What is a ground wire?
The definition of ground wire that everyone learns in textbooks is: ground wire is an equipotential body that serves as the reference point for circuit potential.
This definition is not in line with the actual situation. The potential on the actual ground wire is not constant.
If a meter is used to measure the potential between various points on the ground wire, it may be found that the potential of each point on the ground wire may differ greatly. It is precisely these potential differences that cause abnormal circuit operation. The definition of a circuit as an equipotential is only based on people’s expectations of the ground potential. HENRY gave a more practical definition of ground wire, which he defined as a low impedance path for signal flow back to the source. This definition highlights the flow of current in the ground wire. According to this definition, it is easy to understand the reason for the potential difference in the ground wire. Because the impedance of the ground wire is never zero, a voltage drop occurs when a current passes through a finite impedance. Therefore, we should imagine the potential on the ground wire to be like waves in the ocean, rising and falling one after another.
When it comes to the potential difference between points on the ground wire caused by the impedance of the ground wire, many people find it incredible that when we measure the resistance of the ground wire with an ohmmeter, the resistance of the ground wire is often in the milliohm level. How could such a large voltage drop be generated when the current flows through such a small resistance, leading to abnormal circuit operation.
Ground wire interference mechanism: When two circuits share a section of ground wire, the ground potential of one circuit is modulated by the working current of the other circuit due to the impedance of the ground wire. The signal in such a circuit will be 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 exhibits a large impedance. At this point, if different circuits share a common ground wire, there may be a problem of common impedance coupling
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According to the mechanism of ground loop interference, reducing the current in the ground loop can reduce ground loop interference. 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 address ground loop interference.

A. If one end of the device is grounded and the circuit is grounded, the ground loop is cut off, thus eliminating the current in the ground loop. But there are two issues that need to be noted, one is that for safety reasons, circuit floating is often not allowed. At this point, it is possible to consider grounding the equipment through an inductor. This way, the grounding impedance of 50Hz AC current equipment is very small, while for high-frequency interference signals, the grounding impedance of the equipment is large, reducing the ground loop current. But doing so can only reduce the ground loop interference caused by high-frequency interference. Another issue is that although the device is floating to the ground, there is still parasitic capacitance between the device and the ground, which provides lower impedance at higher frequencies and therefore cannot effectively reduce the high-frequency ground loop current.

B. Using a transformer to connect devices can cut off the ground loop current by connecting two devices using a magnetic circuit. However, it should be noted that the parasitic capacitance between the first stages of the transformer can still provide a path for the high-frequency ground loop current, so the method of transformer isolation has poor suppression effect on the high-frequency ground loop current. One way to improve the high-frequency isolation effect of transformers is to set up a shielding layer between the primary stages of the transformer. But it is important to note that the grounding terminal of the shielding layer of the isolation transformer must be at the receiving circuit end. Otherwise, not only does it not improve the high-frequency isolation effect, but it may also make the high-frequency coupling more severe. Therefore, the transformer should be installed on the side of the signal receiving equipment. A well shielded transformer can provide effective isolation at frequencies below 1MHz.

C. Another method of using optical isolators to cut off the ground loop is to transmit signals using light. This can be said to be the most ideal method to solve the problem of ground loop interference. There are two ways to connect using light, one is through optocoupler devices, and the other is through fiber optic connections. The parasitic capacitance of the optocoupler is generally 2pf, which can provide good isolation at very high frequencies. Fiber optic has almost no parasitic capacitance, but it is inferior to optocoupler devices in terms of installation, maintenance, and cost.

D. Using a common mode choke on the connecting cable increases the impedance of the ground loop, so that under a certain ground voltage, the ground loop
The circuit current will decrease. However, attention should be paid to controlling the parasitic capacitance of the common mode choke, otherwise the isolation effect on high-frequency interference will be poor. The more turns the common mode choke coil has, the larger the parasitic capacitance and the worse the high-frequency isolation effect.

There are two ways to eliminate common impedance coupling. One is to reduce the impedance of the common ground wire, so that the voltage on the common ground wire also decreases, thereby controlling the common impedance coupling. Another method is to avoid sharing ground wires with circuits that are prone to mutual interference through appropriate grounding methods. Generally, it is necessary to avoid sharing ground wires with strong and weak current circuits, as well as digital and analog circuits. As mentioned earlier, the core issue of reducing ground wire impedance is to reduce the inductance of the ground wire. This includes using flat conductors as grounding wires and using multiple parallel conductors that are far apart as grounding wires. For printed circuit boards, arranging a ground wire grid on a double-layer board can effectively reduce the ground wire impedance. Although using a single layer as the ground wire in a multi-layer board has a small impedance, it will increase the cost of the circuit board. The grounding method that avoids common impedance through appropriate grounding is parallel single point grounding. 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 a single point grounding. For circuits with less mutual interference, series connection with a single point grounding can be used. For example, circuits can be classified according to strong signals, weak signals, analog signals, digital signals, etc., and then connected to a single point ground in series within similar circuits, while different types of circuits can be connected to a single point ground in parallel.

Summary:
The main reason for electromagnetic interference caused by ground wires in PCB is the impedance of the ground wire. When current flows through the ground wire, voltage is generated on the ground wire, which is called ground wire noise. Driven by this voltage, a ground loop current will be generated, forming ground loop interference. When two circuits share a ground wire, a common impedance coupling is formed. The methods to solve ground loop interference include cutting off the ground loop, increasing the impedance of the ground loop, and using a balanced circuit. The method to solve common impedance coupling is to reduce the impedance of the common ground wire, or to use parallel single point grounding to completely eliminate the common impedance.

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