PCB Blog - Interference and Suppression of PCB Ground Wire

In PCB board design, especially in high-frequency circuits, some irregular and abnormal phenomena caused by ground wire interference are often encountered. This paper analyzes the causes of ground wire interference, introduces three types of ground wire interference in detail, and proposes solutions based on practical experience. These anti-jamming methods have achieved good results in practical applications, enabling some systems to operate successfully in the field. In the single-chip microcomputer system, the PCB board (printed circuit board) is an important component used to support circuit components and provide electrical connections between circuit components and devices. There must be a certain impedance in the conduction process. The inductance component in the wire will affect the transmission of the voltage signal, and the resistance component will affect the transmission of the current signal. The influence of the inductance is particularly serious in the high-frequency line. The effects of ground impedance must be noted and eliminated.

1. The reason for the interference
Resistance and impedance are two different concepts. Resistance refers to the impedance of the wire to the current in the DC state, and impedance refers to the resistance of the wire to the current in the AC state, which is mainly caused by the inductance of the wire. Since the ground wire always has impedance, when measuring the ground wire with a multimeter, the resistance of the ground wire is generally mmΩ. Taking a piece of wire with a length of 10 cm, a width of 1.5 mm and a thickness of 50 μm on the PCB as an example, the impedance can be obtained by calculation. R=ρL/s(Ω), where L is the length of the wire (m), s is the cross-sectional area of the wire (mm2), and ρ is the resistivity ρ=0.02, so the resistance value of the wire is about 0.026 Ω. When a piece of wire is far away from other wires and its length is much greater than its width, the self-inductance of the wire is 0.8 μH/m, then the inductance of a 10 cm long wire is 0.08 μH. Then calculate the inductive reactance of the wire by the following formula: XL=2πfL, in the following formula, f is the frequency (Hz) of the signal passing through the wire, and L is the self-inductance of the wire per unit length (H). Therefore, the inductive reactance values of the wire at low frequency and high frequency are calculated respectively: in the actual circuit, the signal that causes electromagnetic interference is often the pulse signal, and the pulse signal contains rich high-frequency components, so it will generate a large amount of noise on the ground wire. voltage. It can be seen from the above formula calculation that the wire resistance is greater than the wire inductance in low-frequency signal transmission. For digital circuits, the operating frequency of the circuit is very high, and the wire inductance is much larger than the wire resistance in high-frequency signals. Therefore, the impact of ground impedance on digital circuits is very considerable. This is why a large voltage drop occurs when current flows through a small resistor, causing the circuit to work abnormally.

2. Ground wire interference mechanism
2.1 Ground loop interference
Ground loop interference is a relatively common interference phenomenon that often occurs between devices that are connected by long cables and are far apart. The main reason for the electromagnetic interference caused by the ground wire is the impedance of the ground wire. When the 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, resulting in ground loop interference. Ground loop interference: Due to the different ground potentials of the two devices, a ground voltage is formed. Driven by this voltage, current flows between the loop formed by "device 1, interconnecting cable, device 2, and ground". Due to the unbalance of the circuit, the current on each wire is different, so a differential mode voltage will be generated, which will interfere with the circuit. Since the ground loop interference is caused by the ground loop current, it is sometimes found that when the ground wire of a device is disconnected, the interference phenomenon disappears, because the ground loop is cut off when the ground wire is disconnected. This phenomenon often occurs in the occasion of low-frequency interference. When the interference frequency is high, it does not matter whether the ground wire is disconnected or not.
2.2 Common Impedance Interference
In digital circuits, due to the high frequency of the signal, the ground wire often presents a large impedance. At this time, when several circuits share a section of ground wire, due to the impedance of the ground wire, the ground potential of one circuit will be modulated by the operating current of the other circuit, so that the signal in one circuit will be coupled into another circuit, this coupling It is called common impedance coupling. The method to solve the common impedance coupling is to reduce the impedance of the common ground wire, or use single-point grounding to completely eliminate the common impedance, which is a kind of interference phenomenon. Figure 2 is a simple circuit with four gates. Assuming that the output level of gate 1 changes from high to low, the parasitic capacitance in the circuit (sometimes there is a filter capacitor at the input of gate 2) will discharge to the ground wire through gate 1. Due to the impedance of the ground wire, the discharge current will be A peak voltage is generated on the ground line. If the output of gate 3 is low at this time, the peak voltage will be transmitted to the output terminal of gate 3 and the input terminal of gate 4. If the amplitude of this peak voltage exceeds the noise of gate 4 Threshold, it will cause malfunction of door 4.
2.3 Ground loop electromagnetic coupling interference
The "ground loop" will surround a certain area. According to the law of electromagnetic induction, if there is a changing magnetic field in the area surrounded by the loop, an induced current will be generated in the loop, causing interference. The changes of the space magnetic field are everywhere, so the larger the enclosed area, the more serious the interference.

