PCB Blog - The way of grounding in multi-layer PCB board

According to the rule of thumb, a four-layer PCB board is usually used in high-density and high-frequency applications. Under the condition of a four-layer board, a complete ground plane and a complete power plane can often be used. In this condition, only a few The ground wire of the circuit of the group is connected to the ground plane, and the operating noise is treated specially. There are a number of things that can be done to connect the ground wire of the individual circuits 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 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 and suitable for high-frequency grounding.

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

Between low frequency, low power and the same power supply layer, single-point grounding is suitable, and it is usually used in analog circuits; here, the star connection is generally used to reduce the influence of possible series impedance, as shown in the figure Shown in the right half of 8.1. High-frequency digital circuits need to be grounded in parallel. Here, it is generally easier to deal with the ground hole. Generally, all modules will use two grounding methods, and the hybrid grounding method is used to complete the circuit ground and ground plane. connect.

Hybrid grounding method

If you do not choose to use the entire plane as the common ground wire, for example, when the module itself has two ground wires, the ground plane needs to be divided, which often interacts with the power plane. Note the following principles:

1) Align each plane to avoid the 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, the layers will be coupled through the parasitic capacitance of the circuit board;

3) The signal lines between ground planes (such as digital ground plane and analog ground plane) are connected using ground bridges, and the closest return path is configured through the nearest via.

4) Avoid taking high-frequency traces such as clock lines near the isolated ground plane, causing unnecessary radiation.

5) The area of the ring formed by the signal line and its loop is as small as possible, also known as the Z-small rule of the loop; the smaller the ring area, the less the external radiation and the less the interference from the outside world. When dividing the ground plane and signal routing, the distribution of the ground plane and important signal routing should be considered to prevent problems caused by ground plane slotting.

The connection method between the ground, here is some sorting out.

1) Ordinary wiring connection of circuit boards between grounds: 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 there is a voltage difference between the two ends of 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 be zero.

3) Capacitance connection between grounds: The characteristics of capacitors are DC cutoff and AC conduction, which are used in floating systems.

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

5) Inductive connection between grounds: Inductance has the characteristics of suppressing circuit state changes and can cut peaks and fill valleys. It is usually used between two grounds with large current fluctuations.

6) Small resistance connection between grounds: The small resistance adds a damping to prevent the overshoot of the rapid change of the ground current; when the current changes, the rising edge of the inrush current is slowed down on PCB board.