> > To answer the question why is the diode in circuit...I believe to
> > protect from reverse polarity installations.
>
> Consider placing a REVERSE biased diode from input to ground,
> plus a series fuse if the input has any power capability. Approx no
> action on normal voltage, clamps to ground on reverse voltage
>
>             RM

And definitely a Schottky. A 1N40xx will not protect the A/D pin
from negative excursions, you need a diode with a smaller V drop
than the PIC's internal diode

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