2 things quickly 5.0 VDC for a 7805 is 'nominal' it can and will vary +/- 5% so the output can vary +/- .25 V and be in spec. There are 'grades', none A, B, C, that denote various output voltage tolerances. If you need 5.0V foa an A/D measurement then correct it by multiplying the A/D result by the actual Vcc divided by 5 '5/4.9 =' .02 or the A/D measurements are 2% low when the Vcc is 4.9V
Second, @ MarkT How is the internal AVref calibrated/changed..? I would like it to be 1.024V for obvious reasons.
Bob