According to the rule of thumb, four-layer boards are usually used in high-density and high-frequency situations. Multi-layer PCBs are more than 20DB better than two-layer boards in terms of EMC. Under the condition of a four-layer board, a complete ground plane and a complete power plane can often be used. Under this condition, it is only necessary to connect the ground wires of the circuits divided into several groups to the ground plane, and the working noise is specially handled.

　　There are many ways to connect the ground wires of each circuit to the ground plane of a multilayer PCB, including:

　　1. Single-point and multi-point grounding methods

　　1. Single-point grounding: The ground wires of all circuits are connected to the same point on the ground plane, which is divided into series single-point grounding and parallel single-point grounding.

　　2. Multi-point grounding: The ground wires of all circuits are grounded nearby, and the ground wires are very short, which is suitable for high-frequency grounding.

　　3. Mixed grounding: Mix single-point grounding and multi-point grounding.

　　Single-point grounding is the most suitable for low frequency, low power and the same power supply layer, and is usually used in analog circuits; star connection is generally used here to reduce the influence of possible series impedance, as shown in the right half of Figure 8.1. High-frequency digital circuits need to be grounded in parallel, which can be handled more simply through ground holes, as shown in the left half of the figure; generally all modules will use a combination of two grounding methods, and use a mixed grounding method to complete the connection between the circuit ground wire and the ground plane.

　　2. Hybrid grounding method

　　If you do not choose to use the entire plane as a common ground line for a multi-layer PCB, for example, when the module itself has two ground lines, you need to split the ground plane, which often interacts with the power plane. Pay attention to the following principles:

　　1. Align each plane to avoid overlap between irrelevant power planes and ground planes, otherwise all ground plane segmentation will fail and interfere with each other;

　　2. In the case of high frequency, coupling will occur between layers through the parasitic capacitance of the circuit board;

　　3. Signal lines between ground planes (such as digital ground plane and analog ground plane) are connected using ground bridges, and the nearest return path is configured through the nearest through-hole.

　　4. Avoid running high-frequency lines such as clock lines near isolated ground planes to avoid causing unnecessary radiation.

　　5. The loop area formed by the signal line and its loop should be as small as possible, which is also called the minimum loop rule. The smaller the loop area, the less external radiation and the less external interference. When dividing the ground plane and routing the signal, the distribution of the ground plane and important signal routing should be considered to prevent problems caused by grooving the ground plane.

　　The connection methods between multi-layer PCB circuit boards are summarized here.

　　1. Ordinary wiring connection between ground circuit boards: This method can ensure reliable low-impedance conduction between the two ground wires, but it is limited to the connection between medium and low frequency signal circuits.

　　2. Large resistance connection between ground and earth: The characteristic of large resistance is that once a voltage difference appears at both ends of the resistance, a very weak conduction current will be generated. After the charge on the ground wire is discharged, the voltage difference at both ends will eventually be zero.

　　3. Ground capacitor connection: The characteristics of capacitor are DC cutoff and AC conduction, and it is used in floating ground systems.

　　4. Magnetic bead connection between ground and ground: Magnetic bead is equivalent to a resistor that changes with frequency, and it shows resistance characteristics. It is used between ground and ground of weak signals with fast and small current fluctuations.

　　5. Ground inductor connection: Inductor has the characteristic of suppressing changes in circuit state and can reduce peaks and fill valleys. It is usually used between two grounds with large current fluctuations.

　　6. Small resistor connection between ground and earth: Small resistor adds a damping to hinder the overshoot of rapid changes in ground current; when the current changes, it slows down the rising edge of the impact current.

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