Unless you have very good control and experience in PCB layout, do not use a built in ADC over 8 bits. The logic noise in the analog section will kill you. Without an extremely well laid out PCB and very careful separation of analog and digital ground, decoupling to the right of the two grounds etc. you will not get a good reading beyound 8 bits from a built-in ADC.

Absolutely right, Erik. It's interesting to note, that some serious manufacturers of mcu in their datasheet recommend to invoke some 'quiet' mode, when 'real' 10bit resolution shall be achieved.
Newbies often think, that they have 10bit precision when they see 10bit resolution. But when you make a careful analysis of performance of ADC in real application you will often see, that precision is even less then 8bit. One fast edge coupled into only one port line of mcu at the wrong moment can be enough to erode precision of ADC!
It's allways good to have the whole board to be as quiet as possible during aquisition and converting phase. But even with the whole board being quiet, noise of oscillator of mcu still can make enough trouble and can reduce precision of ADC.
I allways suggest careful testing after mounting... {grin}

Bye,
Kai