Voltage Readings Changing When I Have Nothing Connected to the Channel

Updated Dec 15, 2017

Reported In

Driver

  • NI-DAQmx

Issue Details

I am trying to read a voltage channel that does not have a signal connected to it. I was expecting to see it read a value of 0 V, but I see it pulling up to the maximum or minimum voltage for the channel. Is my device damaged?

Solution

This floating behavior is expected for National Instruments devices. We do not specify what the device will read if nothing is connected to the channel. You may also see different results on the channels even if they are on the same device. Some will pull the voltage high for the reading, while others will pull the voltage low. To test out your DAQ card to make sure it is reading correctly do the following:
  1. Attach a known voltage source to the channel, such as 1.5V battery.
  2. In Measurement & Automation Explorer (MAX) open up the Test Panels for your device
  1. Enter values for the Channel Name, Mode, Input Configuration and any other relevant parameter.
  2. Click Start.
The graph should read the voltage level of the device connected to your DAQ card.
 

If you are still getting unexpected results, please contact NI Support.

Additional Information

  • Alternatively, if you don't have a battery, you can short the Analog Input and COM terminals for an RSE device, or the AI+ and AI- terminals for a differential device, to make a 0 V measurement.
  • Similar behavior can bee seen on differential input devices, even when signals of known values, if the signal is not correctly referenced to ground. Refer to your hardware documentation to verify the correct method to connect different types of signals to its analog input channels.

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