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Varying DC Offset

Discussion in 'Electronic Basics' started by bubbles123, Aug 1, 2007.

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  1. bubbles123

    bubbles123 Guest

    Hi!

    I'm getting an output signal (from my transimpedance amp) that has a
    varying DC offset. The maximum size of the offset is about 200mV but
    varies depending on whether there is a current or not. Ideally, I
    would like to stabilise the offset or at least reduce the variation
    down to about 50mV. Can someone suggest how I can do this?

    Thanks!
     
  2. Tom Bruhns

    Tom Bruhns Guest

    Transimpedance, like current to voltage converter? What is the
    source? Is it DC coupled to the amplifier? (If it is, how can you
    tell what's an offset and what's the voltage because of a DC current
    from the source, when there's a source current?) What's the rated
    input bias current of the op amp? What size feedback resistor?

    In other words, what is the source of the offset? Is it caused by op
    amp input bias current, or is it caused by a source resistance that's
    low enough to give you significant voltage gain from the input offset
    voltage to the output, or is it caused by a DC current from outside
    (like from the source)? Or--is is caused by rectification of some
    large RF signal, or some other "parasitic" effect?

    Suggest you first identify the source of the offset, and attack it
    there, rather than putting a band-aid on the wound that may never
    heal, so to speak.

    Cheers,
    Tom
     
  3. bubbles123

    bubbles123 Guest

    Hi Tom,

    This is the circuit I'm using:

    IR Photodiode -> Transimpedance Op Amp -> LPF -> Capacitor for DC
    block -> Voltage Divider to fix offset -> Buffer etc.
    So the problem lies in the voltage divider's offset when the distance
    between the infra-red tranceivers is varied.

    I assume that this is due to a variable current from the
    transimpedance op amp.

    I've tried to use a zener diode instead of a voltage divider to
    overcome this variable offset, but the zener just sinks far too much
    current and is impractical.

    Also the source is an IR photodiode. I'm using Burr Brown OPA380 as
    the transimpedance amp, with a 10k feedback resistor. The input bias
    current is typically 3pA, however I doubt that this is the cause of my
    problem.

    Any help/suggestions would be greatly appreciated!
     
  4. Tom Bruhns

    Tom Bruhns Guest

    It doesn't make any sense to me that you'd be getting a DC offset
    through a capacitor. Is there a way you can post a schematic? It
    _sounds_ simple enough, but something is missing. What kind of
    capacitor is it? Could there be any DC leakage in the capacitor? I'm
    assuming the DC voltage at the transimpedance amplifier end (the LPF
    end) of the capacitor is higher than at the output end...

    Cheers,
    Tom
     
  5. Guest

    Phenomena that don't make any sense around high gain amplifiers often
    make sense when you look at the amplifier's output with a fast scope
    and see that it is oscillating.

    Signficant amounts of AC - more than a volt for FET-input amps, more
    than about 30mV for bipolar inputs - at frequencies higher than can be
    followed by feedback from the output, drive op amp input stages into
    non-linearity, and can produce DC offsets.
     
  6. Phil Hobbs

    Phil Hobbs Guest

    This makes no sense. How do you get a DC offset through that blocking
    capacitor? And what does a voltage divider do to fix it?

    It sounds as though you're trying to use a pot to force the buffer stage
    to have zero offset, and the buffer's input current isn't constant with
    signal. Are you using an emitter follower as the buffer?

    Cheers,

    Phil Hobbs
     
  7. bubbles123

    bubbles123 Guest

    The capacitor I'm using is of the ceramic type. Without the capacitor,
    I get a DC offset of approximately 2.5V. The capacitor removes most of
    the offset so that I get an output signal centred at approximately
    200mV but this varies depending on how much current the photodiode is
    sourcing. Simply put, I want to remove this variation or at least
    minimise it such that it is never more than 50mV. I've used the
    voltage divider to try and stabilise the offset variation (two
    resistors connected to +/- 5V) but it doesnt seem to be working. My
    guess is that the photodiode current varies slightly causing the
    output voltage at the voltage divider to also vary slightly, thus
    producing the slight offset variation. The buffer is being used as a
    voltage follower before the comparator. I anticipate that the buffer
    input current should not be constant with signal, as it depends on the
    amount of current being sourced by the IR photodiode (which is
    dependent on the intensity of IR light received). However, my issue is
    not to do with the buffer, more so how to ensure that the
    transimpedance output signal is centred at 50mV or less.
     
  8. Phil Hobbs

    Phil Hobbs Guest

    If there is no circuit path from the TIA to the buffer that doesn't pass
    through a good quality capacitor, then the DC photocurrent can't affect
    your output signal at all.

    Maybe you'd better post a schematic at alt.binaries.schematics.electronic.

    Cheers,

    Phil Hobbs
     
  9. Tom Bruhns

    Tom Bruhns Guest

    What are the values of the resistors in the voltage divider? What
    kind of buffer amplifier are you using? Have you tried a Mylar/
    polyester capacitor in place of the ceramic? (I've known some
    ceramics in the old days long ago to be quite "leaky" though modern
    ones aren't too bad as a rule.) If the input to the voltage divider
    comes through the capacitor, and the output goes to the buffer
    amplifier, and the "bottom" of the divider is grounded, and you have a
    good high-impedance volt meter, what DC level do you measure at the
    input to the voltage divider? What DC level at the output? Is the DC
    divided by the expected voltage divider ratio? How, exactly, are you
    determining the DC level?

    I ask these questions thinking that (1) the capacitor could be bad, or
    (2) the buffer amplifier input bias current could be causing the
    offset, or (3) the buffer amplifier could be loading the voltage
    divider unequally on positive and negative going parts of the
    waveform, or (4) you're not measuring quite what you think you are.
    There are other possibilities, but these are the first that come to
    mind. There's also, "what's in my head about the circuit doesn't
    match what you actually have," so again, a schematic could be helpful.

    Cheers,
    Tom
     
  10. neon

    neon

    1,325
    0
    Oct 21, 2006
    DC offset on amplifiers come from inheretance of the particular amp. the missmatch of input voltages on the diodes hat will get multiplied by A. The other is miss-match input inpedances. if the f/b resistor is very hi in value then there is an additional offset caused by current offset. without a schematic to look at it can be all of them compounded.
     
  11. Jamie

    Jamie Guest

    How about correcting the route of the problem?

    like for example, Is power supply also moving that much?
    are you using shielded Twisted balanced pairs ? etc...

    questions, questions etc...
     
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