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I would like to be sure I understand well the principle of the AD7746. I have two sensing capacitors with one plate in commun which is grounded. I would like to sense (C2-C1). Is that possible with the AD7746? I see that the AD7746 can be configured in a differential way, does it mean that it can do (Cin1+ - Cin2+)? If so, I read that it is recommended that the plate is floating. However in the AD7747, there is only one input for a capacitance.
I would suggest the AD7747, which is designed for single ended or grounded capacitances. The AD7746 is used for mutual capacitance and requires an excitation signal. The AD7747 does present two inputs for differential capacitance, as you've described; see figure 30 of the datasheet.