Incorrect Test Panel Readings on the NI 9237

Updated Mar 28, 2018

Reported In

Hardware

  • NI-9237

Issue Details

I am attempting to read a known voltage with my NI 9237 using a test panel in MAX, however the voltage reading shown is incorrect. I am attempting to measure .01 V however, the NI 9237 is reading .004 V. Why I am getting this reading?

Solution

This is expected behavior. The NI 9237 returns its readings in volts per volt of excitation rather than in volts. In this case, it is returning .004 V for every volt of excitation. When using test panels the NI 9237 will default to an excitation value of 2.5V. Based on this the actual voltage at the input to the module will be as follows:  

2.5 V * .004 V/V = .01 V

Additional Information

This behavior is preferable for bridge measurements since the voltage across the bridge is directly proportional to the excitation voltage. If the returned measurement were in terms of volts variations and errors in the excitation voltage would be reflected in the measurement; however by returning a ratio, these variations can be factored out. This allows fixed strain to remain fixed even if the excitation voltage fluctuates. 

If this is not behavior you want and would rather directly measure voltage instead of volts per volts of excitation the NI 9215 and NI 9219 are analog input modules that will measure voltage. The product pages for these modules are linked in the related links

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