In PCB design, especially in high-frequency circuits, some irregularities and abnormalities caused by ground interference are often encountered. This paper analyzes the causes of interference caused by grounding, and introduces three types of interference caused by grounding, and proposes solutions according to the experience in practical applications. These anti-interference methods have achieved good results in practical applications, enabling some systems to run successfully on site.
In a single-chip system, a PCB (printed circuit board) is an important component used to support circuit components and provide electrical connection between circuit components and devices. The PCB wires are mostly copper wires, and the physical properties of copper itself also cause them to conduct electricity. There must be a certain impedance in the process. The inductance component in the wire will affect the transmission of the voltage signal. The resistance component will affect the transmission of the current signal. The influence of the inductance in the high-frequency circuit is particularly serious. Therefore, it must be noted in the PCB design. Eliminate the effects of ground impedance.
1 Causes of interference
Two different concepts of resistance and impedance. The resistance refers to the impedance of the wire to the current in the DC state, and the impedance refers to the impedance of the wire to the current in the AC state. This impedance 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 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 wire length (m), s is the wire cross-sectional area (mm2), and ρ is the resistivity ρ = 0.02, so the wire resistance is about 0.026 Ω.
When a length of wire is away from other wires and its length is much larger than the width, the wire has a self-inductance of 0.8 μH/m, and the inductance of the wire of 10 cm is 0.08 μH. Then, the inductance of the wire is obtained by the following formula: XL=2πfL. In the following formula, f is the frequency (Hz) of the wire passing signal, and L is the self-inductance (H) of the wire per unit length. Therefore, the inductive reactance values of the wire at low and high frequencies are calculated separately:
In the actual circuit, the signal causing electromagnetic interference is often a pulse signal, and the pulse signal contains a rich high-frequency component, so a large voltage is generated on the ground. It can be seen from the above formula calculation that the wire resistance is higher than the wire inductance in the low frequency signal transmission. For the digital circuit, the working frequency of the circuit is very high, and the wire inductance is much larger than the wire resistance in the high frequency signal. Therefore, the effect of ground impedance on digital circuits is considerable. This is the reason why a large voltage drop occurs when a current flows through a small resistor, causing abnormal operation of the circuit.
2 Ground interference mechanism
2.1 Ground loop interference
Ground loop interference is a relatively common type of interference that often occurs between devices that are connected by long cables and that are far apart. 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 is generated on the ground wire. This is the ground wire noise. Driven by this voltage, ground loop current is generated, causing ground loop interference. As shown in Figure 1, there are two grounded circuits.
Since the ground potentials of the two devices are different, a ground voltage is formed, and under the driving of this voltage, a current flows between the loops formed by "the device 1 interconnects the cable and the device 2". Due to the imbalance of the circuit, the current on each of the wires is different, so differential mode voltage is generated and the circuit is disturbed.
Since the ground loop interference is caused by the ground loop current, it is sometimes found that when the ground of one device is disconnected, the interference phenomenon disappears because the ground loop is cut off when the ground line is disconnected. This phenomenon often occurs in the case of low frequency interference. When the interference frequency is high, the relationship between disconnecting the ground line or not is small.
2.2 Common impedance interference
In digital circuits, the ground line tends to exhibit a large impedance due to the high frequency of the signal. At this time, when several circuits share a ground line, due to the impedance of the ground line, the ground potential of one circuit is modulated by the operating current of the other circuit, so that the signal in one circuit is coupled into another circuit. It is called common impedance coupling.
The solution to the common impedance coupling is to reduce the impedance of the common ground line portion, or to use a single point grounding to completely eliminate the common impedance. The example of Figure 2 illustrates an interference phenomenon. Figure 2 is a simple circuit with four gates. Assume that the output level of gate 1 changes from high to low. At this time, the parasitic capacitance in the circuit (sometimes the filter capacitor at the input of gate 2) will discharge to the ground through gate 1, and the discharge current will be due to the impedance of the ground. A spike voltage is generated on the ground. If the output of the gate 3 is low at this time, the spike voltage is transmitted to the output of the gate 3, the input of the gate 4, if the amplitude of the spike exceeds the noise of the gate 4. The threshold will cause the door 4 to malfunction.
2.3 Earth loop electromagnetic coupling interference
The "ground loop" shown in Figure 1 will enclose a certain area. According to the law of electromagnetic induction, if there is a changing magnetic field in the area surrounded by this loop, an induced current will be generated in the loop to form interference. . The change of the spatial magnetic field is ubiquitous, so the larger the area enclosed, the more serious the interference.
3 Ways to solve ground interference
3.1 Solving ground loop interference
There are three basic ideas for solving ground loop interference: one is to reduce the impedance of the ground line, 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 the other 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 for solving ground loop interference are proposed.
