Question
I have tried to find some information about the volume control in this circuit.
How it is made. Is it a digital shift of the signal or does it work without
reducing the S/N level? My question is: Does the volume setting influence on
the THD+N specification?
Answer
The volume in the AD1955 is linear.
It is the same as described on page 9 of the AD1853 data sheet.
I have confirmed with our factory audio converter applications engineers that
the THD+N is not measurably affected ( it will be only a 3rd order effect).
In the AD1955 it is a digital scaler and would have a similar effect on S/N as
a traditional potentiometer volume control.
The main contributor to the THD+N is in fact the actual DAC section of the
AD1955 (rather than the interfaces and the "bells and whistles").