Hi peter-h,
Yes, I know this ADC's are not precise, but, the reason to use the external Vref is because the rest of the board (the analog) is supplied with other regulator, not the same regulator that supplies the MCU, this is to avoid noise problems. Because I'm using different regulator for this I want to set Vref from this regulator to get the best precision, and this is the problem with Vref and Vdda.
Thanks