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NI USB-6343 Analog and Digital Grounds

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In the manual for the NI USB-6343 it says that the analog input/output ground and digital ground are tied but by a small trace. For my application I'm tying all 3 these grounds output the box (I'm tying all the grounds with physical wires). This might be a stupid question but this is okay to do, correct?

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Accepted by topic author ashifulk

That should be OK unless there are large currents flowing on the ground conductors.  If you have significant ground currents flowing, you have other problems which need to be resolved before connecting the DAQ device.

 

Lynn

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Thank you for the reply. The current should not exceed few mA.

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I have three different grounds(PGND, DGND, vGND) in my UUT circuit and am using digital and analogue I/O throughout.

My DAQ analogue input channels need to measure a mixture of signals and rails.

Most rails are referenced to PGND.

Most signals are referenced to vGND... though not all!

My UUT PGND is jumpered to DGND (So they can be treated as the same, though perhaps ideally be kept them seperate if possible),

 

Considering that my UUT vGND is 1.4V above PGND,

And the DAQ (NI USB-6343) has a small trace linking Ai GND, Ao GND and DGND,

 

Is it possible for the DAQ to simultaneously supply a signal referencing PGND to the UUT, whilst measuring a UUT test point signal against vGND?

 

Thanks,

Cuttsism

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Cuttsism,

 

I am not sure I followed all those connections.  I diagram would be helpful.

 

My first thought is to make the measurements differentially. Then vGND is an input on the differential pairs for the signals which are referred to vGND.

 

Lynn

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Another +1 for differential measurements.  That's about the only way to do it when you've got multiple ground planes, without creating ground loops through the DAQ/measurement system. 

 

Other alternative is multiple DAQs for each ground plane, through USB isolators.

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Hi SnowMule,

 

Sorry for delay responding (Spam filter must have sucked it up) and thanks for the advice so far.

 

My test box is now nearly fully wired, so it seems I may have a big problem here 😞

I have used differential measurements whenever possible and I've also used NRSE measurements for some test points.

I'd appeciate any further advice on this matter as I've used all the NI-6343 analogue channels already and most of the digital channels too.

 

I'll check through the wiring list and workout how many times I've tried to do anything referencing vGND that isn't differential.

 

(QUESTION 1)      If the single ended measurement requires vGND, then perhaps I can make the measurement against PGND instead and then somehow factor the 1.4V difference in afterwards?

 

(QUESTION 2)      I do have a HPIB rack mount system DC power supply, so maybe if the rail power always comes from that and I use some relays to switch the Ai GND between vGND and PGND in my circuit at different times, I could get away with not using a second DAQ?

 

 

Thanks in advance for any further advice 🙂

Cuttsism

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‎10-15-2013 07:38 AM

Hi SnowMule,

 

Sorry for delay responding (Spam filter must have sucked it up) and thanks for the advice so far.

 

My test box is now nearly fully wired, so it seems I may have a big problem here :smileysad:

I have used differential measurements whenever possible and I've also used NRSE measurements for some test points.

I'd appeciate any further advice on this matter as I've used all the NI-6343 analogue channels already and most of the digital channels too.

 

I'll check through the wiring list and workout how many times I've tried to do anything referencing vGND that isn't differential.

 

(QUESTION 1) If the single ended measurement requires vGND, then perhaps I can make the measurement against PGND instead and then somehow factor the 1.4V difference in afterwards?

 

(QUESTION 2) I do have a HPIB rack mount system DC power supply, so maybe if the rail power always comes from that and I use some relays to switch the Ai GND between vGND and PGND in my circuit at different times, I could get away with not using a second DAQ?

 

 

Thanks in advance for any further advice :smileyhappy:

Cuttsism

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