Talk about ground suppression and interference in PCB design

- Dec 17, 2019-

Talk about ground suppression and interference in PCB design

Executive summary: What is a ground wire? The definition of the ground wire in the textbook that everyone has learned in the textbook is: The ground wire is an equipotential body that serves as a reference point for the circuit potential. This definition is not true. The potential on the actual ground is not constant. If you use a meter to measure the potential between the points on the ground line, you will find that the potentials at the points on the ground line may vary greatly. It is these potential differences that cause abnormal circuit operation. The definition of a circuit as an equipotential is simply what one would expect from a ground potential. HENRY gave the ground wire a more realistic definition. He defined the ground wire as: a low-impedance path for the signal to flow back to the source. This definition highlights the flow of current in the ground. According to this definition, it is easy to understand the cause of the potential difference in the ground. Because the impedance of the ground wire will never be zero, a voltage drop will occur when a current passes through a finite impedance. Therefore, we should imagine the potential on the ground line like waves in the sea, one after the other.

When it comes to the potential difference between the points on the ground caused by the impedance of the ground, it can cause malfunction of the circuit. Many people think it is incredible: when we use an ohmmeter to measure the resistance of the ground, the resistance of the ground is often at the milliohm level , How can such a large voltage drop occur when a current flows through such a small resistance, resulting in abnormal circuit operation.

Ground wire interference mechanism, common impedance interference When two circuits share a section of ground wire, the ground potential of one circuit will be modulated by the working current of the other circuit due to the impedance of the ground wire. The signals in such a circuit are coupled into another circuit. This coupling is called common impedance coupling.

In digital circuits, because the frequency of the signal is higher, the ground wire tends to show a larger impedance. At this time, if different circuits share a section of ground, common impedance coupling may occur.

Ground wire interference countermeasures, ground loop countermeasures From the ground loop interference mechanism, we can know that as long as the current in the ground loop is reduced, the ground loop interference can be reduced. If the current in the ground loop can be completely eliminated, the problem of ground loop interference can be completely solved. Therefore, we propose the following solutions for ground loop interference.

A. Floating the equipment at one end If you float the circuit at one end, the ground loop is cut off, so the ground loop current can be eliminated. However, there are two issues to be aware of. One is for safety reasons, and the circuit is often not allowed to float. At this time, you can consider grounding the device through an inductor. In this way, the ground impedance of the 50Hz AC current device is very small, and for high-frequency interference signals, the equipment ground impedance is large, which reduces the ground loop current. But this can only reduce the ground loop interference of high frequency interference. Another problem is that although the device is floating, there is still parasitic capacitance between the device and ground. This capacitor will provide lower impedance at higher frequencies, so it cannot effectively reduce the high-frequency ground loop current.

B. Using transformers to achieve connection between devices Using magnetic circuits to connect two devices can cut off the ground loop current. However, it should be noted that the parasitic capacitance between the primary and secondary stages of the transformer can still provide a path for higher frequency ground loop currents. Therefore, the method of transformer isolation has a poor suppression effect on high frequency ground loop currents. One way to improve the high-frequency isolation effect of a transformer is to provide a shield between the primary stages of the transformer. However, it must be noted that the ground terminal of the shield of the isolation transformer must be at the end of the receiving circuit. Otherwise, not only the high-frequency isolation effect cannot be improved, but high-frequency coupling may be made more serious. Therefore, the transformer should be installed on the side of the signal receiving equipment. A well-shielded transformer can provide effective isolation at frequencies below 1MHz.

C. Using an optical isolator Another way to cut off the ground loop is to use light to implement signal transmission. This can be said to be the most ideal solution to the problem of ground loop interference. There are two methods for optical connection, one is the optocoupler and the other is the optical fiber connection. The parasitic capacitance of the optocoupler is generally 2pf, which can provide good isolation at very high frequencies. Fiber has almost no parasitic capacitance, but it is not as good as an optocoupler in terms of installation, maintenance, and cost.

D. Using a common mode choke The use of a common mode choke on the connecting cable is equivalent to increasing the impedance of the ground loop, so that under a certain ground voltage, the ground loop current will decrease. But pay attention to controlling the parasitic capacitance of the common mode choke, otherwise the isolation effect of high frequency interference is very poor. The more turns of the common mode choke, the larger the parasitic capacitance and the worse the high frequency isolation effect.

There are two ways to eliminate the common impedance coupling. One is to reduce the impedance of the common ground wire part, so that the voltage on the common ground wire will be reduced accordingly, thereby controlling the common impedance coupling. Another method is to avoid the circuits that are easy to interfere with each other by using a proper grounding method. Generally, it is necessary to avoid sharing the ground wire between strong and weak current circuits, and digital circuits and analog circuits. As mentioned earlier, the core issue of reducing the impedance of the ground wire is to reduce the inductance of the ground wire. This includes using flat conductors as ground wires and multiple parallel conductors that are far apart as ground wires. For printed circuit boards, laying a grid of ground wires on a double-layer board can effectively reduce the impedance of the ground wire. Although a layer of ground wire in a multilayer board has a small impedance, it will increase the cost of the circuit board . The proper grounding method to avoid common impedance is to connect a single point in parallel. The disadvantage of parallel grounding is that there are too many wires to ground. Therefore, in practice, it is not necessary for all circuits to be grounded in parallel at a single point. For circuits with less mutual interference, a series single point grounding can be used. For example, you can classify circuits according to strong signals, weak signals, analog signals, digital signals, etc., and then use series single-point grounding within similar circuits, and different types of circuits using parallel single-point grounding.

Summary: The main cause of electromagnetic interference caused by the ground wire in PCB is the impedance of the ground wire. When current flows through the ground wire, a voltage will be generated on the ground wire, which is the ground wire noise. Driven by this voltage, a ground loop current will be generated, which will cause ground loop interference. When two circuits share a ground, a common impedance coupling occurs. Methods to solve ground loop interference include cutting the ground loop, increasing the impedance of the ground loop, and using a balanced circuit. The method to solve the common impedance coupling is to reduce the impedance of the common ground part, or use a single point ground in parallel to completely eliminate the common impedance.

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