Potential Ground Loop Noise on PCI-4472(B) and PCI-4474

Updated Feb 6, 2018

Reported In

Hardware

  • PCI-4472
  • PCI-4474
  • PCI-4472B

Issue Details

I am seeing noise that is outside of the expected accuracy specifications on my PCI-4472, PCI-4472B, or PCI-4474 device.  Why am I seeing this noise?
 

Solution

For the PCI-447x, it is possible that the pseudodifferential outer shell of the SMB connectors on the device are in contact with the shell of the PC chassis.  Visually examine the PCI device or use a multimeter to see whether the outer shell of the SMB connector has been shorted to the PC chassis.

Noise can be caused by a variety of symptoms.  For more information on possible causes of noise for your PCI-447x device, or for other NI DAQ devices, check out the following articles:

Field Wiring and Noise Considerations for Analog Signals – National Instruments
Five Tips to Reduce Measurement Noise – National Instruments
 

Additional Information

There is no clearly defined standard that all PC manufacturers follow as far as defining the dimensions of their PC slots and their orientation with respect to the mounted motherboard for the computer.  Consequently, some lower quality commercial PC chassis may be far enough out of the typical dimensions for a PC cutout that the connectors of an NI DAQ device may be contacting the edge of the PCI cutout. (It is also possible that the PC chassis is bent or misshapen due to extended use).
 

Figure 1.  PCI chassis rear cutouts.
 
For most PCI DAQ devices, this is not an issue because the outer shell of the connector is already connected to common ground, but since the PCI-447x are pseudodifferential, their outer connectors are designed to be isolated from ground by a 50 ohm resistor, as shown in the excerpt from the NI Dynamic Signal Acquisition User Manual below:

Figure 2. NI 447x Input Connections.
 
Consequently, it is possible that a PC chassis could be touching the outer shell of the connector and creating a ground loop, shorting the 50 Ohm resistor and potentially leading to increased noise due to ground loops.  Review the articles linked in the Solution section above for more information about the impact of a ground loop on your measurement system.
 

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