And no-one has added that using these so called 16 bit parts at 16 bit precision is either impossible or close to it. Quite how one can get 24 bit precision over operating temperature range (0-40 deg C say) ( 24 bits roughly 1 part per million) out of the noisy crap inside the microcontroller beats me. My guess is that you can get no better than 12 bit precision from any of these parts on average, 14 if you're very very careful.
What kind of voltage reference can you use ?

Steve