Please explain the SCXI-1520 �Value� and �Actual Excitation Value� properties.

[dj143]

As part of the process of using the 1520, I need to know the actual applied excitation value. In this case the 1520 is programmed to provide 10VDC excitation for each channel. On one channel I measure 10.000 V and on another channel I measure 9.997 V. I can understand why there is a voltage difference between channels, and for this reason I need to know the actual applied values.

There are two DAQmx Channel property nodes that I can find that deal with excitation values. One is Analog Input >> General Properties >> Signal Conditioning >> Excitation >> Value. When I read this value, I get a constant 9.99756 reading for both channels. As I understand the context sensitive help, this value
is simply the excitation value for which the channel is programmed for excitation voltage. If so, why does it read 9.99756 instead of 10.00000?

And then there is the other property node, which is Analog Input >> General Properties >> Signal Conditioning >> Excitation >> Advanced >> Actual Excitation Value. When I read this value I get a constant 2.50000 for all channels. Where does 2.50000 come from and how does this relate to the excitation voltage?

In summary, is there a way to actively read the actual excitation voltage that each channel is providing? I also have to wonder how the Remote Sense function ties in with all this? (The Remote Sense lines are connected to the Power lines.) Please advise.

[Salvador_Santolucito]

Hi DJ,

Check out the KB's listed below. They should contain all of the information you need. The last link describes the use of the remote sense lines to read the actual excitation voltage.

As far as the excitation voltage property node reading the wrong value, I might need to see your code so I can ensure that everything else in the VI is set up correctly before I can comment on that.


http://digital.ni.com/public.nsf/websearch/DC27851A3F24439386256B27006E72D5?OpenDocument

http://digital.ni.com/public.nsf/websearch/878F659676A941CA86256A77006F15C4?OpenDocument

http://digital.ni.com/public.nsf/websearch/724FDFA5FEA4FE9586256E9000762B64?OpenDocument

[Ben]

I suspect there is a "issue" with monitoring the RS lines at the same time as monitoring the SG inputs.

I am waiting to hear back from the PSE.

NI support should be contact for help on this Q.

Ben

[dj143]

Attached is sample code that shows how I'm reading the actual excitation voltage. Let me know if more information is needed.

[bman]

I modified the DAQmx shipping example so it will monitor the excitation lines as well as the AI channels. Let me know if this does not work for you.

Brian

[dj143]

I assume this vi goes in the initialization sequence (just before the DAQmx Start Task.vi) within the DAQmx Read Values.vi. When I do this, I get error -200452. ("Specified property is not supported by the device or is not applicable to the task.") It's not clear how to apply this vi. Also, how is a specific channel (i.e. channel 6) chosen to read the excitation value?

[bman]

In the new part of the code I added (in the green box) you can see the two new channels, _pPos0 and _pNeg0. These are for the two excitation lines for channel 0. If you are using channel 6, you will need to change them to _pPos6 and _pNeg6.

Brian

[dj143]

I apologize for not making myself clear. I get the same results when I use the code within the green box. For this reason I thought the subSCXI Shunt Cal Channels.vi is needed because of the Measure Actual Excitation function. When I add the subSCXI Shunt Cal Channels.vi, I get the -200452 error. Apparently there is a conflict with the DAQmx functions. Attached shows how I modified my original vi with your code without using the subSCXI Shunt Cal Channels.vi. Is there something else needed, because I still get a consistent 2.5000 V reading for actual excitation?

[bman]

Ah, I see what you are doing. The property nodes will not give you the measured value of the excitation as you are wanting. To do that, you must actually read the data while you are reading your channel data. (Or read the excitation lines before you start your acquisition. This is what the shunt cal VI does.)

I modified your VI so it shows the excitation values. You already had it set up to read them.

Let me know how this works for you. (I could not get the error message you were getting.)

Brian