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Voltage fluctuations using cDAQ 9174 with NI-9220 module

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We are using a cDAQ-9174 with a NI-9220 input module. I am reading from 4 channels with one DAQ assistant express vi to test before inserting into bigger project. This is a small portion of a larger project but we cannot get stable readings with 3 of our sensors. 1 is a power transducer to record watts, another is a voltage transducer for volts and the last is a Monarch tachometer. There is a current transducer that is working properly. The readings do give accurate readouts at certain times but will then spike down to near zero volts randomly, the tach not as often as the others. I switched channels with one of the signals giving "bad" readings with the "good" one and the fluctuations are not particular to a channel but follow the signal being measured. I used the Measurement and Automation Explorer Test Panels and it fluctuates in that as well. I've also checked with a very good DMM at the terminal blocks and on the plug side that plugs into the NI-9220 module and those voltage readings are accurate and do no fluctuate. I'm not sure what else to try so any input and help is greatly appreciated. I have attached an image of how bad the fluctuations are but if you'd like anything else just let me know. Thanks for your help!

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Here are the images I was referring to. The current graph on left is acceptable, however, the voltage and power graphs follow the same trend of stepping down and ramping back up to correct reading. The tachometer (red) just seems to do whatever it wants.


You can see it drops down in a step fashion and jumps right back up to the correct value it should be reading.You can see it drops down in a step fashion and jumps right back up to the correct value it should be reading.LabVIEW Test Panel_Channel AI0.PNG

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Accepted by topic author QC_Justo

Problem is fixed! Ran a negative lead wire from negative signal input from each channel to the COM port on the card. Hope this helps if anyone has the same issue!

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