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Analog Angle Article

Getting ungrounded

We use the term “ground” a lot, as a noun, as a verb, as a concept, and an answer to many design questions. And we should, since ground and grounding are important factors of almost every design.

But lately, I have stopped using that term where it does not apply—and that' a lot of places. In fact, you might say that I have become “ungrounded.” For battery-powered equipment, as well as automotive systems and airborne or space systems, there is a “common” but there is no ground, unless we are using the term as another word for common.

But w shouldn't. A real ground refers to a circuit point which is Earth-connected, and thus can take advantage of, or needs, the huge reservoir of electrons that the Earth offers. This ground not only acts as a signal common (not very good one, admittedly), it also provides a relief path for ESD, overvoltage, and system faults. An ungrounded appliance, for example, can be lethal if its metal enclose accidently connects to the AC mains; a grounded one will pop the local circuit breaker.

Inherently, battery-powered devices are ungrounded. Sometimes this is a virtue, as when you need to make floating measurements across individual cells in a series string of batteries, and the top cell may be at a potential of hundreds of volts above Earth ground. [An aside here: remember that “voltage” is a shorthand tem for “potential difference” and you should always keep in mind the unspoken “with respect to what” part when you say “the voltage is 'x' at this point.”]

The interesting thing is that there are times when you want your system grounded and there are times when you deliberately don't. In the case of the battery-cell string, you want to make sure the instrument is ungrounded (floating) so that it can be protected against high potential in the string and also make a differential measurement across a given cell. But in a reverse example, many AC-powered medical instruments carefully isolate the patient's I/O so that any failures in the instrument itself do not ripple to the patient.

So you stand your ground, and I'll stand mine, but more importantly, don't say “ground” where what you really have is “common”. Not only is ungrounded a design reality in so many cases, but using ground may lead you to unconsciously assume you actually have mother Earth at your disposal, for better and worse.♦

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