The Chronicles of GND (Part 2): Single Supply = Multiple Problems

The Chronicles of GND (Part 2): Single Supply = Multiple Problems

The Filter Wizard's latest installment of "The Front End" looks at how the ancestral “split-supply” PCB layouts have mostly ceded to “single-supply” configurations, and the GND implications that often ensue.

When I get a printed-circuit-board (PCB) layout back for review, there are a few things I look for first. I can usually get a feeling for what kind of board designer has been at work, too, specifically if s/he is (or s/he thinks s/he is) a “digital” designer, an “RF” designer, or a “power” designer.

You might have formed the impression by now that I can be something of a picky customer. I know what I like in my PCB designs. In my younger years, I designed a fair few—over 300, from what I can piece together from the archeological residues of a lifetime in electronic engineering. And while some of these designs had modest digital content, the vast majority of them were for systems intended for low- to medium-frequency analog signal conditioning and processing done to (I hoped) a world-class performance level.

When the first PC-based board design tools appeared, I threw myself into board design with gusto. At last, control over how I wanted things done! No more would I have to lean over the shoulder of the guy with the arcane layout skills, the tape, and the scalpel, and yell “No, route it there! Route it there!” (We are still friends, by the way, after meeting the first time some 40 years ago when he was testing a home-build amplifier while on his lunch hour. I see him every Christmas for drinks.)

Naturally, I didn’t know nearly as much about how to do good layout as I thought I did, and I made plenty of mistakes. “The man who never made a mistake never made anything!” my dad used to preach, and it’s a wise observation. I’ve made plenty of mistakes in the past, in multiple contexts, including in the search for how to get the best performance out of a circuit when it makes that transition from your head to FR4.

Running Aground

So, let’s get back on track (spontaneous and unintended pun alert). What did I look for first in this latest design? Well, since this is another episode of “The Chronicles of GND,” it won’t surprise you to learn that my first checks are GND-related. Specifically, related to this system being, as is nearly every design I encounter these days, a “single-supply” system. What does that mean?

Well, analog circuitry from the Golden Age of Operational Amplifiers, during which I learned much of my craft, was almost always powered from a “split supply,” typically +15 V and −15 V by the time I started in the industry. Instead of having two terminals on your power-supply box (plus and minus), you had three (plus, “nought volts,” and minus).

The reasons for this are lost in the fog of time, but one thing is clear. Much of the development work on the kinds of analog circuits I’m referring to—amplifiers, integrators, and filters—came from early Analog Computers. In such systems, voltages (and sometimes currents) were the “analog” of a variable or result that the computer was required to “compute.” And since that result could be either a positive or a negative number, it was sensible that the circuits could operate with positive or negative voltages, with respect to a reference that represented a result of zero.

The circuits in a typical “split-supply” system draw current from the plus terminal of the supply, and return it to the minus terminal. Sometimes a little current may end up routing to “nought volts,” usually due to resistive voltage dividers and resistors from various circuit nodes to “nought volts.”

However, “nought volts” was never thought of as a supply rail like plus or minus. It’s a reference point, where your scope ground clip or the (mandatory, of course) second lead of your two-leaded voltmeter would connect. It was also likely to be connected to a nice ground plane under the insulating surface of your bench, and to the safety grounds of all the test equipment that kept the lab warm in winter.

Now, of course, “nought volts” was the spiritual precursor to GND. In fact, we often called it “ground,” and things connected to it as “grounded.” Since it was connected to the electrical safety “earth,” it was even possible that it found its way back to some metal buried deep in the actual ground, snuggling up to Mother Earth itself. So fundamental are the mythic origins of GND.

All for Nought?

Fast forward to today’s “single-supply” systems. Here your circuits have a positive supply, but they don’t have a negative supply. Or (pause for theatrical effect…) do they? Well, yes, they do. It just happens to be at (ostensibly) the same potential as GND. Effectively, the value of the negative power-supply voltage is zero. So that means you can just connect it to GND and be done with it, right? No one in their right mind would go to the bother of building a power supply whose output voltage is zero.

Well, maverick that I am, I’m here to tell you that—in principle, at least—that’s exactly what you should do to get the best and most predictable results from your board layouts.

This is because that negative power supply has a job to do. It has to sink the current “coming out” of the negative supply pin of each of your amplifiers, comparators, and whatnot. And this current is likely to be signal- and load-dependent. You do not want this flowing through all of the same conductors that direct your lovely clean GND reference to the sensitive points of your amplifier circuits and converters.

For anything that’s a “power” pin required to be at GND potential, you should connect it to a separate net, often called 0 V (“nought volts!”), to represent this negative power supply of value zero. 0 V will have to be connected to GND at some point; this is best done at the “star point,” where the power actually arrives at the board or subsystem. This connection should not be done as any kind of copper on the board that your software can recognize as conductive. I sometimes define a small copper object that I can place as needed.

The netlist should contain two completely distinct nets, GND and 0 V, so you can eyeball the netlist to be completely sure that pins are connected to the correct nets. You do do a visual inspection of your design netlists, don’t you? Become familiar with one of the simple, easily readable netlist formats and export your netlist into that format to check it.

Some people call 0 V “dirty ground.” But it really has nothing to do with GND at all, except that it’s very close in potential to it.

By this point you might be asking “What’s wrong with letting all of these currents mix and just having a super-low-impedance ground plane to keep all of the voltage drops small?” Well, small isn’t zero, for one thing. In some systems, microvolts can matter. And there are some very specific reasons in op-amp-based circuits why you should be suspicious about the currents coming out of the negative supply pins. For instance… no, only kidding, I’m going to leave you hanging on that thought until next time.

Meanwhile, as usual, agreements and disagreements are welcome through the usual channels. Has something about a single-supply system “ground” you down? Let me know!

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