AI GND vs AI SENSE for NRSE inputs

[cyohpe]

Otis:

Well, it seems we have eliminated the voltage shift problem by adding suppression devices (back to back transorbs) to the AC solenoids on the tester.  This is not the end of the story, however, as we recognize that this is a band-aid, not a solution.  We are ready to redesign the interface board, but we would appreciate some direction on the grounding scheme and overall PCB layout.  Does NI have a reference design (or suggested design rules) for such an interface board?  Can you provide a list of vendors that have experience in the design of boards that interface with E-series DAQ boards?

Regards,

Chris

[cyohpe]

Otis:

Well, it seems we have eliminated the voltage shift problem by adding suppression devices (back to back transorbs) to the AC solenoids on the tester.  This is not the end of the story, however, as we recognize that this is a band-aid, not a solution.  We are ready to redesign the interface board, but we would appreciate some direction on the grounding scheme and overall PCB layout.  Does NI have a reference design (or suggested design rules) for such an interface board?  Can you provide a list of vendors that have experience in the design of boards that interface with E-series DAQ boards?

Regards,

Chris

[Justin_M]

Hi Chris,

Though I can't give you a list of any 3rd party vendors, I can give you a good wiring reference.  The following document gives a good overview of how you should and shouldn't setup your wiring, and it explains the different grounding issues you may run into.  I especially like Table 1, which I use myself all the time to verify my connections.  See the following link:

Field Wiring and Noise Considerations for Analog Signals

I hope this helps!

Thanks,

Justin M.

National Instruments

[massic76]

Dear Otis,

I am not sure if you will get this message since the issue seems to be old. However, I report the message you posted on May 19th, 2006 to Chris problems:
"AISense should not really be tied to ground unless you are using a floating voltage source, then you should connect something called a bias resistor (10k-100kOhms) between AISense and AIGND. This allows electrons to flow, but keeps the signal stable. If you are measuring a grounded source then nothing should be connected to AIGND, period."

Now, my question is what if we have a combination of E-series board and a BNC-2090 to collect a mixed set of floating and ground-referenced signals (GRS)? All are single ended signals. The majority are GRS while the others are floating. Can I still use a 100k Ohm resistor to connect AI-Sense and AI-GND? I saw your comment above, but what should I do with the floating signals?

Any suggestion will be appreciated.

Thank you in advance,
sincerely,

Massimo.

[Chris_D]

Massimo,

Can you give more details about how you are wanting to connect your signals? Will you be connecting both the floating and ground-referenced signals as NRSE? If so, since you are connecting floating signals, then yes you will need to connect bias resistors. This will not affect the ground-referenced signals. Please see the following KB and User Manual for how to configure bias resistors for the BNC-2090 and for more detailed information.

Connecting Signals to the BNC 2090
http://digital.ni.com/public.nsf/allkb/EB5D253CF2DE354786256DE4001FA4A8?OpenDocument

BNC-2090 User Manual
http://digital.ni.com/manuals.nsf/websearch/B25C39693232E88C8625665E0063596D

[massic76]

Hello Chris,

thank you for the reply. Yes as you commented after reading the material posted on the NI website (manuals and forums) I have came up with the conclusion that I will need to set all channel switches on the BNC-2090 to SE and the RSE/NRSE switch to NRSE mode. Now, because I will have both GSR and Floating signals in different channels of the BNC-2090 accessory, then my guess is that I will need a bias resistor between AI-sense and AI-GND. At this point I am not sure how to do this. From what I have read, I need to access to the AI-sense and AI-GND lines somehow and some suggestions were SCB or CB boxes that connect to the 68-pin pass-through port and allow for accessing the signals I need. At this point I don't think I have this option, but I can check, so I am wondering if there is another way to do this in a simpler/temporarily way. Do you have the circuit diagram of the BNC-2090 available so that I can see where to tap to do so? Thank you for your help and look forward to receiving a reply back from you,

sincerely,

Massimo.

[Chris_D]

Massimo,

You are correct. You will need to set you switches to SE and NRSE. Your only options in connecting the bias resistors will be to manually solder the resistors to the circuit board or to connect a terminal block to gain access to the AIGND and AISENSE lines.

Please see the Soldering and Desoldering on the BNC-2090 section of the user manual for information on soldering. You will need to disassemble the BNC-2090 to do this. For example, if you are using channel 0, you will want to solder your resistor at R1.

The other option would be to use a terminal block. This method requires no soldering. Essentially, the terminal block gives you access to the AIGND and AISENSE lines and you can connect them with the resistor. You can see the Bias Resistor Help section of the Connecting Signals to the BNC 2090 KB linked above for more information on doing this.

Also, be aware that connecting AIGND and AISENSE with a ground-referenced signal should be okay, but may have some issues in extreme situations. There could be a potential danger if there is significant voltage difference between the grounds. This could cause the measurements to be off and could even fry the resistors.

[massic76]

Hello Chris,

thank you for the reply and to point out the issue on having different voltages for the AI-Sense and AI-GND. I will make sure to do some testings and see what we get to make sure I design the system properly.

Thank very much for your help,
have a nice holiday,

Massimo.

[E_Coleman]

Hi,

I just have a question relating to this topic.  I was just trying to find out exactly what the AI Sense was.  what are its functions exactly and in what applications would it be useful.

Thank you

[Chon.Mech]

If AI GND is analog input ground.  What is AI SENSE? 

I'm fresh out of college and not in on all the industry lingo yet.