Read datasheet pg 7 "Single ended negative input". Resolution will be 11 bits.
If you need 12bit resolution and unsigned result, it is possible. You can see the datasheet table attached. The procedure you have to follow is
1.Keep Vref select to external
2.Set Vref value to 2.5V
3.Make single ended negative input as Vref
4. You will be able to take single ended result format as unsigned and will be getting 12 bit resolution.
Only overhead here is 2.5V external ref. You have to connect this to Vref pin.
Good heads up. I used VDDA/2 as my external reference instead of an external pin. Results works great.