The Chronicles of GND, Part 1

Beware the one-leaded voltmeter…

So out there on LinkedIn, “single supply” just came up and a bunch of you lovely readers weighed in. It reminded me of the “Chronicles of GND” sequence I wrote quite a few years back for Electronic Design. so, lightly edited, this and the next two posts will tell the story. Gather round…

At the time I wrote this piece, some colleagues were working on a demonstration for the big ‘Embedded World’ show in Germany. They plumbed in some natty speech recognition technology into the super new dual-core processor we’d just launched. The buzz-phrase for this is “wake phrase recognition”, and all you up-to-date people out there have no doubt already used it when you intone “OK, Google” or “Hey, Siri” or “Dang, Alexa, I did not order kitchen towel”. The difference is that for personal-scale embedded devices like watches, rings and earbuds without a permanent connection to “The Cloud”, the processing has to be done locally, and at super-low average power.

I only mention it because this “wake phrase recognition” will be essential for the Engineer’s Smart Watch that I feel I must crowdfund rather urgently. It’s a special Smart Watch to be worn by any engineer who gets involved in electronic circuit or system design. Its unique feature is that it will deliver a painful electric shock to the wearer upon detecting a preset “wake phrase” of my choice. And the very first phrase that will go on its list is:

“voltage at”

as in

“I measured the voltage at the junction of R5 and C3.”

Accepting this phrase at face value is a symptom of a dread disease. I refer to this ailment as “Kirchhoff’s Madness”, and its main carrier is an imaginary beast that I call the One-Leaded Voltmeter. I say imaginary, because none of the meters I’ve ever seen for sale here in the real world have just one lead. Two leads seems to be very much the order of the day.

The problem has become far more prevalent in this age of circuit simulation, where map is assumed identical to territory. Here’s one example I found in the wild after a quick search:

Now, I can live with this hypothetical one-terminal flourish when the currency of your simulation is a signal of unspecified (and irrelevant) physical form. Here’s a less harmful version, also found on the web:

In this type of diagram the signals flowing through the connecting lines are just dimensionless carriers of information in a system-level simulation. No electrons are harmed in the acquiring of that information.

What, then, is the problem that is getting me all hot under the collar? It’s this: voltage is something that exists between two points, not at one point. You may dimly recall from the physics of your youth that the potential between two points is the definite integral of the electric field along a path between those points (any path); the definite integral is required in order to eliminate the constant of integration. Trying to “measure” a “voltage” with a one-leaded meter is like trying to calculate a numeric value from an indefinite integral – you have not defined a range or path over which to do the calculations.

Of course, when pressed, the sufferer of Kirchhoff’s Madness will protest that, of course, the other lead of the voltmeter has been connected to (they may wave a limb or other body part) GND, by some higher authority. The existence of GND is axiomatic to their worldview, and all measurements of voltage require only the one true lead.

Now, there are circumstances where the temptation to take the two leads of a real voltmeter in two hands and go probing between points could be harmful. Any experienced engineer who has worked on high-voltage circuits will tell you that low voltage systems are easy enough that you can work on them with one hand behind your back, but that high voltage systems are hazardous enough that you must work on them with one hand behind your back. In this case, you need to carefully attach “the other lead” of your voltmeter to one carefully-selected reference point and leave it there while you go exploring with the other lead.

Back to the watch and the punishment shock. The moral of today’s tale is this: always make it clear where both leads of your voltmeter are connected, when you declare that you have measured a voltage. If there is an unambiguous place where you feel comfortable in parking “the other lead”, write down what that place is, and try to show it on the circuit diagram of your design. This will, of course, rapidly illustrate that almost every circuit diagram fails correctly to define this one critical node.

A useful tip is to take a photograph of the setup in front of you, so that everyone can see where that other lead went – and, in a goodly percent of cases, inform you that the reason your numbers suck is that this was a bad place to connect that lead. Hashtag not kidding.

Another common symptom of Kirchhoff’s Madness is the belief that current flowing down a wire is akin to water flowing in a pipe. The water company sends water to you down this pipe. You do what you want with it, but it never ever gets back to the water company, it just soaks into your lawn or drains into a sewer somewhere. When (mis)applied to electronic circuits, this commonly leads the incautious into trouble. The temptation is to believe that currents that flow to – look, there it is again – GND, somehow disappear, like the spider that a parent brushes off the bed with a casual “it’s gone now”. Nope. That current is still flowing, in a closed loop that will eventually get it right back to where you first encountered it – after it has had some interesting adventures, causing trouble in places you weren’t looking. A bit like that spider.

More “The Chronicles of GND” shortly. For now, maybe you’d like to fess up about a time when the “voltage at” shock would have worked as a wake-up call to ensure you connected “the other lead” up right!