In a data acquisition system, a single ended earth grounded daq board has inputs from many force transducers and some transducers types.
Daq basically has a common ground for all inputs and multiplexes the inputs.
My question is only about a peculiar observation.
If you look at the diagram above, when any of the force transducer's shield(T1 as an example in the diagram) touches a large floating metallic frame nearby(an aluminum frame where many servos mounted on it but the system is OFF); when any of the transducer's shield touches to this frame, that particular channel picks up large amount of 50Hz and its harmonics noise. But not the other channels. I observe this from the daq software.
I thought if it were a ground loop this noise would appear on all channels, but since it appears only on the channels which is in contact with the frame I thought it cant be ground loop? Does that mean the frame picks up the 50Hz noise through the air?
What can be said about the coupling way of the noise knowing that it only appears on the channel which is in contact with the frame even though all channel grounds are common at the daq side?