How can I activate terminal configurations other than pseudodifferential for a PCI6115 DAQ card?

[Moor]

We're using a PCI 6115 DAQ card with a BNC 2110 board to interface to a dual-balance detector.  The only terminal configuration option available to us in the DAQ wizard is "pseudodifferential", which is giving us a graphical output that is the derivative of what we see on our oscilloscope.  We're using DAQ to measure voltage.  Do we need to find an alternative way to ground our BNC 2110 board in order to use a non-differential terminal configuration?  Is there some setting in DAQ that we're missing?

[Chris_K_]

Hello Moor,

Thank you for using our discussion forums.

There is no way to activate additional input configurations for the PCI-6115.  Have a look at the DAQ S Series manual on pages 3-15 and 3-17 for proper connections for non referenced and and ground referenced configurations.

What type of signal are you measuring, and what is its frequency?  What exactly do you mean when you say “you see the signal's derivative” when acquiring with the 6115?  If you can provide me with a screen shot of what you are seeing it might be more helpful. 

The method in which an oscilloscope and a DAQ card acquire and display data is different and if you are just seeing a phase shift of the signal you are reading, you may be acquiring the correct data, however your scope may be triggering at a different point and you might see a phase shift between the data acquired between the Scope and the DAQ card.

Another thought is that if you have a highly capacitive circuit, and there is an impedance mismatch between terminals, then you may be creating an unintended low pass filter, which may be causing the signal you are seeing.  So check to make sure that your connections are solid. 

Chris_K

[Moor]

Here is the output as measured by an oscilloscope:


And here's out labview output:



The DAQ assistant is set to measure voltage within the 0 to 5 volts range, with N samples, in pseudodifferential configuration.  I don't understand why we have negative values coming out.  The DAQ assistant writes directly to the chart.  Another version of our program saves the values to a text file, and is coming up with the same information.  A suspicion was that perhaps instead of measuring the actual signal, it was 'differentiating' the signal, and we were getting a graph of how the signal was changing over time.

[Moor]

If we feed the program a sine wave, Labview shows us a cosine wave.  If we give it a triangle wave, we get sort of a sawtooth pattern in Labview.

[Moor]

I forgot to mention, in the above graphs, the two spikes on the bottommost graph correspond to a single peak in the oscilloscope.  

[Chris_K_]

Hi Moor

Thank you for the screen shots, they were very helpful.

It looks like the signal that you have acquired with the PCI 6115 is AC coupled, and AC couples signal may resemble a differentiated signal. To fix this, while you are setting up you input task with the DAQ Assistant, under Voltage Input Setup, select the Device Tab and choose DC under coupling mode.

This should fix the signal you are acquiring. 

Just for reference, here is a link that explains what pseudodifferential is.

Chris_K

 

[Moor]

Yep, that was the problem.  It's working beautifully now.  Thanks!