Specifying Excitation Voltage for Quarter Bridge and NI-9219

Updated Aug 14, 2018

Reported In

Hardware

  • NI-9219

Driver

  • NI-DAQmx

Issue Details

I'm currently setting up my acquistion for a quarter-brigde strain gage using an NI-9219 C Series Module. When reviewing the datasheet, I saw that the excitation voltage level for my strain gage (120 Ω) is 50mV.


Then, I created a strain acquisition task in NI MAX and entered 50m in the Vex Value (V) field.


Finally, I clicked Run to start the task, but the task did not start and I got the error

-200077: Requested value is not a supported value for this property. The property value may be invalid because it conflicts with another property.


That error message includes a single possible value of 2.50000. Why is only possible to enter 2.5 for Vex Value (V)?

Solution

Actually, NI 9219 provides excitation by selecting between two internal reference paths, both of which are a reference voltage with a known source impedance. This is put in series with the load to form a voltage divider. Therefore the voltage coming out of the terminals depends on the load resistance and will be called Characteristic Excitaiton Level. That voltage is measured and the known resistance is used to calculate the bridge resistance.

That is the main reason why that parameter is not software-selectable. If you check the NI-9219 Datasheet there is a footer note that states "Excitation level is a characteristic and is not software-selectable."

 

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