> I think ADC as a fixed input Z load! 
> Simple 100K 10 turn pot as divider,
> has below part parallel with ADC's Z,
> it will give accurate enough V dividing!

100k is higher than MC recommendation and you'd
have to adjust the sampling time, as well as consider
the internal leakage

http://ww1.microchip.com/downloads/en/AppNotes/00693a.pdf

For a DC signal it's perfectly possible that you'd get good
results with an "out of spec" source impedance

> By adding a op, you add error as well!
> And I believe, this error is not smaller than 
> above, where simplifying ADC as a pure R.

Not necessarily - the op-amp can be trimmed for off-set

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