PCB Blog - Analysis and Suppression Method of Ground Wire Interference in PCB Board Wiring

1. Definition of ground wire
What is a ground wire on PCB board? The definition of ground wire that everyone learns in textbooks is: the ground wire is an equipotential body that serves as the reference point of the circuit potential. This definition is not in line with the actual situation. The potential on the actual ground wire is not constant. If you use a meter to measure the potential between the points on the ground line, you will find that the potential of each point on the ground line may vary greatly. It is these potential differences that cause the abnormal operation of the circuit. The definition of a circuit being an equipotential body is only one's expectation of the ground potential. HENRY gave a more realistic definition of the ground wire, he defined the ground wire as: the low impedance path for the signal to flow back to the source. The flow of current in the ground wire is highlighted in this definition. According to this definition, it is easy to understand the cause of the potential difference in the ground wire. Because the impedance of the ground wire is never zero, when a current flows through a finite impedance, a voltage drop occurs. Therefore, we should imagine that the potential on the ground wire is like waves in the sea, one after another.

2. The impedance of the ground wire
When it comes to the potential difference between the points on the ground wire caused by the impedance of the ground wire, many people find it inconceivable: when we measure the resistance of the ground wire with an ohmmeter, the resistance of the ground wire is often at the milliohm level, How can such a large voltage drop occur when the current flows through such a small resistor, causing the circuit to work abnormally. To understand this problem, we must first distinguish the two different concepts of resistance and impedance of the wire. Resistance refers to the resistance 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. Any wire has inductance, and when the frequency is high, the impedance of the wire is much greater than the DC resistance. In practical circuits, the signals that cause electromagnetic interference are often pulse signals, and the pulse signals contain rich high-frequency components, so a large voltage will be generated on the ground wire. For digital circuits, the operating frequency of the circuit is very high, so the impact of the ground wire impedance on the digital circuit is very considerable. If the impedance at 10Hz is approximately considered as DC resistance, it can be seen that when the frequency reaches 10MHz, for a 1-meter long wire, its impedance is 1000 to 100,000 times that of the DC resistance. So for RF current, when the current flows through the ground wire, the voltage drop is large. It can also be seen from the table that increasing the diameter of the wire is very effective in reducing the DC resistance, but has a limited effect on reducing the AC impedance. But in electromagnetic compatibility, people are concerned about the AC impedance. In order to reduce the AC impedance, an effective way is to connect multiple wires in parallel. When two wires are connected in parallel, the total inductance L is: L = ( L1 + M ) / 2; where L1 is the inductance of a single wire, and M is the mutual inductance between the two wires. It can be seen from the formula that when two wires are far apart, the mutual inductance between them is very small, and the total inductance is equivalent to half of the inductance of a single wire. Therefore, we can reduce the ground impedance through multiple ground wires. But it should be noted that the distance between multiple wires cannot be too close.

3. Ground wire interference mechanism
3.1 Ground loop interference
Because of the ground wire impedance, when current flows through the ground wire, a voltage is generated on the ground wire. This voltage can be large when the current is large. For example, when a high-power electrical appliance is started nearby, a strong current will flow through the ground wire. This current creates a current in the connecting cable between the two devices. Due to the unbalance of the circuit, the current on each wire is different, so a differential mode voltage will be generated, which will affect the circuit. Since this interference is generated by the loop current formed by the cable and the ground wire, it becomes the ground loop interference. The current in the ground loop can also be induced by the external electromagnetic field.

3.2 Common Impedance Interference
When two circuits share a ground wire, the ground potential of one circuit will be modulated by the operating current of the other circuit due to the impedance of the ground wire. Signals in such a circuit are coupled into another circuit, and this coupling is called common impedance coupling. In digital circuits, due to the high frequency of the signal, the ground wire often presents a large impedance. At this time, if there are different circuits sharing a section of ground wire, the problem of common impedance coupling may occur. 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.

4. Ground wire interference countermeasures
4.1 Ground loop countermeasures It can be known from the mechanism of ground loop interference that the ground loop interference can be reduced as long as the current in the ground loop is 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 to solve the ground loop interference.
1) Floating the equipment at one end If the circuit at one end is floated, the ground loop is cut off, so the ground loop current can be eliminated. But there are two problems that need to be paid attention to. One is that for safety reasons, the circuit is often not allowed to float. In this case, consider grounding the device through an inductor. In this way, the grounding impedance of the 50Hz AC current equipment is very small, and for the interference signal with a higher frequency, the grounding impedance of the equipment is relatively 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 the transformer to realize the connection between the devices. Use the magnetic circuit to connect the two devices, which can cut off the ground loop current. However, it should be noted that the parasitic capacitance between the primary and secondary of the transformer can still provide a path for the high-frequency ground loop current, so the transformer isolation method has a poor suppression effect on the high-frequency ground loop current. 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. A well-shielded transformer can provide effective isolation at frequencies below 1MHz.
3) Using an optical isolator Another way to cut off the ground loop is to use light to transmit signals. This can be said to be an ideal solution to the ground loop interference problem. There are two methods for optical connection, one is optocoupler device, and the other is optical fiber connection. The parasitic capacitance of the optocoupler is generally 2pf, which can provide good isolation at very high frequencies. Optical fibers have almost no parasitic capacitance, but they are inferior to optocouplers in terms of installation, maintenance, and cost.
4) Using a common mode choke coil The use of a common mode choke coil on the connecting cable is equivalent to increasing the impedance of the ground loop, so that the ground loop current will decrease 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.
4.2 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 easy to interfere with each other through appropriate grounding methods. Generally, it is necessary to avoid the shared ground wire of the strong current circuit and the weak current circuit, and the digital circuit and the analog circuit to share the ground wire. As mentioned earlier, the problem with reducing the impedance of the ground wire is to reduce the inductance of the ground wire. This includes the use of flat conductors for grounding and multiple parallel conductors spaced apart for grounding. For printed circuit boards, laying a ground wire grid on a double-layer board can effectively reduce the ground wire impedance. Although one layer is used as a ground wire in a multi-layer board, it has a small impedance, but it will increase the cost of the circuit board. . The grounding method to avoid common impedance by proper grounding is to ground 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 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, and use parallel single-point grounding for different types of circuits.
5. Summary
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. Common impedance coupling occurs when two circuits share a segment of ground. The methods to solve the ground loop interference are to cut off the ground loop, increase the impedance of the ground loop, and use a balanced circuit. The solution to the common impedance coupling is to reduce the impedance of the common ground wire, or use a single-point ground connection in parallel to completely eliminate the common impedance on PCB board.