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

(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.

Early isolation techniques used standard transformers and analog isolation amplifiers. Their small bandwidth, high power-consumption, and single-channel design present significant limitations to modern electronic designs.

Today’s data acquisition and data transmission systems often require isolating multiple signal channels, i.e., for data and control signals. The easiest and least-expensive approach to signal isolation, however, lies in the digital domain. Hence, modern isolator design is focused mainly on digital isolators.

While there are a wide variety of isolation technologies such as optocouplers, inductive isolators, giant magnetoresistance (GMR) isolators, and capacitive isolators, the digital, capacitive isolator has proven to be the most reliable solution for harsh, industrial environments.

Internal construction of the isolator consists of a transmitter and a receiver chip, with the isolation barrier being provided through a small high-voltage capacitor (Figure 2 ). Its inter-level dielectric is a 16 μm level of silicon dioxide (SiO2 ), one of the most robust isolation materials. SiO2 has the least aging effect, thus, extending the life expectancy of capacitive isolators well beyond those of competing technologies.



Figure 2: Internal construction of a digital, capacitive isolator.

(Click on image to enlarge)

Digital isolators are available as single-, dual-, triple-, and quad-channel devices for unidirectional and bidirectional operation. Because their design utilizes 3V/5V, high-speed CMOS technology, they do not conform to any specific interface standard, but are designed to isolate digital, single-ended data lines only.

While at first this might appear to be a design limitation, Figure 3 shows how to isolate a variety of digital and analog interfaces; all having in common that the isolator is placed in the single-ended, 3V/5V section of the isolated interface.



Figure 3: Digital isolators must be placed within the single-ended section of an isolated interface.

(Click on image to enlarge)

For more information on digital isolators please refer to the application literature in the reference section.

Please join us next month when we will introduce the basics of active filters.

References

About the Author


Thomas Kugelstadt is a Senior Applications Engineer at Texas Instruments where he is responsible for defining new, high-performance analog products and developing complete system solutions that detect and condition low-level analog signals in industrial systems. During his 21 years with TI, he has been assigned to various international application positions in Europe, Asia and the U.S. Thomas is a Graduate Engineer from the Frankfurt University of Applied Science.

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