Jeff,

If the input voltage exceeds the reference, the A/D will read FF.

There are other complications, though.

Having an overrange sample can affect the next sample, if you
are cutting the sample time too close, so be sure you are giving the
A/D section sufficient time between selecting the channel and
starting the conversion.  "Sufficient" depends on many factors.
You'll have to check the data sheet to see what applies to you.

You said Vref is 5 V.  What's Vdd? If Vdd is only 5 V and you apply a
6 V signal, the input protection diodes will (try to) clamp the
input to about Vdd + 0.6 volts.  If your signal has a very low series
resistance (like the direct output of an op-amp or the like) you can
fry the PIC's input by over-current.  The max current spec for these
protection diodes is 20 mA. (Spec sheet Iik) Adding a series
resistor to protect the PIC is a good idea if the signal source
impedance is very low.

Far better to prevent the overvoltage if possible.  Where does the 6
V signal come from?  If there is a 6V power supply available, can you
let the PIC use that for Vdd?  I use external clamps and series
resistors on some signals to protect the PIC, too.

------------
Barry King, KA1NLH
Engineering Manager
NRG Systems "Measuring the Wind's Energy"
Hinesburg, Vermont, USA
barry@nrgsystems.com
"The witty saying has been deleted due to limited EPROM space"