The inner workings of NRSE

[katiemo]

Hey,

I am a little nervous that I may cook my BNC 2090 board with a large voltage, so I wanted to know with confidence that I understand the different modes.  I have set up a voltage divider circuit to measure voltage drop over one resistor.  The total voltage into the system is 120 VAC.  The multimeter readings I am getting across reference resistor range from 1-3 V.  Ideally, I would like to plug a BNC with the center pin reading one side of the resistor and the shielding on the other.  Would collecting data using NSRE ensure that I will not fry the system?  Are there special considerations to take when collecting an AC voltage?

Thanks,

Katie 

[JÞB]

Likely the BNC 2090 will not fry if you have the jumpers configured correctly and do not use more than 1 channel.  Since 16 inputs share a common AIsense line and there is only 1 RSE/NRSE switch connecting more than 1 of those returns together would cause a short between them and may draw enough current to burn traces on the 2090.  I know- don't ask, it was a bad day long ago and I'm not likely to forget.

On the other hand the DAQ device has its own specifications on the maximum voltage between any input and GROUND. so a 3V difference accross the resistor is only one measurement you need to be aware of.  for instance the overvoltage protection spec for the 6251 M series MFD analog input is:

"Overvoltage protection (AI <0..79>, AI SENSE, AI SENSE 2)
Device on ....................................±25 V for up to
four AI pins
Device off ...................................±15 V for up to
four AI pins
Input current during
overvoltage condition ......................±20 mA max/AI pin"

Good Idea to read the following manuals:

NI 625x Specifications

M Series User Manual

BNC-2090A User Manual

[katiemo]

Thank you Jeff!  I am having trouble understanding how to jumper the signal correctly.  My signal is a floating signal.  According to the manual, I need to provide a return path my placing a 10-100k ohm resistor to AI GND.  I took the DAQ out of its case the relavant onboard components, but I don't see a way to modify or add to it.  Do you have experience measuring a floating analog input?  Thank you for your help!

Katie

[Jake_H]

Like Jeff said, the main voltage you need to be aware of is the differential between your channel and ground to stay within the overvoltage protection of the device. The article here talks about connecting signals to the BNC 2090 and the differences between differential, RSE, NRSE. The manual that Jeff linked above also includes connection diagrams and more information on measuring the signal.

[JÞB]

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@katiemo wrote:

Thank you Jeff!  I am having trouble understanding how to jumper the signal correctly.  My signal is a floating signal.  According to the manual, I need to provide a return path my placing a 10-100k ohm resistor to AI GND.  I took the DAQ out of its case the relavant onboard components, but I don't see a way to modify or add to it.  Do you have experience measuring a floating analog input?  Thank you for your help!

Katie

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Yes I do,  Jake may have more--- do not be afraid to contact your local NI rep.  In my experience they have been good people to work with.

@Jake, nice! Kudos