cDAQ 9172 chassis ground

[bon2b]

cDAQ9172 user guide (page 15, figure 6) says that to properly ground the chassis:

" attached a ring lug to a 14 AWG wire. Connect the ring lug to the ground terminal on the side of the cahssis using the ground screw. Attach the other end of the wire to the system safety ground".

So I've connected the ring lug to the ground screw on side of 9172. Can anyone show me where I should attached the other end of this wire in order to properly ground the chassis?

Thanks a lot!

[Seth_B.]

The other end should be tied to the common ground of your measurement system.  In most environments, this is building ground or earth ground.  If the system was operating in a vehicle, system safety ground is the metal chassis.  If the system is at a large potential (electrified railroad rails for instance) the measurement system ground may be a terminal at a very large potential relative to actual earth ground.

If you are just in a normal lab setup, find a metal beam or pipe that runs to the foundation or use the ground pin on the electrical plug (be careful not to short to one of the other two!)  I recommend checking out this article: Earth Grounding for Test and Measurement Devices.

[bon2b]

The cDAQ9172 manual also mentions:

"Additionally, attach a wire with a ring lug to all other C Series I/O module cable shields. You must connect this wire to the ground terminal of the chassis using the ground screw"

I use NI9234 modules on this cDAQ 9172 chassis to measure 0-3V voltage signals with pseudo-differential connection. NI9234 references to chassis ground through a 50ohm resistor.

Shall I connect the cable shields on the cable to NI9234 to the chassis ground?

How about the senor end of cable shield? I'm not tying this end to anything now.

Thanks a lot for any inputs!

[Seth_B. · Accepted solution]

If you are using a cable with shielding, the shielding also needs to be tied to ground in order for it to actually be effective.  Connecting the shield at the cDAQ end should be sufficient.  Please note that the cable shield is different than the + and - connections in a cable.  If you are using the BNC connectors of a 9234, it's likely you don't also have a shield that you need to connect.  Most of the time, wire shielding is for twisted pair wires.