In the process of PCB outsourcing design, especially in high-frequency circuit design, some irregular and abnormal phenomena caused by ground wire interference are often encountered. In this paper, the causes of interference caused by ground wire are analyzed, three types of interference caused by ground wire are introduced in detail, and solutions are put forward according to practical experience. These anti-interference methods have achieved good results in practical application, and made 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, and the physical characteristics of copper itself also lead to certain impedance in the conductive process. The inductance component in the wires will affect the transmission of voltage signals, while the resistance component will affect the transmission of current signals, especially in high-frequency circuits. Therefore, the influence of inductance must be paid attention to and eliminated in PCB design.
1. Causes of interference in pcb design and painting
Resistance and impedance are two different concepts. Resistance refers to the resistance of a wire to current in DC state, while resistance refers to the resistance of a wire to current in AC state, which is mainly caused by the inductance of the wire. Because there is always impedance in the ground wire, when measuring the ground wire with a multimeter, the resistance of the ground wire is generally mmω.
Take a 10 cm long, 1．5 mm wide and 50μm thick wire on PCB as an example, the impedance can be obtained by calculation. R = ρ l/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 0.026ω.
When a section of wire is far away from other wires and its length is much larger than its width, the self-inductance of the wire is 0.8 μ h/m, so the inductance of a 10 cm long wire is 0.08 μ h. Then, the inductance of the wire is calculated by the following formula: XL=2πfL, where f is the frequency (Hz) of the signal passing through the wire, and l is the self-inductance (h) per unit length of the wire. Therefore, the inductance values of the wire at low frequency and high frequency are calculated respectively:
In practical circuits, the signal that causes electromagnetic interference is often pulse signal, which contains rich high-frequency components, so it will produce a large voltage on the ground. From the above formula calculation, it can be seen that the wire resistance is larger than the wire inductance in low frequency signal transmission. For digital circuits, the working frequency of the circuit is very high, and the wire inductance is much larger than the wire resistance in high frequency signals. Therefore, the influence of ground impedance on digital circuits is considerable. This is the reason why the current flowing through a small resistor causes a large voltage drop, which leads to the 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 apart. The main reason for the electromagnetic interference of the ground wire is the impedance of the ground wire. When the current flows through the ground wire, it will generate voltage on the ground wire, which is the noise of the ground wire. Under the drive of this voltage, the ground loop current will be generated, resulting in ground loop interference. As shown in Figure 1, there are two grounded circuits.
Because the ground potentials of the two devices are different, a ground voltage is formed. Driven by this voltage, a current flows between the loops formed by “device 1-interconnection cable-device 2-ground”. Because of the imbalance of the circuit, the current on each wire is different, so it will produce differential mode voltage, which will cause interference to the circuit.
Because 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 cut off when the ground wire is disconnected. This phenomenon often occurs in low frequency interference situations. When the interference frequency is high, it doesn’t matter whether the ground wire is disconnected or not.
2.2 Common impedance interference
In digital circuits, because of the high frequency of signals, 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 that the signal in one circuit will be coupled into another circuit. This coupling is called common impedance coupling.
The solution to the common impedance coupling is to reduce the impedance of the common ground or to use 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 gate circuits. Assuming that the output level of gate 1 changes from high to low, the parasitic capacitance in the circuit (sometimes the input of gate 2 has filter capacitance) will discharge to the ground through gate 1. Due to the impedance of the ground, the discharge current will generate a peak voltage on the ground. If the output of gate 3 is low at this time, this peak voltage will be transmitted to the output of gate 3 and the input of gate 4. If the amplitude of this peak voltage exceeds the noise threshold of gate 4, it will cause the misoperation of gate 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. The change of magnetic field in space is everywhere, so the larger the enclosed area, the more serious the interference.
3. Solution to the interference of ground wire
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, thus 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, 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 floated to the ground, the ground loop is cut off, so the ground loop current can be eliminated. However, there are two issues that need attention. One is that the circuit is not allowed to float on the ground for safety reasons. At this time, consider grounding the equipment through an inductor. In this way, the grounding impedance of the equipment with 50 Hz AC current is very small, while the grounding impedance of the equipment with high frequency interference signal 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 on the ground, there is still parasitic capacitance between the equipment and the ground. This capacitance will provide a lower impedance at higher frequencies, so it cannot effectively reduce the high-frequency ground loop current.
