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Transimpedance bandpass w/o inductors?

Discussion in 'Electronic Design' started by Asa Cannell, Sep 25, 2004.

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  1. Asa Cannell

    Asa Cannell Guest

    I am trying to design a transimpedance circuit for very high
    sensitivity. I have pulled a circuit directly out of the OPA129
    datasheet. Its a basic transimpedance configuration using a 1Gohm
    resistor for feedback. I have built the circuit and am very impressed
    by how sensitive it is. However, it is useless in any kind of ambient
    light whether static (sunlight) or dynamic (flourescents at 60hz). I
    need a way to block low frequency (DC to 60Hz) signals.

    I thought about putting an inductor in the feedback loop, but I don't
    think they make inductors with an impedance equal to a 1Gohm resistor
    at around 1khz.

    I tried putting a capacitor in series with the input, and this blocks
    DC, but the photodiode will easily saturate in sunlight or other
    static light sources.

    I was thinking there might be some way to make a bandpass
    transimpedance amplifier and maintain my 1Gohm feedback resistance,
    without using inductors. Anyone done this?

    I am using this for outdoors flouroscopy (of oil), it will be facing
    concrete and/or fairly level water in bright sunlight, so it needs to
    be very immune to strong DC signals.

    The signal will be at ~1khz. (its a pulsed uv light)

  2. I would try putting a noninverting integrator or band reject filter in
    parallel with the feedback resistor, perhaps with a current output.
  3. Jim Thompson

    Jim Thompson Guest

    1GOhm? I have a couple of those between my ears ;-)

    The only way to avoid sunlight type biasing is to utilize a (low-pass)
    current feedback loop to null the "DC" component.

    Do you really need the 1GOhm effective transconductance? Maybe do it
    in several stages separated by high-pass (active) filters.

    ...Jim Thompson
  4. John Larkin

    John Larkin Guest

    Why no inductors? I'd think that dumping the pd current into a
    parallel-tuned LC would be ideal; that would extract the signal but
    let arbitrary amounts of DC flow without saturating any signal stages.

    OK, no inductor: you don't need 1G feedback. Go with a value that
    sunlight won't saturate, then highpass or bandpass filter to let the 1
    KHz through, then more gain as needed.

    A UV-passing, narrowband optical filter would be helpful, and a
    synchronous detector later on will optimize s/n.

  5. The whole setup is called lock-in and is based on extremely
    high gain for the ~1khz. Meaning you rather have multiple
    filter stages, ahem bandpass filters with 1kHz passband
    each. Make sure, the ~1khz are stable and accurate and
    you can narrow the noise with steep filters. A common
    Q value for the ~1khz bandpass of 1000 up seems reasonable.

    A common practise in physics, BTW.

  6. R Adsett

    R Adsett Guest

    If you are unfamiliar with lock-in amplification google for lockin amp

    That turned up a microcontroller implementation for measuring low
    reistance and the manual for a commerical lock-n amplifier. Both had
    reasonable outlines of the process.

    Another thought, if the frequency of light coming from the flouroscopy is
    reasonably narrow (I would expect it to be) an optical badpass filter in
    front of the detector should also improve your signal to noise ratio.

  7. Flourescents lights also put out a lot of 120 hz
    Forget all the filtering and just a Schott or similar pass band
    optical filter centered on your source's wavelength or use a high
    quality low pass optical to cut off everything above your source's
    maximum wavelength of interest.

    If you are pulsing your source are you also using a reference from
    your modulator or chopper to provide a reference for lock-in or
    synchronous detection and recovery of the incident light? If so you
    will inherently remove most all of any residual artifacts from indoor
    or outdoor sources.

    I have designed an oil contamination system for a client years ago but
    it was a transmissometer-based design looking at a specimen sheet with
    the oil under test in assigned locations on a sheet of absorptive
    analytical paper. Based on absorption at proprietary wavelengths the
    oil's level of contamination was measured.

    There is considerable prior art regarding devices that look at oil
    using spectrophotometry in both transmissive and reflective
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