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How to wire ground in differential measurement

Currently I am using USB6009 to collect 2 signals from a circuit board designed by myself. I meet problems on how to connect the wires.
First, I will introduce my signal source. My circuit board is powered by 8 A-size batteries so I have V+, SGND,  and V-. My two signals are produced by a temperature sensor (having 3 terminals, V+, SGND and Vout) and a pressure sensor(having two terminals, Vout and SGND). I designed a 4-pin connector on the circuit board for connecting with USB6009. The 4-pin connector contains V1 (amplifed from temperature sensor) , V2 (amplified from pressure sensor), SGND, and SGND. I put two SGND here for providing two wires to connect with USB6009 in differential mode. Because I didn't use the 110V power so I think my circuit board system is  not grounded.
Now I will talk about the USB6009 side. I connect V1 to AI0+, a SGND to AI0-, V2 to AI1-, another SGND to AI1-. Should I connect the GND port on USB6009 to SGND too? or I should connect GND with SGND via a resistor, eg. 10K, or just leave GND there ?
I reviewed the explain in LABVIEW help. I cannot understand that.  For example, In the figure of "Grounded Signal Sources". How to find the Earth Ground in my system? In the figure of "Referenced and Nonreferenced Single-Ended Measurement Systems", what is the difference between AIGND and the reference GND?
Anyway, I am confused with the GROUND.
East star 
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Message 1 of 5

East star,

Do read this link.

It should answer most of your queries

Any more doubts, do get back

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Message 2 of 5
To east star:
There are three types of terminal. One is RES(Referenced Single-Ended Configuration), another is DIFF(Differential Mode) and NRSE(No Referenced Sigle-Ended Configuration). Before connect the wires, making sure which's kind of your sensors.
Judging from what you told, the temperature sensor maybe 3-wires RTD or Thermister. Both temperature sensor and presure sensor have SGND terminal. So the tyep of the terminal is RES.
You need connect Vout to AI+, SGND to GND and V+ to Power Supply+.
Only the type of DIFF need connecting Vout+ to AI+ and Vout- to AI-.
Hope it will help you!

Feel Good Inc. Love forever love is free. Let's turn forever you and me.
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Hi, devchander  and NangSgi,
Thanks very much. Fortuantely, NangSgi is online currently.
I read the recommended link and got more sense on theory. Actually I have read those information in Labview help for at least three times before. However, I don't know how to do it practically.
In terms of my above case, I think my battery power is independent of Building ground. I put the battery pack on the table and didn't use 110V AC power so I think my source signal is non referenced (floating), right?
However, because I use USB6009 digital port to control the CD4051 (multiplelexer) on my circuit board, so I have to connect the USB6009 digital ground with the ground of my circuit board. My output signals are based on the reference of my circuit board ground (which is directly connected to the battery negative terminal). All in all, in my user circuit board system, I have nothing with the building ground, that is Earth Ground.
The signal for measurement is tranported by two  wires, one is Vout and another is the ground signal (denoted by VGnd) of my circuit board. I want to use differential measurement for reducing noise so I connect Vout to USB6009 CH1 V+ and VGnd to CH1 V-. The problem is what I should do for the AIGND of USB6009. I found all the AIGND port of USB6009 are connected together by manufacturer. Moreover, the digital port of USB6009 is also connected to the AIGND. Well, you may see that the CH1 v- is actually linked to AIGND via my user circuit board. In this case, how to wire a differential mode?
I tried several wiring mode, for my best understanding of differential and RSE, I found the measured value fluctuate between +-20mV.  I measured the battery output too, it seems at the same level.
Could you give me any suggestions? Thanks.
East Star
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Hi East Star,

It is hard to describe a setup properly, and it is so especially online. But, let me give you some clarifications.

First, make sure your entire setup only refers to one ground. No more than one. If you have more, you will have ground loops and it will cause erratic readings. So if you decide to use the PC chassis as your system ground, then make everything refer to that.

Now from what i read, your floating source should also be indirectly connected to your system gound. In the diagrams in the help file, you'll notice that there are Bias resistors to ground to provide a path for bias currents. This also prevents a high common-mode voltage which could damage your DAQ card.

The fluctuation of +-20mV seems to be something that could quite possibly be caused by a gound loop. You will have to find what's the components in the system that's adding a second or even more grounds.

Hope this helps,

Nathan Yang
Applications Engineer
National Instruments

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