>> Carbon pots do tend to get crackly over time

> I thought that was the cause too, but the replacement pot I
> tried had the same issue.

Have you looked at the ADC signal to see the nature of it ? eg
is it noise noise or perhaps more wavy ? At the extremes of the
pot the signal will be more influenced by the impedance of the
supply, and you say the signal seems less noisy at the extremes
so that does point the finger at the pot. It could be that the pot
is not particularly good quality, eg has too few wiper fingers
and those it has may have tarnished. But you go on to say a
replacement is similarly bad. It would be worth going back to
the two fixed resistors and noting what happens. If the problem
goes away it's probably the pot, if it doesn't then it's the ADC
or something associated with it, like Vcc

>> A capacitor at the ADC input should help integrate the jitter
>> although the RC effect will delay the correct reading when the
>> pot is intentionally moved.
>=20
> Any rule of thumb on the cap value?

Not exactly. 100n is often used between wiper and 0V. It's a
compromise between smoothing the noise and not delaying the
signal readiness due to a very long time constant. Crackly noise
will be a fairly high frequency, or have HF components, due to
its fast rise/fall times so a smallish capacitor will do. A large
electro with perhaps a small (10 ohm) series resistor is going to
have a long settling time but will smooth most signals. More
likely to be seen on Vcc as a high pass filter rather than on the
ADC though.


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