Thank all of you for your replies. Maybe I should give details on my application.
I want to do absorption spectroscopy with photodiode and pulsed laser (10 MHz or lower frequency). I deal with very weak absorption variation (delta ~0.001) and this is why i'm affraid that 16 bits will be not enough. But if it is cheap, I can try if I catch some signals variation.
To answer at groundFungus, I use an Arduino Mega, data will be stored on a computer and transfered by USB. I have only 10 to 15 points per data set.
Ho and 12 bits come from my error, I thought it was 12 instead of 10.