Make sure you look at the 'fine print' in the parameter tables.
I found that the "2.56V reference" varied from ~2.3V to ~2.7V
across devices in the CAN64 parts I have been using.
The values still fell within what the table said where acceptable.
If you need repeatability from device to device, then use
an external reference.
Our programmer decided to use an A/D input
to show the battery voltage for a level indicator.
That resulted in Quality Control constantly coming
to me saying "this unit shows the battery is 'full',
while the unit besides it is not 'full', but they have
the same battery voltage" for the first few dozen
units. They never went to the programmer to
tell him to get rid of the gauge, that was
not part of the design specification.
Who also didn't consider that voltage
is not the best indication of charge state.
Programmer and management thought the guage
was a nice addition, so we are now "dead bugging"
a external reference on each board, until the new board rev.
comes in, so the battery gauge works consistently, from
unit to unit.