Engineering and Isolation: It's Not What You Think

As engineers, we must concern ourselves with isolation. Sooner or later we have to deal with it. It's not your fault. It's not anyone's fault. But sometimes two circuits are just so incompatible there is no choice but to isolate them for the safety of all concerned.

Oh, there are the usual excuses. We must avoid ground loops. It’s needed to improve noise immunity. One of the most common is we have to keep the high voltages safely away from everyone.

So, let's agree to optically isolate. It's not you, it's me. Really, you're too high voltage for me. Or am I the high voltage one? But don't worry, optical isolation doesn't mean we still can't see each other. No, quite the opposite. Optical isolation means "Look but don't touch."

So now we are engaged in optical isolation. No, it's not what you think - let me explain.

Optical isolation means that while two circuits are physically separated, the actual technique used to transmit data while achieving isolation is optical ("Look but don't touch"). So, an LED shining light that represents digital data through a clear isolation material received by a physically separated photoreceptor is referred to as optical isolation (Figure 1). This is because the optical data transmission method between the two independent circuits also isolates the two independent circuits from each other.

Figure 1: An optical isolator uses an LED on the left to optically transmit digital data in the form of light to a photodiode on the right. The two circuits communicate optically while being isolated. (Image source: Bill Giovino)

So, when referencing isolation, the mechanism that separates the two circuits is also how the two circuits communicate. Nowhere is this more confusing than when referencing the term “galvanic isolation”.

Just as optical isolation refers to transmitting data optically while maintaining isolation between the two circuits, galvanic isolation refers to transmitting data galvanically (by induced electric charge) while maintaining isolation. We can argue that this isn’t the exact definition of the word galvanic, but at the same time it’s the strange use of the word isolation that confuses engineers and technical writers and editors and distributors, which drives the technical marketing people crazy because let’s face it, engineers love to argue the details.

Galvanic is named after the brilliant Italian physicist Luigi Galvani who discovered that although passing an electric charge through a frog’s leg will make the leg twitch, it will still taste like chicken. Galvanic isolators are sometimes called digital isolators. Optical isolators are also sometimes called digital isolators and also very incorrectly called galvanic isolators just in case some people are not totally confused.

Semiconductor galvanic isolators induce an electric charge in one of two ways, either between the two plates of a capacitor or between the coils of two inductors. Single package semiconductor galvanic isolators are available in either capacitive coupling or inductive coupling. The dielectric used for the capacitor is silicon dioxide. The two inductors are in close proximity inside the package and are also separated by silicon dioxide. Semiconductor galvanic isolators have been tested to handle voltages as high as 6,000 Vrms and temperatures as high as 150°C.

Texas Instruments makes a line of silicon dioxide galvanic isolators that use capacitive coupling to separate the two circuits. The ISO7810FDWR is a single-channel galvanic isolator that is specified to handle as much as 5,700 Vrms and can handle surge voltages as high as 8,000 volts (Figure 2).

Figure 2: This simplified block diagram of the Texas Instruments ISO7810F shows how the two circuits are separated and isolated. Each side has its own separate power domain. (Image source: Texas Instruments)

The ISO7810F shown uses On/Off Keying (OOK) to transmit data across the isolation capacitor where a logic high is a high frequency signal and a logic low is 0 volts. The OUT pin always reflects the logic state of the IN pin. The ISO7810F targets serious applications where people must be kept isolated from dangerously high electric voltages to prevent physical harm.

An important safety example is medical equipment. Probes placed on the skin use very low voltages, usually less than 5 volts. These probes must be kept isolated from the 110 VAC or 220 VAC wall power that runs the diagnostic equipment. A galvanic isolator chip separates the battery-powered probe circuits from the high voltage diagnostic equipment so that if a malfunction or physical damage to the equipment occurs, the mains power won’t harm the patient.

In the end, it’s a matter of reading the technical specifications closely. Regardless of what it is called, when sourcing a single-package isolator make sure that the rated voltage specifications exceed the voltage of the application by a safe margin. That’s not just for you or me, that’s for everyone.

About this author

Image of Bill Giovino

Bill Giovino is an Electronics Engineer with a BSEE from Syracuse University, and is one of the few people to successfully jump from design engineer, to field applications engineer, to technology marketing.

For over 25 years Bill has enjoyed promoting new technologies in front of technical and non-technical audiences alike for many companies including STMicroelectronics, Intel, and Maxim Integrated. While at STMicroelectronics, Bill helped spearhead the company’s early successes in the microcontroller industry. At Infineon Bill orchestrated the company’s first microcontroller design wins in U.S. automotive. As a marketing consultant for his company CPU Technologies, Bill has helped many companies turn underperforming products into success stories.

Bill was an early adopter of the Internet of Things, including putting the first full TCP/IP stack on a microcontroller. Bill is devoted to the message of “Sales Through Education” and the increasing importance of clear, well written communications in promoting products online. He is moderator of the popular LinkedIn Semiconductor Sales & Marketing Group and speaks B2E fluently.

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