>> Think solar site where the instrument is powered from the same
>> batteries that you are measuring voltage from
> > 
> > Can you do anything about that ?
> > 
> 
> No, unfortunately.
> 
> because the only source of power is the battery array, this will be
> powered by the battery array

What I was thinking was for the PIC, powered by the solar array, to
have a charged cap for its Vcc and disconnect itself from the solar
array power

For example a normally-open relay (or FETs), passing Vcc and Vss,
held closed by the PIC when not measuring, and released to measure,
isolating the PIC completely from everything. Temporary connections
to lines to be measured could be via FETs controlled by the PIC. With
the PIC electrically isolated it could take differential readings across
shunts etc

Or something along those lines

An alternative would be V-to-F conversion using a 555 or LM2917,
opto-coupled to the PIC

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