Mixed Signal recording (Floating and GRS)

[massic76]

Hello,


I have a technical question about the E-series in combination with the BNC-2090 to record SE analog signals. The issue that we have is that we have to record some signals that are ground-reference signals (GRS) and some others that are floating with the same BNC-2090. I have read carefully a lot of the information on the NI website and user manuals for the E-series and BNC-2090. My understanding is that for SE-GRS's I should set the Input configuration on the E-series board as NRSE and leave the BNC-2090 in factory-default configuration and select NRSE on the RSE/NRSE switch of the BNC-2090. However, for the floating signals I have found on table 2-6 of the E-series manual that if I connect AI-sense to AI-GND using a bias resistor (10k-100k Ohm) that would be ok. Now, my question is: what will happen to the GRS's is I connect the AI-sense to the AI-GND? If R-bias is large (~100k Ohm) should this help in reducing recording problems on the GRS's?

Please let me know about this. Also, if you think that I should go ahead with my original plan, how can I connect the AI-Sense to the AI-GND? Can you help me?

Thank you in advance,
sincerely,

Massimo.

[Chris_K_]

Hello Massimo,

Please see https://forums.ni.com/t5/Multifunction-DAQ/AI-GND-vs-AI-SENSE-for-NRSE-inputs/m-p/628660#M35888

I agree with Chris_D that the bias resistor connecting AI SENSE to AI GND will not affect the GRS.  Another NI document that is worth while reading is the Field Wiring and Noise Considerations for Analog Signals (https://www.ni.com/en/shop/data-acquisition/measurement-fundamentals/field-wiring-and-noise-consider...). It gives a thorough explanation of the different signal sources, and measurement configurations.  To reiterate the configuration for your application, you should set all the channel switches set to SE, and the RSE/NRSE set to NRSE.

Since there is no access to the AI GND or AI SENSE on the connector pane of the BNC 2090, connecting the bias resistors is difficult.

Connecting Signals to the BNC 2090 KB (https://knowledge.ni.com/KnowledgeArticleDetails?id=kA00Z000001DdSnSAK&l=en-US) explains how to connect these bias resistors, but without having access to a 68-pin break out box you will need to open the BNC 2090 as indicated in the manual and solder the resistors in.

Have a good evening

Chris_K

[massic76]

Hello Chris,

thank you for the informative reply. I have a sort of silly question for you. Is there any way to verify if a signal is a GRS or floating? The reason I am asking this is that since I did not have any way to connect the AI-sense and AI-GND with a bias resistor, I have tested the system the way it is, which is RSE on the BNC-2090 and on the E-card software. Then, I tried to switch to NRSE for both and I have not found a noticeable difference. So now I am not really sure that the signals I thought were GSR are such signals at all. I may have to contact the company that make the device we collect the signal from but I was just wondering if you have any suggestion regarding how to determine if a signal is floating or GSR.

Thank you in advance,
sincerely,

Massimo.

[Chris_K_]

Hello Massimo

The information for the type of signal source of the signals that you are using to take measurements can be found in their respective manuals. 

The following image details the difference between an ground referenced signal, and a floating signal. 

  

A battery or thermocouple would be a floating signal because it doesn’t explicitly reference building ground.  A sensor where the negative terminal is connected to ground would be a ground referenced signal.  You could check continuity between the negative terminal of your sensor and ground and if there is continuity then it is a GRS, if not, then it is floating. 

Have a good day

Chris_K_

Message Edited by Chris_K_ on 12-20-2007 09:20 AM