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Integrator question

Discussion in 'Electronic Design' started by Jim Miller, Oct 5, 2004.

  1. Jim Miller

    Jim Miller Guest

    I have been looking at using an integrator to measure a current. The
    integrator would be the typical opamp with about 1nf of feedback cap. My
    problem is that the current source is a photodiode with about 300pf
    capacitance.

    Am I correct that the integrator output as a function of time in response to
    the current source will be larger due to the "gain" created by the ratio of
    the feedback to source capacitance?

    thanks

    jtm
     
  2. John Larkin

    John Larkin Guest

    No. The integration rate depends only on the pd current dumped into
    the integration cap. A very large pd capacitance can cause ringing or
    oscillation in the opamp, but your numbers are probably safe.

    John
     
  3. Tim Wescott

    Tim Wescott Guest

    The way to understand this is to add the following rule to your toolkit:
    An op-amp will do everything it can to keep the sum of all of the
    currents flowing into the V- node equal to zero. So if the only
    connections are the op-amp (assume zero current for now; that's not
    always valid but pretend), the feedback cap and the photodiode, then the
    output voltage _must_ change at a rate sufficient to keep the capacitor
    current equal to the photodiode current. It ends up looking like the
    photodiode is charging the cap (which is what you want).
     
  4. Jim Miller

    Jim Miller Guest

    hi john

    so the input capacitance wouldn't act as a divider to reduce the feedback
    from the "feedback" cap and thereby require the opamp output to be greater
    in order to achieve a null at the summing junction?

    i should have added some additional information on the original post: the
    photodiode has a gigohm impedence other than the capacitive element if this
    has any importance.

    if you're right then since the input current is in the picoamp range and the
    slew rate of output is consequently in the volt per minute range i probably
    shouldn't have any issues with stability.

    thanks
    jtm

    message
    No. The integration rate depends only on the pd current dumped into
    the integration cap. A very large pd capacitance can cause ringing or
    oscillation in the opamp, but your numbers are probably safe.

    John
     
  5. Tim Wescott

    Tim Wescott Guest

    Because the photodiode's capacitance (and resistance, for that matter)
    are in parallel with the apparent current source, and because the opamp
    is holding the voltage across the diode constant, the PD capacitance
    will affect stability but not DC precision.

    These effects do cause issues: the parallel resistance _could_ cause you
    problems if you get more dark current from the PD than signal current,
    and the parallel capacitance could cause stability problems if you try
    to lower the feedback capacitance for better response speed or gain.
     
  6. John Larkin

    John Larkin Guest

    The opamp forces the voltage across the pd to be zero, or at least to
    be constant if the diode is biased. At any rate, photocurrent doesn't
    change the potential across the pd, so the pd capacitance is not
    charged, so all the photocurrent must flow into, and charge, the opamp
    feedback cap. That's the cool thing about an opamp!
    The slew rate doesn't factor into the stability analysis... the thing
    could oscillate wildly irregardless of the charge rate. You do have a
    230 pF load on the opamp output, among more obvious other things that
    could make trouble. This loop does rate proper analysis.

    John
     
  7. Jim Miller

    Jim Miller Guest

    thanks!

    jtm

    message On Tue, 5 Oct 2004 13:03:42 -0400, "Jim Miller"



    You do have a 230 pF load on the opamp output, among more obvious other
    things that
    could make trouble. This loop does rate proper analysis.

    John
     
  8. Jim Miller

    Jim Miller Guest

    thanks!

    jtm
     
  9. Jim Miller

    Jim Miller Guest

    spiced it.

    simulation says you were right!

    tnx
    jtm


    message On Tue, 5 Oct 2004 13:03:42 -0400, "Jim Miller"


    The slew rate doesn't factor into the stability analysis... the thing
    could oscillate wildly irregardless of the charge rate. You do have a
    230 pF load on the opamp output, among more obvious other things that
    could make trouble. This loop does rate proper analysis.

    John
     
  10. If your light source has no sharp spikes in output you might improve
    stability by putting a resistor in series with the detector, before
    the summing junction. This will hurt the high frequency response but
    stabilize the opamp. Something on the order of 1k might be enough.
     
  11. John Larkin

    John Larkin Guest

    That brings up an interesting point: PDs have some series resistance,
    and that may be a major contributor to loop stability.

    John
     
  12. Perhaps. I modeled the ones I am using on a project at the moment
    http://www.osram.convergy.de/upload/documents/bulkload/infrared/bpx61.pdf
    and the model that matched tested forward drop data indicated about 1
    ohm series resistance at DC. I don't know how that would vary at the
    instability frequency.
     
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