2) Use transformer
The most basic way to solve the ground loop interference in PCB outsourcing design is to cut off the ground loop. The isolation transformer plays this role, and the signal transmission between the two devices is carried out by magnetic field coupling, thus avoiding direct electrical connection. At this time, the interference voltage on the ground line appears between the primary stages of the transformer, but not at the input of the circuit. One way to improve the high-frequency isolation effect of the transformer is to set shielding layers between the primary stages of the transformer. However, it must be noted that the grounding end of the shielding layer of the isolation transformer must be at one end of the receiving 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 one side of the signal receiving equipment.
The method of transformer isolation has some disadvantages, such as inability to transmit DC, large volume and high cost. Because of parasitic capacitance between the primary stages of the transformer, the isolation effect at high frequency is not very good.
3) Use optical isolation elements.
Optical transmission signal is an ideal method to solve the problem of earth loop. As shown in Figure 3, the parasitic capacitance of the optocoupler 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 perfect isolation effect can be obtained. However, the use of optical fiber will bring other problems, such as the need for more power, more peripheral devices, the linear and dynamic range of optical connection can not meet the requirements of analog signals, and the installation and maintenance of optical cables are complicated, so attention should be paid when using them.
4) Use common mode choke.
The ground voltage is actually a common-mode voltage. Driven by this voltage, the current flowing in the cable is a common-mode current. Using common-mode choke on the connecting cable is equivalent to increasing the impedance of the ground loop, so that the current of the ground loop will decrease under a certain ground voltage. However, attention should be paid to controlling the parasitic capacitance of common-mode choke, otherwise the isolation effect of high-frequency interference will be poor. The more turns of the common-mode choke, the larger the parasitic capacitance and the worse the effect of high-frequency isolation.
5) Suppression of the interference of the ground loop by the balanced circuit
The definition of a balanced circuit is that two conductors and their connected circuits have the same impedance relative to the ground or other reference objects.
Time-frequency balance is very difficult, and the actual circuit will have many parasitic factors, such as parasitic capacitance, inductance, etc. These parameters play an important role in circuit impedance when the frequency is high. Because of the uncertainty of these parasitic parameters, the impedance of the circuit is also uncertain, so it is difficult to ensure that the impedances of the two conductors are 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 high frequency ground loop current interference.
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 line, so that the voltage on the common ground line also decreases, thus controlling the common impedance coupling. Another method is to avoid the common ground wire of circuits that are easy to interfere with each other by proper grounding. Generally, it is necessary to avoid the common ground wire between strong and weak electric circuits, and the common ground wire between digital circuits and analog circuits. The disadvantage of parallel grounding is that there are too many grounded wires. Therefore, in practice, it is not necessary for all circuits to be connected in parallel with single point grounding. For circuits with less mutual interference, series single point grounding can be adopted. For example, circuits can be classified according to strong signals, weak signals, analog signals, digital signals, etc., and then series single-point grounding is used inside similar circuits, as shown in Figure 4, and 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 single-point grounding method can be adopted to prevent it from forming a loop. When the signal frequency is higher than 10 MHz, it is best to use multi-point grounding to minimize the impedance of the ground wire. The power line and the ground line should be routed as close as possible to reduce the area of the enclosed loop, thus 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 core problem of reducing the impedance of the ground wire is to reduce the inductance of the ground wire. A flat conductor can be used as the ground wire, or a plurality of parallel conductors which are far apart can be used as the ground wire. For PCB, laying the ground grid on the double-layer board can effectively reduce the impedance of the ground wire, and in the multi-layer board, one layer can be specially used as the ground wire to reduce the impedance.
4 Conclusion
Anti-interference design is an important part of single chip microcomputer system design, and 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 the ground wire interference should also be found and eliminated through tests. This paper introduces the causes and solutions of 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 theory, through a large number of experimental processes and experience accumulation, can we better master the design method and interference elimination means of the grounding system, so as to better improve the reliability of the circuit work.