analog output ground loops

[scalpas]

Hi, 

I'm using a power supply (kepco BOP 100-4 if it helps) and controlling it using a PCI-6221 board. I'm also measuring the current and voltage in the load circuit using the same board.

The power supply is a floating source, but I've earthed it, so for the AI measurements  I use the differential mode. On the power supply the input and outputs have the same ground, so I was wondering if there was any risk of a ground loop due to the AO GND ?

 Thanks,

[Chris_W.]

Hi scalpas,

When you're using the differential connection for measurements you should not have any ground loop issues, as long as you're not actually grounding either of those connections. I am unclear on what your system looks like. If you could create a layout and connection diagram of your system that would really help me see exactly how your system is set up.

Also, when you say that the power supply is a floating source, what exactly do you mean? Is it not powered by a grounded plug?

Chris W

[scalpas]

Hi Chris, 

Thanks for your answer. I've enclosed a diagram of the setup. 

The power supply is powered by a grounded plug, so the chassis is grounded. However, the inner circuitry is isolated from the chassis ground, so the output is a floating source. I've grounded it 'manually'. I've drawn the power supply as an op amp, because basically it acts that way : it amplifies the signal it receives from the daq board.

I know there's no risk of ground loops on the measurement/power supply output side, because there's only one ground connection. However, is the AO GND output on the board actually grounded ? Or is it just called that because it's a reference potential ? 

 Regards,

[AnalogKid2DigitalMan · Accepted solution]

scalpas:

I believe all the grounds (AI, AO, digital) are tied to earth ground within the PC. If you have an ohmmeter, with the PC off and unplugged, check for continutiy between a GND pin on the DAQ and the metal case portion of the PC (case is usually wired to AC mains earth ground).

-AK2DM

[Chris_W.]

Hi scalpas,

AK2DM is correct, the AI, AO and digital grounds are all tied together at one point and grounded through the PC bus. There is a knowledgebase article that basically states the exact same thing.

 http://digital.ni.com/public.nsf/allkb/02A451C0A88A3EAE8625702F0059C98E?OpenDocument

Chris W

[scalpas]

Ok, so I probably have a ground loop right now on my setup, through AO GND and the cable I used to ground the power supply.

Thanks for your help.

[scalpas]

I just checked the circuit with an ohmmeter, and yes, the whole circuit is grounded through the AOGND channel.

Thanks! 

[hvo123]

Hi,

I am running into a similar problem.  I am using the PCI-6221 to set/read voltage and current on a power supply.  All my analog inputs are set to differential to minimize ground loop noise, however i have no choice but to reference my analog output to AO GND.  The power supply's analog input return is refereced to it's GND.  Their recommendation is to use a floating or isolated source.  Can I effectively create an floating source by using two analog outputs, AO0 set to 0V and AO1 set to 10V?

Best regards

hv

[Carisa_L]

Hi hvo123,

Using the two AOs should work since neither is a ground connection. I'm curious to know why you want to connect a floating source to your power supply though. I've also included links to a few resources that may be helpful.

Field Wiring: http://zone.ni.com/devzone/cda/tut/p/id/3344#toc2

M series manual: http://www.ni.com/pdf/manuals/371022k.pdf (page 5-6)

Best,

Carisa Leal

[johnsold]

hv,

Using two AOs like that will NOT create a floating source.  AO0 will be zero volts from AO Gnd and AO1 will be 10 V from AO Gnd.

Lynn