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Compensating tempcos of a photodiode

Discussion in 'Electronic Design' started by Jim Thompson, May 31, 2004.

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  1. Jim Thompson

    Jim Thompson Guest

    Responsivity is **gain**, so you'd need to compensate for that using
    (perhaps) a thermistor. You could probably rig the extra diode to
    compensate for the dark current, but I'd have to see your setup before
    I could say if the inverting terminal is the place for it.

    What I usually do with photo-link situations is use a PWM signal
    source and a DC loop to keep the output centered.

    ...Jim Thompson
     
  2. Greetings,

    I'm running a photodiode with an op amp in a photovoltaic mode. From the
    datasheets I can find that the responsivity tempco is 0.35%/C and dark
    current tempco is 2X/10C. I wonder how to best compensate for these
    effects. Since I have an extra and so far unused ~identical photodiode,
    would it be possible to use it (blinfolded) to cancel the temperature
    effects of the other one by connecting it to inverting terminal to
    cancel the effects of the other one?

    Jack Middleton
     
  3. Phil Hobbs

    Phil Hobbs Guest

    The other thing is that the actual tempco of responsivity is strongly
    dependent on wavelength and bias voltage, because of at least three things:

    1. (for narrow line illumination, e.g. lasers) etalon fringes in the
    photodiode window, which cause sensitivity variations of a few percent,
    varying sinusoidally with temperature. The rapid drift of these fringes can
    cause responsivity slopes of ~1% per kelvin--10,000 ppm/K--which makes
    absolute nonsense of a precision laser measurement if it isn't cured.

    2. Absorption depth of the light in the silicon. Carriers generated below
    the depletion region give rise to much less actual photocurrent, because they
    mostly recombine instead. This is strongly dependent on reverse bias,
    because the depletion depth can change by a factor of ~10 with bias.

    3. (for high light levels and large diodes) Resistivity variation in the
    silicon changing the lateral voltage drops in the epi layer, causing
    different parts of the diode to run at different bias levels.

    How much these things matter depends a lot on what you're trying to do with
    the photodiode. What is it?

    Cheers,

    Phil Hobbs
     
  4. Joerg

    Joerg Guest

    Hi Jack,

    If things get too complicated there is still the "not so elegant but
    efficient" method: Measure temp with whatever device is handy, then feed
    that info into a uC. Dual slope conversion is often adequate and any uC
    can do that.

    Now the uC can hold several transfer functions that can be as
    complicated as they need to be. It can adjust offset, gain and other
    parameters according to temperature and a function for each. With an
    MSP430 this scheme doesn't even require much in terms of power.

    Regards, Joerg
     
  5. I'm sorry for the delay in responding, I've been kept away from my desk
    these couple of days, now it is looking brighter again. :)

    It is just a regular general purpose light meter. I have an old folder
    camera that needs some work. The first phase is to measure/fix shutter
    speeds (mostly done) and aperture values. The next goal (after fixing
    the camera) is to use it in photography to determine exposures. It is
    connected to a microcontroller via 10-bit adc (0 - 5v). Once I get the
    measurement data in, the rest is easily handled with software and
    displayed with lcd.

    Due to the limitations of adc I have about 9 bits to play with to
    measure from darkness to sun. That should be enough for ordinary
    photographic purposes (assuming logarithmic measurement).

    Since measurement is mostly done in natural environment I assume that
    the effects of tempcos in voltaic mode are significant.
    They would be visible through adc within 2-3 degrees change of
    temperature even without amplification.

    I can either measure the temperature separately and compensate in
    software (a lot more hardware) of try to eliminate the effects before
    adc (hopefylly less hardware).

    As you already may have surmised, I'm more at home with the digital part
    of the operation than with the analog and need some help there.

    Basically the operation looks like this (taken from earlier post, thanks
    John!)

    .----------------------------.
    | |
    .-------------. |
    | | | |
    .-|\ | |
    | \-->|-----+--/\/\--+--/\/\--+
    | / | |
    .-|/| | ___
    | | | -
    | | \\ | |
    .------>|--------------+--/\/\--+
    | |
    .---------------------------.

    Reading what you both said about the natures of tempcos, it doesn't look
    like its too easy to compensate for them, even with an almost identical
    part. I did a search on some application notes and couldn't find any
    standard solution. I wonder if that is the reason why the
    photoconductive mode is advertized so much. But then I suffer with the
    adc. One can never win (the second law of thermodynamics :).

    Jack
     
  6. Phil Hobbs

    Phil Hobbs Guest

    OHHHH. That's much less complicated. For a wide-range photometer, I'd
    certainly do the photovoltaic thing, because you'll need the logarithmic
    response. Film latitude is very wide--even a stickler doesn't worry about
    quarter-stops, which is a 19% change in photocurrent. Even at 0.35%/K, you'd
    need a 50-kelvin temperature swing to have anything to worry about. In
    photovoltaic mode, you don't need to worry about dark current, so it's just
    the shift in the forward voltage of the silicon that's going to matter. Save
    the photodiode, digitize the Vf of a 1N914 run at constant current (say 10
    uA), and use that for first-order temperature compensation. Just subtract
    the two voltages (with the same gain, of course). If you do the subtraction
    in analogue, it will also do your level shifting for you. The simple method
    is like this:




    Your PD will be reasonably accurately logarithmic over 4 or 5 decades of
    light intensity, which is better performance than your film. Nine bits of
    resolution over that range will give you 2% resolution, which is about 1/40
    stop, much more accurate than the ISO rating of your film.

    A more serious problem is that the PD's spectral sensitivity is not a good
    match for film--bad enough in daylight and *atrocious* under tungsten light.
    Cad sulphide is much better for this, but of course it has its own
    problems, so a blue-filtered photodiode is the usual medicine.

    Cheers,

    Phil Hobbs
     
  7. That would be a good thing. The idea is to create a meter that can be
    used reasonably over the range of natural light in different contexts.
    The first being photography, but the rest being mainly dependent on the
    software in the microcontroller. Maintaining accuracy over changes in
    temperature is my main concern. I will expect that 50-kelvin temperature
    swing to happen quite easily. That 2% resolution might come handy in
    some other context.
    I have an access to good source of filter material. I was thinking of
    trying to optimize the spectral response to daylight since that camera
    will be used mainly in natural light photography.

    I've seen different diagrams for photovoltaic mode and wonder which is
    the most appropriate in this situation. It is curious that none of them
    has included temperature compensation - assuming that 25 degrees Celcius
    is maintained universally. Anyone seen one in an application note?

    Jack
     
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