Grounding question using a PXI-6225 and an SCB-68 TB.

[THEEDGE714]

Hi all!

I'm using the configuration in the title of this post to apply signals to a test setup.  The SCB-connections have an analog ground that is placed next to each of the analog outputs (i.e. AOGND, AO0, AOGND, AO1...A0GND, AO7, AOGND).  I have connected an analog ground (SCB-68 pin 31) and a digital ground (SCB-68 pin 12) to the negative side of my power supply.  I belive this will give me a good ground reference for all the cards as I have my other cards grounds tied to this ground as well. 

I'm just curious as to why they have so many grounds for the analog signals.  I can only assume that it is for EMI or susceptibility.  Are those grounds there to help keep the actual waveforms shielded?  If I just use the two grounds mentioned I should have a good enough reference, right?  I just don't want to mess up another card because I originally set this up and was driving signals with it, but didn't have any of the grounds hooked up to the 6713 card.  It worked for some time until the conditions were right and then I had some offset that occurred, which burned up my card.

Any information is appreciated as to the use/need for all these grounds!

Thanks,

DB 

[Peter-E]

Hi DB!

Thanks for contacting NI support! 

The reason that each analog output channel has a separate ground is because each output channel has it's own Digital to Analog Converter (DAC). Each one of those DACs has its own ground line.

The ground next to each AO channel is physically the closest to its particular DAC. Ultimately all of these grounds are tied together internally, but if one ground is used for a different channel, the physical line between them is much longer and has an inherent resistance that can affect the measurement slightly.

The reason that analog input channels don't use as many ground lines is because they are multiplexed internally. That means that one DAC handles multiple input channels.

[THEEDGE714]

Thanks for you response!

OK, so it will probably work if I don't hook everyone of those grounds up, but it I am outputting fast, intricate waveforms, it may give me a cleaner output?

I am going to be driving the analog outputs with waveforms that are square wave's with pulse on times of 10ms and off times of ~20ms(shortest off time).  I have the grounds hooked up from the card to the terminal block(SCB-68), but currently only have one ground connected from the terminal block to the board that I am driving the signals with. 

What is your recommnedation with this speed of signal for the grounds?

[Peter-E]

First of all, be sure that you are using the correct pins for each of the analog output channels of the PXI-6225. The PXI-6225 only has 2 analog outputs, and the SCB-68 is a generic connector block for 68-pin cables. I have attached the pinout for the PXI-6225 for your reference when hooking up these grounds.

After you are sure you are using the correct ground pins on the SCB-68, the grounds should be fine with that speed of signal. However, I would recommend using the individual grounds for the AO channels for continuity.

[THEEDGE714]

Crap...that was a typo...sorry.  I meant the title to say PXI-6713.  I used the 6225 card on my last project and must have had it stuck in my head.

I am talking about the 8 analog outputs on the 6713.  I am having to make some concessions on my setup because I only have 8, but your last comment peaked my interest.  I guess I could use the 8 on the 6713 and then also use the 2 on each of the 6225 cards I have.  I will be playing the waveforms back that I have stored in an array.  I would be able to assign the 4 extra waveforms to the outputs on the two 6225 cards and they should synch in time with the outputs on the 6713 I believe.

I guess this is once that a typo may have helped me in the long run!! 

So...the original question about the grounds was concerning the 6713 and if I need to connect every one that is next to an analog output to my product.  The second question now is if I can synch up 12 waveforms being output on a 6713(8 outputs) and two 6225's(2 analog outputs each)??

Thanks for all your help!!

[Peter-E]

Haha I am glad we could figure out a better way! Especially through a typo.

Regardless of the module, the grounding method internally for analog outputs is the same. So I would still recommend using each individual ground for the different channels just for continuity, and we don't want that many channels referencing the exact same pin. It will be beneficial to spread the return out among the different DACs.

As far as synchronization between PXI modules, we have shipping examples that show how to do multi-device synchronization for DAQmx. They can be located in LabVIEW's Example Finder (Help>>Find Examples) under the "Hardware Input and Output>>DAQmx>>Synchronization>>Multi-Device" folders.

If you have any specific questions on how to use these examples, I would suggest starting a new thread on the forums. That way, as a new issue, it gets more recognition from our community and will hopefully be responded to quickly!

Have a great day!

[THEEDGE714]

Hey,

Thanks so much for all the information!  You guys really do a great job helping your customers out by answering our questions on the forums.  It is so nice to know that you always can find help here from one of you experts or even from the community.

Bravo on your customer service and support!!

Thanks again,

Doug