> the PIC internal 10 bit ADC really is good for only 8 bits

If you're meticulous about the environment the ADC has to work
in you can do better than 8 bits. Conversion in sleep, filtered
supply and reference, grounding etc. All good design practices

See AN546 and others

http://ww1.microchip.com/downloads/en/AppNotes/00546e.pdf

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