1) Floating the equipment on one side
If one side of the circuit is floating, the ground loop is cut off, so the ground loop current can be eliminated. But there are two issues to be aware of, one is for safety reasons, not allowing the circuit to float. At this point, consider grounding the device through an inductor. Thus, the grounding impedance of the 50 Hz AC current device is small, and for the higher frequency interference signal, the device ground impedance is larger, which reduces the ground loop current. However, this can only reduce the ground loop interference of high frequency interference. Another problem is that although the device is floating, there is still a parasitic capacitance between the device and the ground. This capacitor provides a lower impedance at higher frequencies and therefore does not effectively reduce the high-frequency ground loop current.
2) Using a transformer
The most basic way to solve the ground loop interference is to cut off the ground loop. This is done with an isolation transformer, and the signal transmission between the two devices is done by magnetic field coupling, avoiding electrical direct connections. At this point, the interference voltage on the ground line appears between the primary and secondary of the transformer, not at the input of the circuit. One way to improve the high-frequency isolation of the transformer is to place a shield between the primary and secondary of the transformer. However, it must be noted that the grounding end of the isolation transformer shield must be at the receiving circuit end. Otherwise, not only can the high-frequency isolation effect be improved, but also the high-frequency coupling can be made more serious. Therefore, the transformer is to be mounted on one side of the signal receiving device.
The method of transformer isolation has some shortcomings, which cannot transmit DC, is bulky, and has high cost. Since there is parasitic capacitance between the primary and secondary of the transformer, the isolation at high frequencies is not very good.
3) Use optical isolation components
Transmitting signals with light is an ideal way to solve the ground loop problem. As shown in Figure 3, the optocoupler device has a parasitic capacitance of about 2 pF, so it can be isolated at very high frequencies. If the fiber is used, there is no problem of parasitic capacitance, and a perfect isolation effect can be obtained. However, the use of optical fiber brings other problems, such as: requiring more power, requiring more peripheral devices, the line shape and dynamic range of the optical connection are not up to the requirements of the analog signal, and the installation and maintenance of the optical cable are complicated. Be careful when using it.
4) Use common mode chokes
The ground voltage is actually a common mode voltage. Under this voltage, the current flowing through the cable is the common mode current. The use of a common mode choke on the connecting cable is equivalent to increasing the impedance of the ground loop so that the ground loop current is reduced by 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 larger the number of turns of the common mode choke, the larger the parasitic capacitance and the worse the effect of high frequency isolation.
5) Suppression of ground circuit interference by balanced circuit
A balanced circuit is defined as two conductors and their connected circuits having the same impedance relative to ground or other reference object.
It is very difficult to balance at high frequencies. Actual circuits have many parasitic factors, such as parasitic capacitance and inductance. These parameters play a large 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 ensure that the impedance of the two conductors is exactly the same. Therefore, at high frequencies, the circuit balance tends to be poor, which means that the balance circuit has a poor effect on the ground loop current interference suppression of higher frequencies.
3.2 Elimination of 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 is also reduced, thus controlling the common impedance coupling. Another method is to avoid the common grounding of the circuit which is easy to interfere with each other by an appropriate grounding method. Generally, the grounding circuit of the strong electric circuit and the weak electric circuit is avoided, and the digital circuit and the analog circuit share the ground line. The disadvantage of parallel grounding is that there are too many wires grounded. 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 single point grounding in series can be used. For example, the circuit can be classified according to strong signals, weak signals, analog signals, digital signals, etc., and then grounded in series with a single point in the same type of circuit, as shown in Figure 4, different types of circuits are connected in parallel with a single point, as shown in Figure 5. Shown. When the signal frequency is lower than 1 MHz, a single-point grounding method can be used so that it does not form a loop. When the signal frequency is higher than 10 MHz, it is better to use multi-point grounding to minimize the ground impedance. The power and ground lines should be as close as possible to the traces to reduce the loop area enclosed, thereby reducing the external field magnetic field interference caused by the loop cutting, and also reducing the external electromagnetic radiation of the loop.
As mentioned earlier, the core problem of reducing the ground impedance is to reduce the inductance of the ground. You can use a flat conductor as a ground wire, or use multiple parallel conductors that are far apart to make a ground wire. For the PCB, the ground grid on the double-layer board can effectively reduce the ground impedance. In the multi-layer board, a layer can be used as a ground line to reduce the impedance.
Anti-interference design is an important part of the design of single-chip system, and its design often determines the success or failure of the whole system. Regarding grounding, many of the monographs on electromagnetic compatibility are discussed in detail, but the best grounding method should be selected by experiment, and the grounding interference should also be found and eliminated through experiments. This paper introduces the causes and solutions of the ground-induced interference, and explains the general methods and principles in the grounding design. Only under the guidance of the theory, after a large number of experimental processes and experience accumulation, the design method of the grounding system can be better grasped. And interference elimination means, so as to better improve the reliability of the circuit work.