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SAR ADC by architecture is differential in nature. When its single ended negative input is connected to ground, the range is limited to half of the actual range. Kindly see the following table in the ADC component datasheet.
Now if you want 12 bit resolution, In ADC settings change the single ended negative input to Vref, let Vref be VDDA/2 so that you will cover the complete ADC range. I hope this answer helps.