A PCI data-acquisition board has 16 referenced single-ended channels. Here is the board: http://www.mccdaq.com/pdfs/manuals/PCI-DAS6034-35-36.pdf
I'm using all 16 channels with BNC cables. The two channels are used as output and the rest 14 channels as inputs. When I make single-ended measurements with all channels plugged, I get 50Hz noise and its harmonic in the signals. For low signals this causes a problem for data analysis.
Problem is: when all the 16 channels are plugged, the data from channel 1 has 50Hz noise, similarly other channels as well. But if I only plug channel 1 and plug off all the other channels there is no 50Hz noise.
All the input channels have isolated power supplies. I couldn't figure our where the noise can come from?