Russell McMahon wrote:
>>> How about a capacitor on the ADC input charged by the resistor
>>> divider? If I gave it a few minutes to charge the cap, might that
>>> work?
>>
>> No, because it doesn't address the problem.  The reason for the
>> impedence upper limit is because of leakage currents thru the A/D
>> input pin.  This leakage current times the source voltage's resistance
>> creates an offset voltage.  This offset voltage can exceed 1 LSB for
>> source resistances that are out of spec.
>
> He's only after a very coarse reading here ( go/no-go battery voltage.
> Something around 4 or 5 bits would suffice based on the stated required
> resolution.

That's fine, but a capacitor won't make any difference in the accuracy
either way.  The only thing a capacitor can help with is decrease the
acquisition time, but he's got plenty of time so that's not his problem.
The long term voltage error is still the source resistance times the
leakage current, regardless of any capacitance.


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