BNC cables are used as inputs to a data-acquisition hardware. If long BNC cables are picking up the 50/60Hz mains hum through the air like antenna, how can I filter this? Would connecting the BNC conenctors' GNDs to the earth help? Or using a capacitor?
Co-ax and twisted pairs are usually good at removing common mode noise.
Ground loops in coax installations can be a problem as they cause unbalanced noise. This becomes more of a problem if the co-ax shield carries any stray or intended 50/60Hz current.
Make sure signal co-ax screen is not used or involved in current paths that include conductors other than internal core. Running the Co-ax parallel with unbalanced high current 50/60Hz conductors could ealisy cause stray currents to flow in the screen if it is grounded at both ends. Any current on the screen that is not on the core will show up as noise.
Also not that with co-ax capacitive pickup should be minimal if the sensor is shielded. Electromagnetic pickup should be cancelled out except at the terminal points.
You should make sure the grounding at the aquisition end is as good as possible and the co-ax screen is isolated from ground at the sensor end. Grounding the sensor housing locally is usually still a good idea (unless it is small and can be left to float with at co-ax screen potential) but having the co-ax shield grounded to two unequal grounds is always a bad idea.
There are a few things you can do (any one of which may or may not work, and I would try them in this order - don't remove previous steps, just keep adding on):
- Ground everything to a common ground
- ferrite chokes
- isolation transformers
- invest in higher quality cables if shielding is inadequate
- use a 55Hz 20Hz-wide notch filter to remove the hum (not preferable, but as a last resort)
Also, see this website: ARRL RFI. I am a member of this organization and they know what they are talking about.