How Do I Take Non-Ratiometric Voltage Measurements with the PXIe-4339?

Updated Apr 24, 2018

Reported In

Hardware

  • PXIe-4339

Issue Details

I have a voltage signal I want to measure or a powered sensor that requires DC voltage excitation and returns a non-ratiometric voltage signal. How do I configure the PXIe-4339 to take these raw voltage measurements (i.e., not scaled ratiometrically in mV/V to the excitation voltage)? 

Solution

The PXIe-4339 supports taking regular voltage measurements (i.e., not ratiometric) with the AI Voltage DAQmx task type. As of NI-DAQmx 14.5, however, the PXIe-4339 can also perform voltage measurements while providing voltage excitation, as in the case when using DC powered sensors that return a non-ratiometric voltage. To enable this type of measurement, configure the DAQmx Create Virtual Channel VI to AI Custom Voltage with Excitation mode, and set the use excitation for scaling input to False as shown below.


Setting this input to False will prevent the PXIe-4339's ADCs from scaling the measured voltage ratiometrically to the excitation voltage. The table below covers each possible combination of AI channel type, voltage excitation source input value, and use excitation for scaling input value. The cases highlighted in red will throw DAQmx errors, and the reasoning for the errors is provided in the Use Case & Notes column of the table.

Additional Information

When making a bridge measurement with external excitation and setting the use excitation for scaling input to True, the PXIe-4339 will use the excitation value that the user sets with the DAQmx driver for scaling the ratiometric input.  The device will not use the voltage present on the remote sense lines for scaling as it does when providing internal excitation.  Thus, while the driver will support using external excitation for ratiometric measurements, this is not a recommended configuration.

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