POT jitter on ADC???

[The Old Crow]

  The problem is the LSB error spec. of the ADC.  They all have one, and
it usually manifests itself at the edge of a quantizing 'window'.  For
example:  the binary for a value of 7 is 0111 and the binary for a value
of 8 is 1000.  If you set the pot to be right on the edge of 7 and 8, the
ADC sometimes reads one or the other.  This is the jitter that is
observed.  Most ADCs have an LSB error of 1/2 or smaller, but a number of
things can contribute to degrading this spec.  Power supply noise, digital
noise, and so on.

  The best solution is to use software to either do an averaging of
several values over time or to use a deadband around a given value.  A
deadband of 2, for example, requires that the current value moves by at
least two units either postive or negative before being considered a valid
change.  The Prophet-5 knob digitizer does this, for example.

  Filter caps and buffering of the analog input certainly help, but only
software can help overcome the LSB error of the ADC.

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