PXI-6115 measurements differ from PXI-4070 measurements

[A Wooten]

I am receiving different measurements with my PXI-6115 (DAQ) device than I receive with my PXI-4070 (DMM). 
 
I am attempting to measure a signal wich is supplied by the unit I am testing.

- The + side of the signal is input into the + input of the PXI-6115 or PXI-4070.

- The - side of the signal is tied to a common return (in the test equipment) which is then wired to the - input of the PXI-6115 or PXI-4070. 

The PXI-6115 is measuring a value of 0.0 (which is what I expect to see)
The PXI-4070 is measuring a value of 0.60 which is out of range for the test. (I've also measured the signal with a hand held DMM and measured 0.6 volts)
Any Ideas why I am seeing these differences in measurements?
I've read some of the NI tutorials on how Differential and Pseudodifferential measurements minimize common mode voltages caused by ground loops.  I understand that the PXI-6115 is a pseudodifferential device which is capable of rejecting common mode voltages.  Isn't the PXI-4070 (DMM) also a differential measurement (which should also reject common mode voltages) ?
On a side note, I've also observed (using a desktop oscilloscope) that a signal's voltage will drop (on the scope) when I connect the signal to my PXI-6115.  What's really interesting is that the size of the drop depends on how I've configured the input ranges of the PXI-6115.  The voltage drops more when the PXI-6115 input range is set to +/-20v and less when the input range is set to +/-10v ( perhaps this is related the the issue described above).
Any help would be appreciated.
Thanks,
Anson

[StevenH]

Hi Anson,
Have a look at the following link, it could help explain the issue.
<a href="http://digital.ni.com/public.nsf/allkb/7C804DA236250FAE862570F9007D3E7A?OpenDocument" target="_blank">http://digital.ni.com/public.nsf/allkb/7C804DA236250FAE862570F9007D3E7A?OpenDocument</a>
It would seem that the problem lies within DAQmx 8.0. Here's a link to download the new version of the driver.
<a href="http://joule.ni.com/nidu/cds/view/p/id/1087/lang/en" target="_blank">http://joule.ni.com/nidu/cds/view/p/id/1087/lang/en</a>
Post back to let me know if it helps
Regards
Steve

[A Wooten]

Steve,
&nbsp;
Thanks for the response.
&nbsp;
I'm a little hesitant to update my DAQ driver, there are other NI devices on this system that use it.
&nbsp;
I am currently using DAQmx 8.6 ( which may already have this fix, the artical that you&nbsp;directed&nbsp;me to was dated January 2006 ) and I am only using a +/-10v input range, not the&nbsp;+/-42v range&nbsp;that the artical identifies as the problem.
&nbsp;
I can't help but wonder if the problem is with how the measurement is wired.
Even though this is a pseudo differential device it is wired up as a single ended measurement.&nbsp;
&nbsp;
So the analog channel 1 input wiring is:

- The + signal to AI 0 +

- System Gnd (Chassis Gnd) to AI 0 -

- System Gnd (Chassis Gnd) to AI 0 gnd

Similarly the DMM input wiring is:

- The + signal to AI 0 +

- System Gnd (Chassis Gnd) to AI 0 -

With this wiring the analog card is measuring rought 300mv less than the DMM.&nbsp;
&nbsp;Is there a problem wiring a Pseudo Differential device as a single ended measurement?
Thanks again for your response, any help is appreciated.
Anson
&nbsp;

[MXI Master]

Anson,I would not worry about upgrading your driver. If your cards are all listed in the <a href="http://ftp.ni.com/support/softlib/multifunction_daq/nidaqmx/8.7.2/readme.html" target="_blank">Read Me</a> the functionality should only be improved by updating the driver. The problem Steve pointed out may still be at play here. But I would highly recommend you revisit the way you have wired your setup, there is some potential for incorrect signals the way you have your system wired. I would recommend looking at<a href="http://zone.ni.com/devzone/cda/tut/p/id/3344" target="_blank"> this document</a> which discussed the different in signal connection types. Cheers,Message Edited by MXI Master on 08-12-2008 04:25 PM