ADC does not need Calibration... well... how pedantic do you want to be...

It will always give you steps of 1/255 or 1/1023 (depending if you are in 8 or 10 bit mode) of VREF. So if VREF is 5v, you get steps of 5*(1/255) or 5*(1/1024) accordingly.

If VREF=VDD for example, then you rarely get 5.0000v, you'll usually have some arbitrary figure between 4.9 and 5.1v. That can throw your math, if you're making a thousand units and each one has to be precisely identical to the next. It's something that either usually you can live with, or design out with more precise Voltage References or circuitry.

For high speed, I probably wouldn't chose to use ADCIN. It only takes a tiny number of instructions to grab Data from the ADC anyway, and it's just as easy to do it manually...