Is the PCI6229 AIGND connected to PC power earth ?

[MogaRaghu]

Whenever a PCI DAQ card like the 6229 is inserted into a slot in the PC what happens to the DIGND and AIGND terminals ? Do they get linked to the power earth ( the third pin in the mains plug) or do they float with reference to that earth point ?

The reason for this query is a DAQ system where the analog input channels are quite noisy and when we checked up, the DIGND showed a short to the power earth pin. Is this common or do we have a problem in the setup ?

Thanks

Raghunathan

[PBear]

Hi Raghunathan,

On NI DAQ boards, the AIGND, AOGND, and DGND are all tied together and all referenced to you computer ground. Information about this can be found in the following Knowledge Bases:

Are the Grounds on My M Series and E Series DAQ Devices All Tied Together?
http://digital.ni.com/public.nsf/allkb/02A451C0A88A3EAE8625702F0059C98E?OpenDocument

How Are E Series DAQ devices Grounded?
http://digital.ni.com/public.nsf/allkb/10AF3F86965DA0CEC1256BBB005103A2?OpenDocument
As far as the noise that you are seeing, you may be experiencing a ground loop. This is caused by your instrument being at a slightly different (perhaps in mV) ground potential. Look at these two articles to determine if this applies to you and some steps that you can take to resolve this noise. Avoiding Ground Loops
http://zone.ni.com/devzone/cda/tut/p/id/3035

Inconsistent Readings Measuring Ground-Referenced Signal in Referenced Single Ended (RSE) Mode
http://digital.ni.com/public.nsf/allkb/DE02FF8099B8427686256297005224B2?OpenDocument

[MogaRaghu]

Hi Patrick,

The fourth link posted by you on RSE mode had this to say :

.....it is not recommended to configure a DAQ board in RSE mode when measuring ground-referenced sources due to the possible difference in ground potential between the signal source and the wall outlet where the DAQ board draws its reference.

The underlined portion answers my query directly. Thank you very much. So the digital ground of the PC motherboard to which the DAQ card DIGND connects, is linked to the power plug earth pin.

And maybe it also says more : If that wall outlet ground is dirty ( I mean electrically) then all my readings are goig to be dirty irrespective of whether I use RSE, DIFF, NRSE ??? Correct?

Raghunathan

[PBear]

Hi Raghunath,

I'm glad you were able to find the answer. As far as your dirty signal related to a dirty ground, this would be unlikely. Mainly because a computer's power supply filters out much of the noise from a wall outlet (which are generally not the cleanest sources available). I want to reiterate that you probably have a ground loop and recommend reading the previous links, specifically the field wiring diagram. Using some simple circuitry, you should be able to clean up the readings considerably.

Thanks and have a great day.