Isolate or denigrate
Martin Rowe, Senior Technical Editor -- Test & Measurement World, 3/1/2005
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Internal and external data-acquisition instruments have a measurement section and a communications section (see figure). The measurement section may also divide into analog and digital sections. To produce the most accurate measurements, you should make sure the analog section has a separate path to ground.
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| Data-acquisition modules contain separate measurement and communications sections. |
If your measurement instrument isolates its analog, digital, and communications sections, then it will minimize errors caused by ground loops. If you use external instruments connected through a USB port, you should understand that the USB does not require electrical isolation from the PC. Connecting a USB-based instrument to a computer brings the computer's power ground to the instrument. Therefore, you may need to add isolation between signal sources and measurement instruments to prevent ground loops and improve safety. There is one exception—laptop computers running on battery power. They have no connections to earth ground and are thus inherently isolated.
Your options for isolating signal sources include optoisolators, transformers (included in instrumentation amplifiers), and capacitors. Each has advantages and disadvantages. To learn more about how isolation can improve your measurements, download "Gotchas for USB measurements" by Steve Connors, design engineer at Data Translation, and "Isolation boosts safety and integrity" from the August 2002 issue of Test & Measurement World.
