Confusion on NRSE and RSE single ended systems

[name444]

Hello I'm planning to buy a new data-acquisition board from NI but I'm confused about some issues about single-ended configurations which a friend of mine uses.
Here is your link:

http://zone.ni.com/reference/en-XX/help/370466V-01/measfunds/refsingleended/

In RSE case, the AIGND is the point where all channels' grounds are connected. This AIGND is directly connected to the measurement system ground. I was having some offset issues and found out the offset comes from AIGND and measurement system ground's potential differences.

In NRSE case, the AISENSE is the point where all channels' grounds are connected. But this AISENSE is not connected to the measurement system ground.

I've been told NRSE case can reduce the offset.

I have two questions here:

1-)Why in RSE case there is potential difference between AIGND and measurement system ground?(Why this wouldn't be an issue in NRSE case?)

2-)Why would one use RSE if NRSE would introduce less offset?(Does NRSE has some other disadvantages?)

[Jun']

Hello Name444,

You are asking a good question. The way you connect your measurement device to your signal will not only depends on how you want to reject noise or avoid offset, but also on how grounded is your signal source. I believe the bellow document shall answer your questions :

Measurement System Types and Signal Sources

1-)Why in RSE case there is potential difference between AIGND and measurement system ground?(Why this wouldn't be an issue in NRSE case?)

>> If your measurement system is connected to the ground and signal source is also connected to the ground, there may actualy be a difference between those 2 grounds and create a ground loop with unwanted excessive current that can dommage both the devices. As AISENSE is a floating ground it will be connected to the ground of the signal source, avoiding having 2 different ground potentials connected together.

2-)Why would one use RSE if NRSE would introduce less offset?(Does NRSE has some other disadvantages?)

>> Not all interfaces have a AI SENSE input, I believe this to be the strongest answer to your question. Also, using AI SENSE, you may consider adding a resistor to the ground.

Let me know if you still have some questions,

Have a nice day,

M-Junior

[name444]

Unfortunaely your answer didnt help me. Because in both questions my the inputs are already Floating Signal Sources. No need to examne gorund loops due to earth gnd connection.

So lets forget about ground loops since input signals have not contact to earth, they are Floating Signal Source.

Now please re-examine my questions and lets compare RSE and NRSE.

1-) For "Floating Signal Source" cases! --->Why in RSE case there is potential difference between AIGND and measurement system ground?(Why this wouldn't be an issue in NRSE case?)

2-)For "Floating Signal Source" cases! ---> Why would one use RSE if NRSE would introduce less offset?(Does NRSE has some other disadvantages?) Why would I use NRSE instead of RSE(or the other way around) if my signal sources are floating already.

[Jun']

I don't think I understand your questions :

1-) For "Floating Signal Source" cases! --->Why in RSE case there is potential difference between AIGND and measurement system ground?(Why this wouldn't be an issue in NRSE case?)

>> AIGND actually is the measurement system ground so I don't understand the potential difference you are talking about.

2-)For "Floating Signal Source" cases! ---> Why would one use RSE if NRSE would introduce less offset?(Does NRSE has some other disadvantages?) Why would I use NRSE instead of RSE(or the other way around) if my signal sources are floating already.

>> I have already answered the first part of your question : not all DAQ device have an AISENSE input and NRSE makes you add a resistor.

>> Second part : NRSE is seen like a differential measurement. If noise is on the input, it is probably going to be the same noise on AIs channels and on AISENSE input, therefor the noise will cancel itself like in a differntial measurement.

Have a nice day,

M-Junior