## Ground Loops Multiple Return Paths

Ground loops occur when there are multiple pathways that ground currents can take.

Ground loops are usually only a problem when the loops are large, like those that can occur when connecting two separately powered products. Figure 12.22 shows two products connected by a two-conductor signal path.

Both of the products are powered by mains (wall socket) voltage, and hence are referenced to Earth. One of the conductors of the interface is connected to ground at each of the products. If a signal is sent across the cable, it can return by the second wire, as intended, or it can return

Figure 12.15 Simple changes can make a big difference with high-frequency signals. Here are two examples of routing the ground to an IC. In the first example, the ground currents are forced to traverse a rather large loop to get from the IC to the ground plane underneath the traces. The second example provides much shorter ground paths.

Figure 12.15 Simple changes can make a big difference with high-frequency signals. Here are two examples of routing the ground to an IC. In the first example, the ground currents are forced to traverse a rather large loop to get from the IC to the ground plane underneath the traces. The second example provides much shorter ground paths.

via the ground connection (i.e., through the power supply ground wire and through wall wiring to the other power cable and into other products). This second return path causes a ground loop to be formed, as shown in the figure. At low frequencies (under 1 MHz is a decent rule of thumb), some of the return signal will follow the intended path and some will follow the longer path. The current that follows the longer path creates a rather large loop. This large loop creates large stray fields, and can create large radiated fields. At higher frequencies, the large path presents too high an impedance (due to its inductance) for any current to follow it; high-frequency current will follow along the intended return path, close to the signal current. This is one of the few exceptions to the general rule that high-frequency signals are more problematic than low-frequency signals.

Another problem associated with ground loops is that of susceptibility. The large loop area in conjunction with small loop resistance effi-

Figure 12.16 Here are two examples of routing a signal trace and its return to a connector. In the first example, the current forms a rather large loop. In the second example, the current loop is small.

ciently captures the ambient magnetic fields. This loop can also serve as a loop antenna for receiving radiated fields. For coaxial cables, this problem is again alleviated at high frequencies for two reasons. First, when the frequency of the interfering signal is high, the outer conductor of the coaxial cable becomes comparable in thickness to the skin depth of the metal. Thus, the signal return current flows on the inside of the outer conductor, and the interference current flows on the outside of the outer conductor. Interference between the two circuits is greatly reduced because it is as if the return current and interference currents are flowing in separate circuits, like a triaxial cable (refer to the section on cables). Second, the mutual inductance of the inner and outer conductors presents a large impedance to high-frequency common mode currents, making it difficult for high-frequency ground loop currents to flow. In summary, ground loops are mostly a low-frequency problem,

poor layout

good layout

Figure 12.17 Two examples of power-supply decoupling on a two-layer board. Light gray denotes bottom-layer traces. The first layout creates a large loop for the decoupling path, which will provide poor performance for high-speed signals. The second layer is better because the decoupling loop is reduced.

Figure 12.17 Two examples of power-supply decoupling on a two-layer board. Light gray denotes bottom-layer traces. The first layout creates a large loop for the decoupling path, which will provide poor performance for high-speed signals. The second layer is better because the decoupling loop is reduced.

causing noise in audio systems and other low-frequency applications, including control systems and sensor measurements.

## Solar Panel Basics

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