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Acquire the difference between two channels.

I know on some scopes you can have derived channels, for example you could have a trace  that's the difference between CH 0 and CH 1. Is that possible with a PXIe-5122 or would you have to calculate that in software after the acquisition?

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Yes, you can perform a channel subtraction on the PXIe-5122. There is a shipping example that implements this behavior that can be found by opening LabVIEW, then Help -> Find Examples... -> Hardware Input and Output -> Modular Instruments -> NI-SCOPE -> Measurements -> niSCOPE EX Advanced Measurement Library.vi


You can select "Subtract Channels" for the Processing Step Measurement property on the front panel. The channel that you specify in the "Channel" control on the front panel is the active channel. The other channel on the PXIe-5122 will be subtracted from your active channel.


Hope this helps!

Mena S.
Applications Engineering
National Instruments
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Mena S. is mostly correct.  That example is the right example, but it needs to be slightly modified to do a 2 channel difference.  In the example, you will need to add the property "Other Channel" (Waveform Measurement >> Other Channel) to the niscope Property Node, which specifies the 2nd channel that will be used for the subtract channels array measurement.  Thus if you wanted to perform CH0 - CH1, you would specify "0" in the "Channel" control, and "1" in the "Other Channel" property.  


Since this property defaults to "0", you could do a CH1 - CH0 without any code modification, just enter "1" in the "Channel" control on the front panel.  


Note that this caluclation is still done on the computer in the driver, and not in the firmware on the device. 


I hope this helps!


Product Support Engineer
National Instruments
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Ahh, I was trying to do it in the driver to reduce PCI bandwidth. Sounds like I can't do that.

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Hi Oligarlicky,

If you wanted to do it in the firmware on the device, the only product we have that would be able to do that would be the PXIe-5170R/5171R, which features a programmable FPGA using LabVIEW FPGA.  Due to this architecture, you can completely customize the firmware to behave any way you'd like, our most flexible product yet!  




Product Support Engineer
National Instruments
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Or you can add a PXI 5900  and do it in hardware 😉

Well or use any other differential (active) scope probe.

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