Can DGND be isolated from Chassis on M series / SC-2345 system?

[icebear]

I have a system set up using both motion control (7344) and a SC-2345 in conjunction with a 6251 M series card.  Using a DMM, I find that chassis is tied to DGND as soon as the 6251 is connected, this introduction of common mode noise to my test setup will not work, I have sensitive instrumentation and it has effected test results for the encoder being driven / tested with this system.  Is there a way to isolate this, on the benchtop I found this to be true even with the 6251 not plugged into the PCI bus.  Once I can isolate my signal ground from chassis ground on the measurement side (seems to be tied to AI sense, as well), I will need to accomplish the same on the 7344 as it appears to have similar issues.  Any help or schematics are appreciated.  -Stefan

[Matt_A]

Hello Stefan,

The M Series User Manual provides the following description of the D GND signal:

"Digital Ground—D GND supplies the reference for
P0.<0..31>, PFI <0..15>/P1/P2, and +5 V. All three
ground references—AI GND, AO GND, and
D GND—are connected on the device."

In addition to these internal connections, the ground signals are also connected to the power supply ground (earth ground). In most cases, depending on your system, the chassis will also be connected to earth ground. So, though the ground signals are not directly connected, they are connected through earth ground. Sharing this common ground is important to reduce ground loops in your system. As discussed in this document, ground loops can induce noise and can cause significant drift on the signal. This topic is also discussed in another discussion forum here. Could you provide some more information about your system? Are you using PCI devices in a desktop PC? Or are you using PXI devices in a PXI chassis?