The input range of the A/D is from Vss (chip ground) to Vref.  If you use Vdd
(+5V) as the reference, that will set the range to 0 to 5V.  You can use a Vref
lower than Vdd but not higher.  If you use Vref lower than Vdd, be aware that
some noise components do not scale with the reference and will therefore be
larger relative to the full scale range than with Vref=Vdd.  These remarks apply
to the 8 bit successive approximation converter used in the 16C7x devices.  More
resolution and flexibility is available with the PIC14000 device, at the cost of
slower conversion rate and more money.

- Mark Sullivan -