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SIGNAL CHAIN BASICS #42: The Digital Isolator, a new member of the signal chain family

June 08, 2010 | Thomas Kugelstadt | 222900801
SIGNAL  CHAIN BASICS #42: The Digital Isolator, a new member of the signal chain  family Thomas Kugelstadt, Texas Instruments, considers the need for signal and ground isolation, and why digital isolation may be the best choice
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(Editor's note: click here for a complete, linked list of all previous installments of the Signal Chain Basics series.)

Continuous changes in legislation with regards to designing electronic systems in industrial applications call for the implementation of galvanic isolation into almost any electronic circuit design. The two main reasons for using galvanic isolation are:

  • increased safety, which basically aims to prevent potential damage to equipment or humans due to high-energetic current or voltage surges, and
  • highly reliable data transmission, which focuses on the increased signal robustness of remote data links with different ground potentials and to prevent disruptive ground loop currents.

While the first case is easily understandable, the second one requires some further explanation. The various nodes of a communication network commonly use local grounds as their reference potential. Thus, remote-located nodes draw their supply from different points in the electrical installation system. Remote-located power sources, however, can experience large ground-potential differences due to multiple, non-standardized, earthing techniques, which are also the cause for multiple ground paths.

When providing a direct connection between the transmitter ground and a remote receiver ground, for example by the means of a ground wire, an unintentional ground loop is created. Ground loop currents can be extremely high, because they connect different ground potentials via low-impedance wire (Figure 1).

Figure 1: Eliminating ground loops through galvanic isolation.
(Click on image to enlarge)

These ground loop currents then induce voltages into transmission signal wires, causing signal distortion and possible data errors. Therefore, breaking ground loops through galvanic isolation not only prevents loop currents, but also presents the most reliable method of dealing with high ground potential differences.

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