Absrtact: in PCB design, especially in high-frequency circuits, we often encounter some irregular and abnormal phenomena caused by ground wire interference. This paper analyzes the causes of ground wire interference, introduces three types of ground wire interference in detail, and puts forward solutions according to practical application experience. These anti-interference methods have achieved good results in practical application, making some systems run successfully in the field.
In the single chip microcomputer system, PCB (printed circuit board) is an important component used to support circuit components and provide electrical connection between circuit components and devices. PCB wires are mostly copper wires. The physical characteristics of copper also lead to certain impedance in the conductive process. The inductance component in the conductor will affect the transmission of voltage signals, while the resistance component will affect the transmission of current signals, especially in high-frequency lines, Therefore, we must pay attention to and eliminate the influence of ground impedance in PCB design.
1. Causes of interference
Resistance and impedance are two different concepts. Resistance refers to the impedance of the conductor to the current in the DC state, while impedance refers to the impedance of the conductor to the current in the AC state. This impedance is mainly caused by the inductance of the conductor. Since the ground wire always has impedance, when measuring the ground wire with a multimeter, the resistance of the ground wire is generally mm Ω.
The length of one section on PCB is 10 cm, the width is 1.5 mm, and the thickness is 50 μ M conductor as an example, its impedance can be obtained by calculation. R= ρ L / S (Ω), where l is the length of conductor (m), s is the sectional area of conductor (mm2), ρ Is resistivity ρ= 0.02, so the resistance value of this wire is about 0.026 Ω.
When a section of wire is far away from other wires and its length is far greater than the width, the self inductance of the wire is 0.8 μ H / m, then the inductance of 10 cm long conductor is 0.08 μ H。 Then the inductive reactance of the conductor is calculated from the following formula: xl=2 π FL, where f is the frequency of the signal passed by the conductor (Hz), and l is the self inductance of the unit length of the conductor (H). Therefore, the inductive reactance values of the conductor at low frequency and high frequency are calculated respectively:
In the actual circuit, the signal causing electromagnetic interference is often pulse signal, which contains rich high-frequency components, so it will generate a large voltage on the ground. Through the above formula calculation, it can be seen that in low-frequency signal transmission, the conductor resistance is greater than the conductor reactance. For digital circuits, the working frequency of the circuit is very high, and in high-frequency signals, the conductor reactance is much greater than the conductor resistance. Therefore, the influence of ground impedance on digital circuits is considerable. This is the reason why a large voltage drop occurs when the current flows through a small resistance, resulting in abnormal operation of the circuit.
2 ground wire interference mechanism
2.1 ground loop interference
Ground loop interference is a common interference phenomenon, which often occurs between devices connected by long cables and far away from each other. The main cause of 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, the ground loop current will be generated, forming ground loop interference. As shown in Figure 1, there are two grounded circuits.
Because the ground potentials of the two equipment are different, a ground voltage is formed. Driven by this voltage, there is current flow between the loops formed by "equipment 1 - interconnection cable - Equipment 2 - ground". Due to the imbalance of the circuit, the current on each wire is different, which will produce differential mode voltage and cause interference to 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 disappears because the ground loop is disconnected when the ground wire is disconnected. This phenomenon often occurs in the case 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, because of 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 working current of another circuit, so the signal in one circuit will be coupled into another circuit. This coupling 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 a single point grounding to completely eliminate the common impedance. The example in Figure 2 illustrates an 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 through gate 1. Due to the impedance of the ground, the discharge current will generate a spike voltage on the ground. If the output of gate 3 is at a low level, the spike voltage will be transmitted to the output of gate 3 and the input of gate 4. If the amplitude of the spike voltage exceeds the noise threshold of gate 4, It will cause the misoperation of door 4.
2.3 electromagnetic coupling interference of ground loop
The "ground loop" shown in Figure 1 will surround a certain area. According to the law of electromagnetic induction, if there is a changing magnetic field in the area surrounded by this loop, it will generate induced current in the loop and form interference. Space magnetic field changes everywhere, so the larger the surrounding area, the more serious the interference.
3. Methods for solving ground wire interference
3.1 solving ground loop interference
There are three basic ideas to solve the ground loop interference: one is to reduce the impedance of the ground wire so as to reduce 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 by connecting the ground wire of one chassis to another chassis and grounding through another chassis. This 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 to ground loop interference are proposed.
