Incorrect Voltage Readings on the NI 9237 Hardware: CompactRIO>>Analog Input Modules>>NI 9237
Problem: I am attempting to read a known voltage with my NI 9237 using a test panel in MAX, and the voltage reading is incorrect. I have verified that the voltage is 10 mV with a DMM or other measurement hardware; however, it seems the NI 9237 is reading 4 mV. Why I am getting this reading? Solution: This is expected behavior. The NI 9237 returns its readings in millivolts per volts of excitation rather than in volts. In this case it is returning 4 mV for every volt of excitation. If you configured the NI 9237 to supply 2.5 V then the actual voltage at the input to the module will be: 2.5 V X 4 mV/V = 10 mV 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 varies. The 9237 is designed specifically for measuring bridge sensors and the hardware is built to measure mV/V directly rather than making a conversion. It is not ideal for taking measurements of sources that are not directly proportional to the excitation voltage. Modules such as the NI 9215 or NI 9219 are better suited for taking independent voltage measurements. Related Links: NI 9237 Operating Instructions Product Page: NI 9219 24-Bit Universal Analog Input Product Page: NI 9215 4-Channel, 100 kS/s, 16 bit, ± 10 V Simultaneous Sampling Analog Input Attachments:
Report Date: 03/15/2007 Last Updated: 04/11/2007 Document ID: 47EAGLIH |
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