PXI-1044 Grounding

[eclipsen]

I am using a PXI-1044 chassis and logic ground is connected to chassis ground somewhere in the chassis.  Is it possible to disconnect these grounds from each other?  If so, how?

I am running into noise issues and having my logic ground tied to chassis ground is causing major issues.

Thanks, Aaron

[lucaso3]

Hello Aaron,

I would not disconnect the ground of it. This grounding is connected to the backplane connections of the chassis, the power supply, to the back plate of the chassis, and to the power plug. If you disconnect this, I am afraid that the grounds will be lost. My question for you would be, what are the noise issues you are experiencing? Which are the mayor issues? I think that we will require much more information. In the meantime, I would like to show you a couple of articles that I hope helps.

• How Do I Reduce EMC Noise Produced From a PXI Chassis?

http://digital.ni.com/public.nsf/allkb/0139475834CA70AB86256C8E005EDEBC?OpenDocument

• Are the JTAG Pins on my PXI Chassis Grounded?

http://digital.ni.com/public.nsf/allkb/E1F8C3571DEF24EB86256CED007736B7?OpenDocument

Please let me know if this might help addressing properly the situations you are having.

Regards,

Luis

[eclipsen]

Within the PXI chassis, we have relay cards, a DMM, and an FPGA card.  The FPGA is controlling various things in our test rack using digital I/O.  During testing, we are experiencing noise spikes on the digital lines which in causing false triggering of digital signals.  We believe these spikes are due to noise on chassis ground which is tied to our logic ground.  If we could disconnect the chassis ground from logic ground, hopefully the noise spikes on our digital lines will go away.  

[ZX81]

Try:  put a ferrite core on your neutral for incomming power (or the entire cable).  That may help reduce some noise.  Or on the I/O (put it on BOTH the signal and common or +V).

Almost no-one will recommend disconnecting the ground from your power source to the PXI chassis.  That would almost always be a bad thing (mainly, it reduces protection from incidental static discharge).  If you try an ungrounded extension cord to see if it helps, be very careful with static protocols, and don't use it long-term.

Disconnecting 0VDC from chassis ground is debatable, mainly for the reason you're seeing.  A lot of I/O cards have isolated I/O channels, just for this reason, but not usually TTL or high-speed discrete I/O.  If you don't need high-speed, you may be able to add relays (solid state or traditional) to isolate the PXI I/O from the devices.