1) Float the equipment on one side
If one side of the circuit is floating to the ground, the ground loop is cut off, so the ground loop current can be eliminated. However, there are two problems to be noted. One is that the circuit is not allowed to float for safety reasons. At this time, the equipment can be grounded through an inductor. In this way, the equipment grounding impedance is very small for AC current of 50 Hz, while for interference signals with high frequency, the equipment grounding 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 equipment is floating to the ground, there is still a parasitic capacitance between the equipment and the ground. This capacitance will provide a lower impedance when the frequency is high, so it can not effectively reduce the high-frequency ground loop current.
2) Use transformer
The most basic method to solve the interference of ground loop is to cut off the ground loop. The isolation transformer is used to play this role. The signal transmission between the two devices is carried out through magnetic field coupling, and the direct electrical connection is avoided. At this time, the interference voltage on the ground wire appears between the primary stages of the transformer, not at the input of the circuit. One way to improve the high-frequency isolation effect of transformer is to set a shielding layer between the primary stages of transformer. However, it must be noted that the grounding end of the shielding layer 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 also the high-frequency coupling may be more serious. Therefore, the transformer shall be installed on one side of the signal receiving equipment.
The method of transformer isolation has some disadvantages, such as unable to transmit DC, large volume and high cost. Due to the parasitic capacitance between the primary stages of the transformer, the isolation effect at high frequency is not very good.
3) Using optical isolation elements
Optical signal transmission is an ideal method to solve the problem of ground loop. As shown in Figure 3, the parasitic capacitance of the optocoupler device is about 2 PF, so it can play an isolation role at a very high frequency. If optical fiber is used, there is no parasitic capacitance problem, and a very perfect isolation effect can be obtained. However, the use of optical fiber will bring other problems, such as the need for greater power, the need for more peripheral devices, the linear and dynamic range of optical connection can not meet the requirements of analog signals, and the complexity of optical cable installation and maintenance, which should be paid attention to during use.
5) Suppression of the interference of the balance circuit to the ground loop
The definition of a balanced circuit is that two conductors and their connected circuits have the same impedance relative to the ground wire or other reference objects.
It is very difficult to balance at high frequency. The actual circuit will have many parasitic factors, such as parasitic capacitance, inductance and so on. These parameters play an important role in the circuit impedance at higher frequencies. Because of the uncertainty of these parasitic parameters, the impedance of the circuit is also uncertain, so it is difficult to ensure that the impedance of the two conductors is exactly the same. Therefore, at high frequency, the circuit balance is often poor, which means that the balance circuit has poor suppression effect on the current interference of the ground loop with high frequency.
3.2 eliminating 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, so as to control the common impedance coupling. Another method is to avoid the common ground wire of circuits that are easy to interfere with each other through appropriate grounding methods. Generally, it is necessary to avoid the common ground wire of strong current circuits and weak current circuits, and the common ground wire of digital circuits and analog circuits. The disadvantage of parallel grounding is that there are too many grounded conductors. Therefore, in practice, it is not necessary for all circuits to be grounded in parallel. For circuits with less mutual interference, series single point grounding can be used. For example, the circuits can be classified according to strong signal, weak signal, analog signal, digital signal, etc., and then the series single point grounding is used in the same kind of circuits, as shown in Figure 4. The parallel single point grounding is used for different types of circuits, as shown in Figure 5. When the signal frequency is lower than 1 MHz, the method of single point grounding can be adopted to prevent it from forming a loop. When the signal frequency is higher than 10 MHz, it is better to adopt multi-point grounding to minimize the ground wire impedance. The power line and ground wire should be as close to the wiring as possible to reduce the enclosed loop area, so as to reduce the electric field interference caused by the external magnetic field on the loop cutting, and also reduce the external electromagnetic radiation of the loop.
As mentioned earlier, the core problem of reducing the ground wire impedance is to reduce the inductance of the ground wire. The flat conductor can be used as the ground wire, or multiple parallel conductors far away from each other can be used as the ground wire. For PCB, laying a ground grid on a double-layer board can effectively reduce the ground impedance. In a multi-layer board, a special layer can be used as the ground to reduce the impedance.
4 Conclusion
Anti-interference design is an important part of single-chip microcomputer system design. The quality of its design often determines the success or failure of the whole system. As for grounding, many monographs on electromagnetic compatibility have detailed discussions. However, the best grounding method should be selected through tests, and ground wire interference should also be found and eliminated through tests. This paper introduces the causes and solutions of interference caused by ground wire, and explains the general methods and principles in ground wire design. Only under the guidance of theory, through a large number of test processes and experience accumulation, can we better master the design methods and interference elimination means of grounding system, so as to better improve the reliability of circuit work.
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