3. Methods to solve the ground wire interference
3.1 Solve the ground loop interference
There are three basic ideas to solve the ground loop interference: one is to reduce the impedance of the ground wire, thereby reducing the interference voltage, but this has no effect on the ground loop interference caused by the second reason. The second method is to change the grounding structure, connect the ground wire of one chassis to another chassis, and ground through the other chassis, which is the concept of single-point grounding. The third is to increase the impedance of the ground loop, thereby reducing the ground loop current. When the impedance is infinite, the ground loop is actually cut off, that is, the ground loop is eliminated. Therefore, the following solutions are proposed to solve the ground loop interference.
1) Float the device on one side
If one side of the circuit is left floating, the ground loop is cut off, thus eliminating the ground loop current. But there are two issues that need attention. One is that for safety reasons, the circuit is not allowed to float. In this case, consider grounding the device through an inductor. In this way, the grounding impedance of the 50 Hz AC current equipment is very small, and for the interference signal with a higher frequency, the grounding impedance of the equipment is large, which reduces the ground loop current. But doing so 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 capacitance provides lower impedance at higher frequencies and therefore does not effectively reduce the high frequency ground loop current.
2) Use a transformer
The basic method to solve the ground loop interference is to cut off the ground loop. An isolation transformer is used for this purpose, and the signal transmission between the two devices is carried out by magnetic field coupling, avoiding the direct electrical connection. At this time, the interference voltage on the ground wire appears between the primary and secondary stages of the transformer, not at the input end of the circuit. One way to improve the high-frequency isolation effect of the transformer is to set up a shielding layer between the primary and secondary stages of the transformer. However, it must be noted that the grounding end of the shielding layer of the isolation transformer must be at the receiving end of the circuit. Otherwise, it will not only fail to improve the high-frequency isolation effect, but also make the high-frequency coupling more serious. Therefore, the transformer should be installed on the side of the signal receiving device. The transformer isolation method has some disadvantages, it cannot transmit DC, it is bulky, and the cost is high. Due to the parasitic capacitance between the primary and secondary of the transformer, the isolation effect at high frequencies is not very good.
3) Use Optical Isolation Elements
Transmitting signals with light is an ideal solution to ground loop problems. As shown in Figure 3, the parasitic capacitance of the optocoupler device is about 2 pF, so it can be isolated at very high frequencies. If an optical fiber is used, there is no problem of parasitic capacitance, and a very perfect isolation effect can be obtained. However, the use of optical fibers will bring other problems, such as: the need for greater power, the need for more peripheral devices, the linearity and dynamic range of optical connections cannot meet the requirements of analog signals, and the installation and maintenance of optical cables are more complicated. When using, pay attention to the use of optical isolation components
4) Use a common mode choke
The ground wire voltage is actually a common mode voltage, and the current flowing in the cable is the common mode current driven by this voltage. Using a common mode choke on the connecting cable is equivalent to increasing the impedance of the ground loop, so that the ground loop current will be reduced under the action of a certain ground voltage. But pay attention to control the parasitic capacitance of the common mode choke, otherwise the isolation effect on 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.
5) Suppression of ground loop interference by balanced circuit
A balanced circuit is defined as two conductors and the circuit they connect to have the same impedance with respect to a ground wire or other reference object. It is very difficult to balance at high frequency, and the actual circuit will have many parasitic factors, such as parasitic capacitance, inductance and so on. These parameters play a greater role in circuit impedance at higher frequencies. Due to the uncertainty of these parasitic parameters, the impedance of the circuit is also uncertain, so it is difficult to guarantee that the impedance of the two conductors is exactly the same. Therefore, at high frequencies, the circuit balance is often poor, which means that the balanced circuit has a poor suppression effect on the ground loop current interference with higher frequencies.

3.2 Eliminate common impedance coupling
There are two ways to eliminate the common impedance coupling. One is to reduce the impedance of the common ground wire, so that the voltage on the common ground wire is also reduced, thereby controlling the common impedance coupling. Another method is to avoid the common ground wire of circuits that are prone to mutual interference through appropriate grounding methods. Generally, it is necessary to avoid the shared ground wire of strong current circuits and weak current circuits, and the shared ground wire of digital circuits and analog circuits. 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 at a single point in parallel. For circuits with less mutual interference, single-point grounding in series can be used. For example, circuits can be classified according to strong signals, weak signals, analog signals, digital signals, etc., and then use series single-point grounding within similar circuits, as shown in Figure 4, and different types of circuits use parallel single-point grounding, as shown in Figure 5 shown. When the signal frequency is lower than 1 MHz, a single-point grounding method can be used to prevent it from forming a loop. When the signal frequency is higher than 10 MHz, use multi-point grounding to reduce the ground wire impedance as much as possible. The power line and the ground line should be as close as possible to the traces to reduce the enclosed loop area, thereby reducing the electric field interference caused by the external magnetic field to the loop cutting, and also reducing the external electromagnetic radiation of the loop. As mentioned earlier, the problem with reducing the impedance of the ground wire is to reduce the inductance of the ground wire. You can use a flat conductor as the ground wire, or use multiple parallel conductors that are far apart as the ground wire. For the PCB board, laying the ground wire grid on the double-layer board can effectively reduce the ground wire impedance. In the multi-layer board, a special layer can be used as the ground wire to reduce the impedance.

4. Conclusion
Anti-interference design is an important part of single-chip system design, and the quality of its design often determines the success or failure of the entire system. Regarding grounding, many monographs on electromagnetic compatibility have detailed discussions. However, the grounding method should be selected through experiments, and the ground wire interference should also be found and eliminated through experiments. This paper introduces the reasons and solutions for the interference caused by the ground wire, and explains the general methods and principles in the design of the ground wire. Only under the guidance of the theory, after a lot of test process and experience accumulation, can we better grasp the design method of the grounding system And interference elimination means, so as to better improve the reliability of the printed circuit boards work.