ArticleImpedance is the Bottom LineWhatever is said about the mysteries of grounding, the driving issue is still ground impedance. If you keep the ground impedance low enough, nothing else matters, not single points, not multi-points. Let's make things as simple as we can. The Very BasicsBasically, everything you really need to know about grounding is given by the very powerful relationship: Ohm's Law. In Figure 1, we have two circuits sharing a common ground path (the resistance indicates the impedance in the path). Supposing one leg is a noisy high current circuit, and one leg is a low level op amp. The noise voltage generated on ground is given by: 
Figure 1. Common ground impedance path Vn = IZ, where Vn is voltage, I is current, and Z is the impedance in the path. Vn appears as signal to the op amp input, ideally zero voltage, a condition that can be achieved by making either the impedance or the current equal to zero.  Figure 2. Common impedance reduced to zero If the impedance is zero (Figure 2), then Vn drops to zero and nothing else matters. This is fairly realistic if we are on a ground plane, but difficult to achieve without a ground plane. If we can't use a ground plane between the source and load, we may have difficulty getting the ground impedance low enough to meet our needs. In that case, we have to attack current, the second parameter in the equation. Presumably, we can't reduce the current in the noisy load (and, hence the return path) to zero, but we can steer the return currents along separate paths (Figure 3) — the noise voltage, Vn, is still there, but it does not appear as signal to the op amp. This, as you might expect, is the hallowed single point ground. Single-Point and Multi-Point GroundsPerhaps nothing in the grounding world has received as much attention as the single point ground. It originated in the early days of audio electronics, including telephony, the purpose being to keep power line frequencies out of the audio system. The basic approach is to run your sensitive circuit grounds all to one point in your installation, preventing noisy facility currents from flowing through your circuits. It works wonders when working with audio frequencies, which is why it is so popular.  Figure 3. Separate ground paths avoid common noise voltage Unfortunately, the single point ground became the Holy Grail, and has been extended to higher frequencies, mostly without understanding why. Single point grounds make one basic assumption — that the speed of light is infinite. Or, less picturesquely, that everything in the circuits of interest occurs simultaneously — propagation delays are negligible. This is basically the difference between lumped circuit and distributed behavior — when run lengths are such that propagation delays are significant, you are in distributed circuit territory, and, therefore, in multi-point ground territory. To give an example, suppose we have ground wire ten inches long, or about 25cm. This is a quarter wavelength at 300MHz, at which frequency the ground vanishes. If we expect this ground to be effective, it needs to be significantly shorter than a quarter wave — our criteria is at 1/20 wavelength. The second point is that at high frequencies, parasitic paths become dominant — even though you have a single point metallic ground, the high frequency currents follow other paths. Low frequencies stick to the wire, so you can pretty much figure out where the current is going. At higher frequencies, series inductance in the path discourages currents from following the wire, opting for lower impedance alternate paths, both capacitive and planar paths. So we don't have a single point ground, even if we try. Audio frequencies can generally be considered low frequency, but once we get to about one MHz, parasitic paths become significant: single point grounds are not in the RF engineers' vocabulary, and it shouldn't be in the digital engineers' vocabulary either. Multi-Point GroundsSo, if single point grounds are inappropriate or don't work, we need to concentrate on getting a low impedance ground path. This is best approximated with a ground plane which provides as low an impedance as you are going to get. If the impedance of the path is zero, nothing else matters. There are two situations where ground impedance becomes a problem, in cables and in ground terminations. Once you have left the protection of a ground plane and step out onto a cable, your ground impedance rises by several orders of magnitude, making it much tougher to achieve a low impedance ground. Handling Multi-point GroundsSo, in most cases, a multi-point ground is necessary, and the goal is to get the ground impedance as low as feasible. How do we do this? Here are your choices:
SummarySingle point grounds are not appropriate for most of modern electronics — the only exception is the low-level audio frequency input signals. For the rest, use multi-point grounds, and fight to get the impedance as low as possible, making direct planar contact if possible or using multiple connections with short, fat straps. | advertisement |
