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SC-2345 and AI SENSE

When taking measurements from an SC-2345 with LP04 modules, what should AI SENSE be connected to?  DAQmx forces NRSE mode for any SCC module inputs so I would think AI SENSE at the terminal block should be connected to something.  The SC-2345 manual doesn't talk about this and the M series manual doesn't seem to mention what AI SENSE is connected to internally when in NRSE mode (it seems to stay pretty close to AIGND when not connected).

 

Thanks,

 

XL600

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XL600,

 

AI SENSE is the common point that your instrument references when taking a single ended measurement (in NRSE mode). That said, your measurement will be taken between your channel of interest and the signal provided at AI SENSE. If your application has no demand otherwise, you could connect AI SENSE to ground. With this, you would essentially be taking measurements in Ground RSE mode (referencing your measured signal against ground).

 

Otherwise, what you connect to AI SENSE strictly depends on your application and the reference point you want to establish when taking measurements in NRSE mode. 

 

You may find this page helpful: http://www.ni.com/white-paper/3394/en/#toc3

 

Hope this helps!

 

-Will

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I found an interesting article here where NI describes the AI SENSE within the SC-2345 as not being connected to the AI SENSE of the M series device.  In fact, it sounds as though AI GND, AO GND, DI GND, and AI SENSE are all tied together within the SC-2345.  That makes me wonder if the AI SENSE terminal in the SC-2345 has any function at all?

 

From the article:

Similarly, pin 62 is the AI SENSE pin on most E/M Series cards, but does not match pin 62 on the SC-2345. Because the SC-2345 is acting as a device for the DAQ card that is providing the signal as Reference Single Ended (RSE), it will route it's own ground to pin 62 of DAQ card. All three grounds (D GND, AI GND, AO GND) are tied together on the SC-2345. Also, a continuity test between the AI SENSE and D GND pins on the SC-2345 will pass suggesting that the pins are internally jumpered. They are different signals in an M-Series, but the SC-2345 will tide those together.

 

Regardless, I think it's best to just leave AI SENSE disconnected at the SC_2345 terminal block.

 

Thanks,

 

XL600

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Hi XL600,

 

From that article you posted, it definitely looks like AI SENSE is tied into the grounds of the 2345 (D GND, AI GND, AO GND).  From that same article, the associated E- or M-series DAQ card looks for an RSE signal, and having the instrument ground (in this case, AI SENSE) connected to the other grounds allows for a reference voltage to be used by the card.  

 

I'd recommend connecting AI SENSE to something or it will likely float as you are taking your measurement, causing inaccurate results.  

Matt | NI Systems Engineering
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DAQmx forces any channel from the SC-2345 to be in NRSE mode.  I believe that is due to the connection of the DAQ card's AI SENSE to the SC-2345 AIGND.  From my measurements, the SC-2345 AIGND and DGND is only a few mV different from the chassis of the DAQ cards but it still makes sense to use NRSE to null out that slight error.

 

I still wonder though what the internal AI SENSE inside the SC2345 is connected to?  The article mentions that it will ohm out to AIGND, but not why or what would happen if it were connected to something else.  I don't want to create a ground loop by connecting it to the wrong thing.

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