proper shielding/grounding of NI 9209

[Blokk]

Just a very basic / noob electronics question about proper grounding/shielding method of signal wires.

I will use a NI 9209 16 CH module with D-SUB37 connector (in a cDAQ slot), i am going to use two wires shielded cables (so I will use differential AI pins, and there is a "third wire" which is the shield around the two internal wires). These shielded cables and the NI9209 D-sub connector will be connected by a Phoenix Contact cable (female-male D-SUB37) and a distributor screw terminal ( https://www.phoenixcontact.com/online/portal/us?uri=pxc-oc-itemdetail:pid=2315191&library=usen&tab=1... ).

The question is, should I connect the shielding wires of my signal cables (the shielding of the signal cables are connected to the metal housings of pressure sensors, so not to a COM pin!) to the Ground pin of the screw terminal, thus to the shielding of the D-SUB37 cable? Also, should I connect this overall shielding network to one of the COM pins of the NI9209?

So in this way the sensor housings, the sensor cables' shieldings and the D-SUB37 cable's shielding would be common, and also connected to the COM pin of the NI9209...

Is this the proper cabling, or I should only "common" the shieldings of the different cables, but not connecting them to the COM pin of the DAQ hardware?

Usually before I just used simple BNC cables (single lead wire and shield wire as reference, so it was obvious how to connect them). But now I have to deal with signal cables with two internal signal wires + shielding.

Thanks!

[aputman]

My understanding of shielded cable is that the shield wire is only connected to ground on one end so that it acts as an antennae.  Any signal that it does pickup is then sent to ground rather than causing interference/noise.  I usually don't have issues with noisy environments so I typically cut off the shield wire. 

[Blokk]

I see, so in my case the single one end ground connection would be the NI9209. Since the other end is the metal house of the sensors...

But of course I can just try without COM- pin connection...hopefully it will not be noisy...

edit: of course it is also a question how the cDAQ itself is grounded via its PSU or via the USB? Hmm...

[aputman]

I would think that as long as both ends are properly grounded, it doesn't matter which end is connected.  If one end of the wire is in a noisy environment and the other is not, there may be benefits to keeping that path to ground as short as possible.  I can't say for sure.  

I know just enough about electricity to get me killed so take my comments with a grain of salt.  

[OEM_Dev · Accepted solution]

It sounds like you have a grounded differential connection, shown on page 13 of the 9209 getting started guide. I'd try that to start. Also, I seem to recall that c-series modules use the cDAQ chassis ground which should have a lug on the side.

[Blokk]

Thanks folks, I will connect the shieldings to the com pin and will test the signals when I get all the orders...

Actually I will need to use like 20 thermocouples also on my rig, I have got a NI 9212 (8 CH with thermal block) and a 9213 module. I will also check out whether I should use or not shielded thermocouple wires (or simple unshielded ones).