The PIC A/D will only work from 0 to Vdd. If Vdd = +5V, then you need to limit +V on the A/D input-pin to +5V.

You'll need to tap into the V divider where +V on the A/D input-pin is never > +5V or Vdd.

Look here for your answer http://www.doctronics.co.uk/voltage.htm