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A/D converter problem

Discussion in 'Electronic Design' started by Shlomi, Jan 3, 2006.

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  1. Shlomi

    Shlomi Guest

    I have a problem using an A/D converter:

    I am connecting a 10-bit A/D input to a voltage from a calibrator and
    doing a histogram over 10,000 samples.
    The results I get is two separate lobes, each one is ~4.5K samples
    The lobes are 2 to 3 counts far from each other and for unknown reason
    the counts in between them are around 200 to 500 samples high.

    What can be wrong in the implementation to cause this phenomenon ?
    Can anybody help?

    Many thanks,
  2. Anno Siegel

    Anno Siegel Guest

    That's consistent with a DC voltage overlaid with a small square wave.

  3. Ken Smith

    Ken Smith Guest

    Effects like you are seeing can have many causes.

    Take a look at the diff. nonlinearity spec. for the ADC. This would have
    to be a bad one for it to be this.

    If the output is parallel, is the ADC's output driving the previous value
    as you convert the next? The digital sides currents can be making

    Plot the data and look at it, there may simply be a step in it or a square
    wave or the like.

    If it has an external reference, is the reference to the ADC good and

    If the ADC has an external clock, does this clock come from a circuit that
    puts systematic jitter on it? A divider chip that has bits below the one
    used as the clock can do this.

    Are you reading the ADC before it is really finished?
  4. Ben Jackson

    Ben Jackson Guest

    Are the lower bits of the output being transposed somehow?
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