What Is The Grounding Classification Of High-speed PCB Design?

With the development of electronic technology, the product functions of electronic products are becoming more and more powerful. PCB design plays an important role in the design of electronic products, because the good or bad design of PCB will directly affect the realization of product functions.

In the design of electronic products, it is not difficult to design a PCB circuit to achieve its function. The difficulty is that it is not affected by various effects, such as changes in temperature and humidity, changes in air pressure, mechanical shock, corrosion, etc. In order to achieve continuous and normal work, we will adopt various design methods or manufacturing process measures to eliminate or reduce these effects. Everyone knows that grounding design is the foundation of system design. Good grounding is a prerequisite for safe and stable operation of the system. So today we will talk about the relevant knowledge of the grounding method in high-speed PCB design.

PCB grounding design:

Broad grounding includes two meanings, namely solid ground and virtual ground. Solid ground refers to the connection with the earth; virtual ground connection refers to the connection with the potential reference point, when this reference point is electrically insulated from the ground, it is called floating connection. There are two purposes of grounding: one is to ensure the stable and reliable operation of the control system and prevent interference caused by the ground loop, which is often called the working ground; the other one is to avoid the risk of electric shock to the operator due to the insulation damage or drop of the equipment and to ensure the safety of the equipment. This is called protective grounding.

Ground selection principle:

For a given device or system, at the wavelength λ corresponding to the highest frequency of interest, when the length of the transmission line L> λ, it is regarded as a high frequency circuit, otherwise, it is regarded as a low frequency circuit.

(1) Low frequency circuit (≤1MHZ), single point grounding is recommended;

(2) High frequency circuit (≥ 10MHZ), multi-point grounding is recommended;

(3) High and low frequency mixed circuit, mixed grounding, the applicable working frequency range is generally 500kHz-30MHz;

PCB grounding method:

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

Single-point grounding is suitable for circuits with lower frequencies (below 1MHZ). If the operating frequency of the system is so high that the operating wavelength is comparable to the length of the system ground lead, there is a problem with the single-point grounding method. When the length of the ground wire is close to 25 percent length of wavelength, it is like a transmission line with short-circuited terminals. The current and voltage of the ground wire are distributed as standing waves. The ground wire becomes a radiating antenna and cannot play the role of "ground".

In order to reduce the ground impedance and avoid radiation, the length of the ground wire should be less than 5 percent length of wavelength. In the processing of the power circuit, a single point of grounding can generally be considered. For PCBs used in a large number of digital circuits, it is generally not recommended to use a single-point grounding method because of its rich high-order harmonics.

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

Multi-point grounding means that each grounding point in the equipment is directly connected to the grounding plane closest to it, so that the length of the ground lead is the shortest.

The structure of the multi-point grounding circuit is simple, and the phenomenon of high-frequency standing waves that may appear on the grounding line is significantly reduced. It is suitable for occasions with high working frequency (≥10MHZ). However, multi-point grounding may cause many ground loops inside the device, thereby reducing the device's resistance to external electromagnetic fields. In the case of multi-point grounding, we need to note intentional loop problems, especially electromagnetic interference caused by ground loop when networking between different modules and devices:

The ideal ground wire should be a physical entity with zero potential and zero impedance. However, the actual ground wire itself has both a resistance component and a reactance component. When a current flows through the ground wire, a voltage drop is generated. The ground wire will form a loop with other connections (signal, power line, etc.). When the time-varying electromagnetic field is coupled to this loop, an induced electromotive force will be generated in the ground loop and coupled to the load by the ground loop, posing a potential EMI threat.

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

Generally, all modules will use two grounding methods in a comprehensive manner, and use a mixed grounding method to complete the connection between the circuit ground and the ground plane.

If you do not choose to use the entire plane as a common ground, such as when the module itself has two grounds, you need to divide the ground plane, which often interacts with the power plane. Pay attention to the following principles:

(1) Align the planes to avoid the overlap between the irrelevant power plane and the ground plane, otherwise it will cause all the ground planes to fail and interfere with each other;

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

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

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

(5) The loop area formed by the signal line and its loop is as small as possible, also known as the loop minimum rule; the smaller the loop area, the less external radiation and the smaller the interference received from the outside world. When splitting the ground plane and routing signals, consider the distribution of the ground plane and important signal traces to prevent problems caused by slotting the ground plane.

4. Floating ground:

Floating ground refers to a grounding method where the equipment grounding system is electrically insulated from the ground.

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