M Series cards. AIGND/AOGND/DGND

[MogaRaghu]

I am using a PCI-6229 card . This is my default multifunction  card for many projects. 

Was just reading the User Manual for a specific topic clarity and was wondering what is the difference between the three grounds as presented by the card. AI GND  / AO GND / D GND 

While which to use for what is obvious there also is a mention in the user manual which says that all these three are connected on the board. 

I always have signal conditioners connected to all my I/Os and so the loading on the pins is well below even the normal rating ... like DO pins are not loaded beyond 2mA at any time. This being the case what will happen if I refer  an Analog Channel with DGND instead of AI GND ? 

Just curious. 

[crossrulz · Accepted solution]

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@MogaRaghu wrote:

This being the case what will happen if I refer  an Analog Channel with DGND instead of AI GND ?

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It will add a little more noise on your signal.  Each of the grounds is their own plane, which gives a really solid reference.  The planes are then connected through a via or a few vias.  These are "loose" connections, ie there is a very small resistance between them.  But this resistance can add ground noise.  Enough for you to care about?  It depends on the signals being written and read.  DIO switching (transitioning between High and Low) can possibly be seen on your AI if you use the AIGND for the DIO or visa versa.

So in the end, you are better off just keeping the grounds as separate as possible.

[Russ_B. · Accepted solution]

...adding to the previous response.  

The grounds are all connected together (since it is non-isolated, ground-referenced) but they service different areas of the board.  For example, if you care about the characteristic impedance of your digital channels you should not reference to AI or AO GND.  

Also, consider that if you are using an NI cable they are designed with the particular ground reference in mind.  Digital channels are twisted with digital ground.  Differential AI channels are twisted and then shielded with AI GND.  

Hope that helps.

Thanks

Russell

[MogaRaghu]

That was good explanation about the different grounds ... from a DC point of view I think they are just the same and things start getting complex when Analog signal need to be converted truly. 

Even though there is no need to mix up the different grounds, just wanted to know if there is anything more than the explanations offered. 

Thanks !!