Regarding this paper, and the following diagram:
We can see above that the non-inverting input of U2 is connected to Vref of the DAC. Signal input goes to the inverting input and Vref to the non-inverting input.
An in this pdf it mentions:
So my questions are:
1-) Why and how does the non-inverting input is used to fix the common mode voltage? What is the reason behind fixing it to a value? And how is not possible by the inverting input. I'm not very into the opamp internals hope a easy explanation is possible.
2-) Regarding the first diagram, in similar situation if we do not have access to Vref but Vdd of the DAC, can that be used to connect to the Vcm pin(to the + input of U2) through a voltage divider? And I really don't get what is the reason non-inverting input is connected to Vref but not to any other reference voltage.(?)