Design experience: Grounding methods in multi-layer PCBs

According to the rule of thumb, four-layer boards are usually used in high-density and high-frequency situations, which are more than 20 DB 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 processed.

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. Here, ground holes can be used to handle this in a relatively simple way, as shown in the left half of the figure. Generally, all modules will use a combination of two grounding methods, and a mixed grounding method will be used to complete the connection between the circuit ground and the ground plane.

Mixed grounding method

If you do not choose to use the entire plane as a common ground line, 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 divisions will fail and interfere with each other;

(2) In the case of high frequencies, coupling will occur between layers through 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-holes.

(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, 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 the important signal routing should be considered to prevent problems caused by the ground plane slotting.

For the connection method between grounds, refer to some practices in Wu Yeqing’s article, and make some arrangements here.

① Ordinary wiring connection between ground circuit boards: This method can ensure reliable low-impedance conduction between the two ground lines, but it is limited to the connection between the ground 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 at both ends of the resistance, a very weak conduction current will be generated. After the charge on the ground line 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, which 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. Applicable to the ground between weak signals with fast and small current fluctuations.
⑤ Inductor connection between grounds: Inductor has the characteristic of suppressing changes in circuit state, can cut peaks and fill valleys, and is usually used between two grounds with large current fluctuations.
⑥ Small resistor connection between grounds: 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.