Question on AD7691: What should be Vref to convert of the maximal number of
bits a difference between two signals which a little from each other differ?
For example, Vin + = 4,5V, Vin-= 4,49V.
As the absolute input range max is =Vref and your signal max is = 4.5volts,
then you can't use a lower ref value less than 4.5volts. So assuming you are
using supplies of 5volts, you should take your Vref from your 5volt supply and
use a common mode of 2.5volts Vref/2.
The main requirement of Vref is that it should be a low impedance source with
recommended decoupling to act as a reservoir of charge.
Driving the part differentially is important in order to get performance.