connecting NRSE signals to 6251 DAQ

[jamoore84]

Hey everyone,

I’ve read several threads on this topic and consulted NI manuals but I still have questions on the correct way to connect Analog I/O to my NI-6251 DAQ (through an SCB-68). I was hoping to probe the community for constructive criticism and information.

My current wiring schematic is attached. The X and Y motor control cards are for a Thorlabs GVS002 galvo scanner. AO 0,1 outputs are control waveforms in X and Y, respectively, and AI 0,1 inputs are for reading the scanner position signal. This wiring scheme is an artifact when we wired things up initially to get it running. Some of the ground connections don’t sit well with me but I'm still a little hazy on grounding practices. This is my primary operating assumption:

*The motor card power supply is connected to wall power and provides +/-V, GND to the boards . As such, the AI signals can be considered a “grounded" source and NRSE.

Is this correct?

From the M-Series User Manual:

"Non-referenced single-ended mode—In NRSE mode, the M Series device measures the voltage of an AI signal relative to one of the AI SENSE or AI SENSE 2 inputs."

Do I need to tie each signal (X and Y) to AISENSE, or just one? if one, should the other signal ground go to AI GND?

From a previous thread:

(Message 8 of 18): "...AISense should not really be tied to ground unless you are using a floating voltage source...If you are measuring a grounded source, then nothing should be connected to AIGND, period."

I agree with the first point. The second statement raises the same question as above: Does one or both signal grounds go to AISENSE?

To stem the posts about how AO GND should not be bridged to AI SENSE, this connection exists on the motor card (Pins J6-8 and J7-8 are on the same ground plane). While it might not look pretty, I don’t think it is technically wrong (though it could be dispensed with).

Regarding tying J7-2 to J7-8, I’m waiting to hear back from Thorlabs on that one. I don’t know what the best practice is here. J7-2 is not connected to ground otherwise.  If I'm driving J7-1 with a +/- waveform, do I need to connect pin 2 to anything?

To muddy the waters further, an addition post (17 of 18) on the same thread states:

"Also, be aware that connecting AIGND and AISENSE with a ground referenced signal should be okay, but may have some issues in extreme situations."

I hardly think my situation would deemed 'extreme' (typical voltage levels +/- 1-3V), but would like a seasoned opinion. Which is correct?

Many thanks for any further elucidation,

jimmy

 

[jamoore84]

Another question regarding M series grounding:

I know that the M-Series cards and pin-for-pin compatible with the older  E-Series, but are the grounds similiarly tied together?

[aNIta B]

Hi Jimmy,

        I'll try to answer your questions as well as I can, but I have a feeling that most of your questions will be better answered by the Field Wiring and Noise Considerations Tutorial.  You'll want to reference Table 1 in that document to see a picture of how your hardware should be set up.  After looking at that document, please let me know if you still have questions.  I believe that you'll want to connect to AI and AI Sense, as shown in Table 1. As for your question about the grounding differences between the E-series and M-series:  the M series grounds are connected similarly in that within the internal circuitry of the board the grounds are all tied together.  Additionally, don't worry- your signals (at =/- 1-3 V) are not extreme.  

Again, please let me know if anything doesn't make sense after looking through that document.  Have a good one!