I would like to ask a possible source of noises when multi AD7746s are being sampled.
Two AD7746s are on my PCB which is connected to a MUX (TCA9548a) due to the overlapped I2C address.
If I sample only one AD7746 (both channels of it were used), I could see a normal level of noise (see the attached figure).
However, the abnormal level of noise was shown when two AD7746s were sampled or one was sampled and the other was just doing a conversion.
At first, I suspected and checked the MUX, but I couldn't see anything wrong with the MUX. Hence, I think that two AD7746s' data were somehow coupled, but I couldn't find any possible sources of those weird noises.
Since the noise was about 4 Hz, I can't filter that out (the interesting capacitance data are in 10 Hz range).
If somebody knows why two AD7746s' data are coupled, please let me know.