I'm wondering why the analogue ground of the ADC and digital ground are connected internally on the boards like Nano, Uno, Due etc. I'd like to use a Nano as a controller for audio circuits and need an absolute isolation of analogue and digital grounds. Why doesn't Arduino allow to build individual grounding concepts?
from my Datasheet it seems ATMega328(p) has nothing like analog GND
AVCC is the supply voltage pin for the A/D Converter, PC3:0, and ADC7:6. It should be externally connected to VCC, even if the ADC is not used. If the ADC is used, it should be connected to VCC through a low-pass filter. Note that PC6..4 use digital supply voltage, VCC.
I'd like to use a Nano as a controller for audio circuits and need an absolute isolation of analogue and digital grounds.
If you look it has two ground pins. One is next to Vcc and the other is next to Aref and the next pin is AVcc. Therefore you would naturally decouple the Vcc to one ground and the AVcc to the other, making one a digital ground and the other an analogue one.