The 30 mV is measured with a multimeter directly across a differential input pair eg. between CH5- and CH5+. An insulated from the circuit 30 V power supply was connected between inputs V13 and V14.
Hmm, do you know if the power supply may be introducing AC noise (most likely, common mode RF) this way? Shouldn't be, since it's behind the filters anyway...
Huh, also, why double filter the signals, why not share each consecutive +/-?
...Does the converter load the inputs differentially at all?
(Does help for probing individual channels, though.)
Do you measure both with DMM and ADC at same time and compare? The DMM will load the front-end with its' own impedance (10meg or something?), reducing the voltage seen by ADC a bunch.
Nahh, out of a ~20k Thevenin source, that can only account for a ~0.2% change.
Tim