nF.
     
  10. Robert Baer

    Robert Baer Guest

    Nope!
     
  11. John Larkin

    John Larkin Guest

    Or do you refer to Johnson noise? Or have you invented perpetual
    motion?

    John
     
  12. Robert Baer

    Robert Baer Guest

    Actually, perpetual motion was invented the day that politicians were
    invented...
     

  13. Politicians weren't invented. They are the spawn of satan.
     
  14. Well, it's obviously Doomsday. Here, in one day, I've found myself:
    (A) laughing out loud at a joke that made Jim Thompson Sno-o-o-ort,
    and
    (B) Agreeing with Michael A. Terrell about politicians.

    Will someone please send me some better drugs?

    Thanks,
    Rich
     
  15. Phil Hobbs

    Phil Hobbs Guest

    So who was the mom?

    Cheers,

    Phil Hobbs
     
  16. Rosemary.
     
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