> What is the purpose of having this Vref?  Why not use the internal 5V
> reference all the time?

1  -  Accuracy.  Precision voltage references are much more accurate than
typical 5V regulators.  A basic regulator like the LM7805 only guarantees
4.8 to 5.2 volts even at 25C.  A PIC 10 bit D/A referenced to that isn't
even accurate to 4 bits.

2  -  Range.  The Vref inputs can be used to narrow the range of voltages
that map to full scale, within limits.

3  -  V+ unknown.  All PICs don't have to run exactly on 5V.  The L versions
have a wide voltage latitude, which can allow them to be powered from
batteries without any voltage regulation.  In that case the power voltage
can't be used as a reference at all.  You either have to make sure input
voltages are meaningful as a fraction of the power voltage (using bridge
circuits, for example), or you need a fixed external reference.


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