In my opinion, there are two things that set the maximum source resistance
driving a PIC ADC. The first is the hold capacitor charge time. If you are
measuring a near DC voltage and not using the PIC input multiplexer to
read multiple ADC inputs, this should have no effect. The source
resistance and the input hold capacitor form a low pass filter, but does
not introduce a steady error.

If the input is multiplexed, this becomes an issue. You'll see crosstalk
between inputs as it takes time for the input holding capacitor to change
voltage from one channel to another. If the signals are slow moving, a way
around this is to have a capacitor on the PIC ADC multiplexer input. The
ADC input hold capacitor charges from this larger capacitor with minimal
voltage change.

The second effect of high source resistance is voltage error due to bias
current. I do not believe there's a workaround on this. If you need fewer
bits resolution, you can round the ADC result to get rid of the error. If
you need the full ADC resolution, I think you have to drive with a
relatively low resistance source, like the voltage follower suggested by
the original poster.

Harold


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