Ground connection processing

According to the rule of thumb, four-layer boards are usually used in high-density and high-frequency applications, which 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, including:
 
single-point and multi-point grounding methods
① Single-point grounding: The ground wires of all circuits are connected to the same point of 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 and 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 layer, and is usually used in analog circuits; here, a star connection is generally used 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, and here it can be handled more simply by using ground holes, as shown in the left half of Figure 8.1; 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.

 If the hybrid grounding method
 does not use the entire plane as a common ground line, for example, when the module itself has two ground lines, the ground plane needs to be split, which often interacts with the power plane. Pay attention to the following principles:
(1) Align the planes to avoid overlap between irrelevant power planes and ground planes, otherwise all ground plane splits will fail and interfere with each other;
(2) At high frequencies, coupling will occur between layers through the parasitic capacitance of the circuit board;
(3) Signal lines between ground planes (such as digital ground planes and analog ground planes) 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 cause unnecessary radiation.
(5) The loop area formed by the signal line and its loop should be as small as possible, also known as the minimum loop rule; the smaller the loop area, the less external radiation and the less external interference. When splitting the ground plane and routing the signal, the distribution of the ground plane and important signal lines should be considered to prevent problems caused by ground plane slots.
For the connection method between grounds, please refer to some practices in Wu Yeqing's article, and here is some summary.
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 the grounds of medium and low-frequency signal circuits.
② Large resistance connection between grounds: The characteristic of large resistance is that once a voltage difference appears across the resistor, a very weak conduction current will be generated. After the charge on the ground wire is discharged, the voltage difference between the two ends will eventually be zero.
③ Capacitor connection between grounds: The characteristics of capacitors are DC cutoff and AC conduction, and they are used in floating ground systems.
④ Magnetic bead connection between grounds: Magnetic beads are equivalent to a resistor that changes with frequency, and they show resistance characteristics. It is used between grounds of weak signals with fast and small current fluctuations.
⑤ Inductive connection between grounds: Inductors have the characteristics of suppressing changes in circuit states, and can cut peaks and fill valleys. They are usually used between two grounds with large current fluctuations.
⑥ Small resistor connection between ground and earth: The small resistor adds a damping to hinder the overshoot of rapid changes in ground current; when the current changes, the rising edge of the impact current slows down.