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Exploring the myth called ground

Posted: 15 Jan 2015     Print Version  Bookmark and Share

Keywords:voltage  voltmeter  PCB  ground  AC 

Several years ago, one my friends told me that there's no such thing as voltage. This was a real shock to me, because I had a voltmeter in the lab. He explained that Maxwell's equations, which are accepted as the basis for all electromagnetic (and therefore circuit) theory, include current, electric field, and magnetic field, but no voltage. In fact,

 

 

 

Most PCB designers talk about things such as DC supply voltage or a signal voltage on traces. If I bring up the idea of current flow, they will accept it. Then we discuss how the return current always flows back to its source on ground.

The term "ground" is probably the most misunderstood and misused term in electrical engineering. I blame the universities. They start their electrical engineering instruction with DC circuits and then progress to AC circuits with resistors, inductors, and capacitors. But the ideas of parasitic and nonschematic effects are seldom discussed in classes. Usually, lab assignments are relatively low-frequency projects, probably designed to ensure parasitic effects aren't encountered.

We learn to read schematics with this magical return current path called ground. At low frequencies, the physical distance between the ground connections is electrically small. This concept of having all ground nodes connected at the same point is reasonable.

In the real world of high-speed circuit boards, the physical distance between ground node connections isn't electrically small, so the distance between the nodes becomes meaningful. Current must travel some distance to return to its starting point. This distance can adds losses that make ground something else entirely.

For example, let's take a microstrip trace over a metal plane, but let's have a PCB stack up where the metal plane is assigned as 3.3 V (not "ground"). A ground plane will exist lower in the PCB stack. The schematic will indicate the return current path is ground, not 3.3 V. Unfortunately, the current does not read the schematic to see where the designer intended the current to flow. It will follow the path of least impedance. At frequencies higher than about 1MHz, this means the path of least inductance. That path will nearly always have the smallest current loop size, so the current will flow on the 3.3 V plane, not the ground plane.

The image below shows the electric field lines for a microstrip. Where those field lines terminate, current will flow (into or out of the page). Even though the signal on the trace was driven relative to a ground plane (which is not shown) below the VCC plane, the field lines terminate on the VCC plane. The return current will always flow in the nearest plane, regardless of its schematic name.

Figure 1: Electric field lines for a microstrip. (Source: Hyperlynx).


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