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.

http://digital.ni.com/public.nsf/allkb/7C804DA236250FAE862570F9007D3E7A?OpenDocument 

It would seem that the problem lies within DAQmx 8.0. Here's a link to download the new version of the driver.

https://www.ni.com/en/support/downloads/drivers/download.ni-daq-mx.html

Post back to let me know if it helps

Regards

Steve

[A Wooten]

Steve,

Thanks for the response.

I'm a little hesitant to update my DAQ driver, there are other NI devices on this system that use it.

I am currently using DAQmx 8.6 ( which may already have this fix, the artical that you directed me to was dated January 2006 ) and I am only using a +/-10v input range, not the +/-42v range that the artical identifies as the problem.

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. 

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. 

 Is there a problem wiring a Pseudo Differential device as a single ended measurement?

Thanks again for your response, any help is appreciated.

Anson

[MXI Master]

Anson,

I would not worry about upgrading your driver. If your cards are all listed in the Read Me 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 this document which discussed the different in signal connection types.

Cheers,

Message Edited by MXI Master on 08-12-2008 04:25 PM