> 1)  The sensor output (up to 5V) is greater than the allowable ADC
> input because the PIC is at 3.3V.  A voltage divider would have to
> use large resistances ~50 - 100 Kohm to limit the current from the
> sensor (max 0.1mA). This exceeds the PIC ADC input impedance

People have reported satisfactory results with a longer sampling time

Another option may be to boost the PIC's Vcc to 5V with a pin-
driven voltage doubler for the duration of the sampling. I'm pretty
sure I've seen that suggested in a Microchip Tricks 'n' Tips or AN

There's the obvious op-amp buffering/amplification of course

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