differential measurements m series usb 6289

[Rsilva]

Hi folks!

I am trying to acquire some voltage differences between channels using an USB 6289 M series.

However after reading the M series manual and Usb 6289 datasheet I have some doubts about the 

way to do this.

I understand that for differential measurements  I need to use AI0:7 coupled with AI8:15, but I don't understand

how to connect the ground...also using a 5 V signal in both AI0 and AI8 but I don't get zero or close to it whenI measure the difference...

If someone could shed some light on what I am doing wrong I would appreciate it.

Cheers!

Rui

[waldemar.hersacher]

If you need to connect ground depends on how your device is powered. There are circuit plans in the manual for most common situations, e. g. battery powered devices.

Not only the the connection must be done correctly also the task or global channel in MAX must be setup with the right  configuration. Check if the the input coupling is differential.

[Rsilva]

The USB 6289 is not powered by batteries, it's pluged-in to the socket wall (I apologize if it's not the correct term, English is not my first language).

My main concern lies on the fact that by measuring the voltage difference between 2 channels (i'm trying to measure the impedance of a domain) 

at this moment I do not have any ground (other than the power supply of the board, which I assume it has ground) connected to AIGND of the board.

When I read the M Series manual, for differential maesurements, the schematics have the AI(-) shunted to the AIGND for floating signals....

This is the part that I'm having some doubts.

Do I need to connect something the AIGND, and if so, the what do connect?

Also I have my code done for differential in the AI Voltage vi.

Hope this helps.

Cheers!

Rui

[NIquist]

http://zone.ni.com/devzone/cda/tut/p/id/3344

You may or may not need shunt resistors.  It depends on whether or not the voltage you're measuring floats around so much that it will go out of the ADCs range.  If not, it doesn't matter if it floats around since you're always looking at the difference between the + and - inputs.  The voltages relative to ground are irrelevant.  If either signal could potentially float out of the ADCs range however, then you must use bias resistors between the inputs and AIGND.  Keep in mind that you will sacrifice a little accuracy for this stability.  Use resistors that are as large as possible, BUT still keep the signals from floating out of range. 

[Rsilva]

I have already read that particular tutorial and my doubts appeared by comparing it to the datasheet of the M series, because

there some differences.

Anyway, like I wrote previously I'm trying to measure voltage drops (impedance) across a domain.

And the domain has a high impedance, so even injecting 2.5 V peak-to-peak I'm getting measurements of mV.

I belive my measurements won't go as for as 1 V, worst case scenario.

So i guess it's OK to leave the all of AIGND in the board unconnected?

Cheers.

Rui

[NIquist]

You shouldn't have a problem floating out of range with that load.  No worries in just trying it out since your voltages are too low to damage your DAQ card.