Grounding – The Essential Facts

By William D. Kimmel, P.E.

and Daryl D. Gerke, P.E.

Kimmel Gerke Associates, Ltd.

We seem to write a lot about grounding – probably because we find so many problems there. The fact is that the overwhelming number of EMI problems, whether emissions, immunity or self-compatibility, have inadequate grounding at the root. That doesn’t say that you will solve your problems by grounding, but it does say that good grounding is a necessary first step.

Grounding started before electronics was on the map – we grounded for lightning and electrical safety, which has little to do with EMI. The early days of electronics was largely the telephone people who discovered the single point ground as a means of controlling low frequency interference to phone lines. The radio people also learned about grounding, too, but at high frequencies – you won’t see a single point ground in the radio business.

Let’s take a look at grounding – its bare essentials. We may not be able to solve your grounding problem, but we can take a lot of the mystery out of it and at least steer you in the right direction.

What Is A Ground?

In electronics, a ground is simply a current path. In most cases, ground has no relationship with “earth.” Earth ground has meaning only in the context of a lightning strike to a surface structure. For our purposes, we can consider ground to be a local reference for common circuits. We may be more descriptive and add an adjective, as “digital ground,” “power ground,” “analog ground,” and so forth.

The Ground Loop

A ground loop exists whenever two or more circuits share ground paths. Figure 1 shows the simplest case, that of two circuits sharing a return path. All you need to know about the effects of grounding is given by Ohm’s Law (substituting impedance for resistance, as we are usually dealing with time-varying signals):

V = IZ

We have indicated an impedance in the ground path to show that there is always an impedance between two separate points. Let’s suppose that circuit A is a noisy circuit pulling a significant amount of current, and that circuit B is a sensitive high impedance circuit. The current drawn by circuit A will drop a voltage Vn across the common return path. This noise voltage shows up as an undesirable signal to the input of circuit B.

The amount of noise voltage you can tolerate is application specific. But once you decide how much Vn you can tolerate, you are then left with two controllable variables, I and Z. If either one can be reduced to zero, Vn is zero, and your problems go away.

We can’t get the common impedance to zero, but for many cases, we can get close enough by using a ground plane which is wide to lower inductive impedance.

Alas, we can’t always get an adequately low impedance ground, especially when we are dealing with systems level grounding. In that case, we turn to the alternate variable – we reduce I to zero. Now that’s an interesting concept – if we can reduce the current in circuit A to zero, then the noise voltage drop across the ground impedance becomes zero, and the problem is also solved. Unfortunately, circuit A is there for a reason, and we have no control over the current that it draws. But we don’t have to reduce the current to zero, we just have to reduce the current to zero in the common return path, as shown in Figure 2. Here, we see that the noisy circuit still exists and draws current, but that current does not flow through the sensitive circuit ground.

Note that you can ground the two circuits at one common point, at either end (or anywhere in between, for that matter) without compromising the solution, hence, the term “single point ground.” This then is the essence of grounding: you can either reduce the ground impedance to an acceptably low level, or you can steer the ground currents along separate paths. As a rule, you work with one or the other variable, but not both.

Single Point Grounds

The single point ground is an elegant solution for sensitive devices in low frequency applications. It is the holy grail of telephones, sound systems and high impedance instrumentation. It is particularly important in systems applications, where cables may run for some distance, and where ground noise is significant. A single point ground is usually desired for power distribution, as well – you want the return current to come back on the wire, rather than on the enclosure or structure.

That concept works well for these needs. Where doesn’t it work?

First, a single point ground assumes the speed of light is infinite – events in the paths of interest occur simultaneously. For radio frequencies, that means that the distances must be a small fraction of a wavelength – anything longer than that and you have the makings of an antenna. Note that ¼ wavelength is a worst case situation – any ground a ¼ wavelength long is an open circuit (we use 1/20 wavelength as an approximate boundary for the onset of distributed behavior). For digital circuits, this includes any path length a significant fraction of a risetime.

Second, a single point ground assumes that you have exactly one ground path. This is achievable at audio frequencies, but at higher frequencies field coupling paths become efficient so that the ground currents don’t necessarily follow the designated conductive path. In short, single point grounds don’t exist at higher frequencies, no matter whether they are desirable or not.

Note that you either have a single point or a multipoint ground – there is no such thing as “almost a single point ground.” Once you have reached the point where a single point ground is not achievable, you must use multipoint grounding in your system.

Multipoint Ground

The most common case of a multipoint ground is on the printed circuit board, especially the multilayer board – the ground plane provides a low impedance. In fact, for high speed digital circuits, we need to use a ground plane in order to keep the ground impedance low or the circuit won’t work at its intended speed. Our general rule of thumb is, if you have a continuous ground plane, you are a good candidate for multipoint grounds.

Once you leave the protection of a single enclosure, your prospects for a multipoint ground diminish but do not become impossible. If you have several enclosures in reasonably close proximity (say, 10 feet), you can use a wide strap for a ground plane. Our rule of thumb is to keep the length of the strap no longer than five times the width. So a 10-foot-long ground strap would need to be two feet wide.

If you find it impossible to achieve a single point ground, your next best bet is to go to as many grounds as you can to reduce ground impedance to a low value. For a systems application, use ground grids and multiple ground straps.

Hybrid Grounds

Often, you need to have a single point ground for low level low frequency signals and a multipoint ground for high frequencies and high level signals and power. Then you turn to a hybrid ground combining the best features of both.

One common hybrid is the frequency-dependent ground, most commonly found in a cable shield, where one end is grounded, the other end is terminated to ground through a small capacitor. At low frequencies the capacitor looks like an open circuit, providing the single point ground. At high frequencies the capacitor looks like a short circuit, providing the multipoint ground.

The second common hybrid segregates sensitive analog circuits, noisy digital circuits and high level power circuits into separate areas. Each area is a multipoint ground of similar levels – they are connected together in a common single point ground. This provides a quiet area for the low level analog circuits.

Summary

In most digital and RF applications, multipoint grounds are necessary. This works well when you can keep the ground impedances low, as would be provided by a ground plane. When you can’t get the ground impedance low enough, single point grounds are appropriate. This would be more common in systems applications, especially where low level analog signals are present.

When deciding on your ground, start with Ohm’s Law. The goal of a good ground is to reduce the noise voltage to an acceptable level, either by reducing the impedance or by steering the currents away from the affected path.