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My Laptop is Reading a Floating Voltage from a Grounded Source in Differential Mode

Hardware: Legacy DAQ Devices>>Multifunction DAQ>>DAQCard-1200, Multifunction DAQ (MIO)>>E Series>>DAQCard-6062E, Multifunction DAQ (MIO)>>E Series>>DAQCard-AI-16E-4, Multifunction DAQ (MIO)>>E Series>>DAQCard-AI-16XE-50, Multifunction DAQ (MIO)>>E Series>>DAQCard-6023E, Multifunction DAQ (MIO)>>E Series>>DAQCard-6024E, Multifunction DAQ (MIO)>>E Series>>DAQCard-6034E, Multifunction DAQ (MIO)>>E Series>>DAQCard-6035E, Multifunction DAQ (MIO)>>E Series>>DAQCard-6036E, Multifunction DAQ (MIO)>>Portable

Problem:
I am reading a floating signal when measuring in differential mode on my laptop. When I plug in my network cable, the problem seems to go away, but plugging my laptop into the power outlet does not solve the problem. What can I do about this problem?

Solution:
You are having this problem because your system does not have a common ground. Laptop computers performing measurements can create a floating situation similar to using a floating source. Because a laptop computer does not have a common ground, you may see floating signals even if your source is grounded. In this situation, instead of the signal source floating, your measurement hardware itself is at a floating voltage.

Even though you connect your laptop power supply to a wall outlet, this problem continues because most power adapters only provide a differential voltage (two-prong) and do not actually connect the laptop to ground. Depending on the way the laptop is built, connecting the laptop's ethernet or modem cable sometimes provides this common ground and stabilizes your system.

Some methods for providing a common ground are the following:
  • Use the Reference Single-Ended (RSE) by connecting a grounding pin from your signal source to the AIGND pin on your DAQ device.
  • If using a differential configuration, use bias resistors (usually between 10k-ohm and 100k-ohm) to connect the positive terminal to AIGND and the negative terminal to AIGND as you would with a floating source.
  • If using a Non-Referenced Single Ended (NRSE) configuration, use bias resistors (usually between 10k-ohm and 100k-ohm) to connect the positive terminal to AIGND and the AISENSE to AIGND as you would with a floating source.


Related Links:
Developer' s Zone "Field Wiring and Noise Considerations for Analog Signals"
KnowledgeBase 0GIA6T03. Voltage Reading of Floating Signal Saturates to the Full-Scale Voltage Value

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Report Date: 01/16/2004
Last Updated: 05/07/2008
Document ID: 35FDGQD8

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