Electronic Design - Notes for engineers in the PCB design process

EMC issues: PCB engineers' considerations and grounding tips

Segmentation and transmission of grounding: Grounding is one of the important means to suppress electromagnetic interference and improve the EMC performance of electronic equipment.

Proper grounding can not only improve the product's ability to suppress electromagnetic interference, but also reduce the product's external EMI radiation.

The meaning of grounding: The "grounding" of electronic equipment usually has two meanings: one is "grounding" (safety), and the other is "system reference" (signal grounding). Grounding refers to the establishment of a low-resistance conductive path between the system and the potential reference surface.

"Connecting to the earth" is based on the potential of the earth, and the earth is at zero potential. The metal shell of the electronic device, the circuit reference point and the earth.

Connecting the ground plane to the ground usually requires consideration of the following factors:

A, improve the stability of the equipment circuit system operation;

B, electrostatic discharge;

C, to provide security for employees.

Grounding purpose:

A, safety considerations, that is, protective grounding;

B, provide a stable 0 potential reference point for the signal voltage (signal or system);

C, the shield is grounded.

Basic grounding methods: There are three basic grounding methods for electronic equipment: single-point grounding, multi-point grounding, and floating ground.

PCB engineer's note:

Single point grounding:

Single-point grounding is the entire system, in which only one physical point is defined as the ground reference point, and other points that need to be grounded are connected to this point. Single-point grounding is suitable for low-frequency circuits (less than 1MHZ). If the operating frequency of the system is very high, and the operating wavelength is equivalent to the length of the system grounding lead, the single-point grounding method has a problem.

When the length of the local line is close to 1/4 wavelength, it is like a terminal short-circuit transmission line, the current and voltage of the ground line are distributed in the standing wave, and the ground line becomes a radiating antenna instead of playing the role of "ground". In order to reduce the ground impedance and avoid radiation, the length of the ground wire should be less than 1/20 wavelength. In the processing of power circuits, single-point grounding can usually be considered.

For a large number of digital circuits used in PCB boards, it is generally not recommended to use single-point grounding mode due to its rich high-order harmonics.

Multi-point grounding:

Multi-point grounding means that each connection location in the device is directly connected to the ground plane closest to it, so the length of the ground lead is the shortest. The multi-point grounding circuit has a simple structure, which can significantly reduce the high-frequency standing wave phenomenon on the grounding line, and is suitable for high operating frequency (> 10MHZ) occasions. However, multi-point grounding may cause many ground loops to be formed inside the device, thereby reducing the device's ability to resist external electromagnetic fields. In the case of multi-point grounding, we should pay attention to the ground loop problem, especially when different modules and devices are established between the networks.

Electromagnetic interference caused by grounding circuit: The ideal ground wire should be a physical entity with zero potential and zero impedance. However, the actual ground wire itself has a resistance component and a reactance component, and when there is a current through the ground wire, a voltage drop will be generated.

When the electromagnetic field is coupled to the circuit, the ground wire will form a loop with other wires (signal, power wire, etc.), induced electromotive force is generated in the ground loop and the ground loop is coupled to the load, thereby creating a potential EMI threat.

Floating ground:

Floating refers to a grounding method in which the ground system of the equipment is electrically insulated from the ground.

Due to some weaknesses of floating ground itself, it is not suitable for general large-scale systems, and its grounding method is rarely used.

The general selection principle of grounding mode: For a given device or system, when the length of the transmission line is l>, the highest frequency of interest (corresponding wavelength) is regarded as a high-frequency circuit, and vice versa, it is regarded as a low-frequency circuit Circuit.

According to PCB design experience, it is best to use single-point grounding for circuits lower than 1MHZ, and for higher than 10MHZ, multi-point grounding is good.

For the frequencies in between, as long as the length of the maximum transmission line L is less than /20, single-point grounding can be used to avoid common impedance coupling.