PCB Technical - High-speed PCB design grounding classification and selection principles

With the development of electronic technology, the product functions of electronic products are becoming more and more powerful. PCB design plays a pivotal role in the design of electronic products, because the quality of PCB design will directly affect the realization of product functions.

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

PCB grounding design:

Broadly speaking, grounding includes two meanings, namely, grounding and virtual grounding. Ground connection refers to the connection with the earth; connection to the virtual ground refers to the connection with the potential reference point. When the reference point is electrically insulated from the earth, it is called a floating connection. There are two purposes of grounding: one is to ensure the stable and reliable operation of the control system and to prevent interference caused by ground loops, which is often referred to as working grounding; the other is to prevent the operator from being exposed to the risk of electric shock due to the insulation damage or drop of the equipment and guarantee The safety of the equipment is called protective grounding.

Grounding selection principle:

For a given device or system, at the highest frequency (corresponding wavelength) of interest, when the length of the transmission line L>in, 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), it is recommended to use multi-point grounding;

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

PCB grounding method:

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.

Single-point grounding means that in the entire system, only one physical point is defined as the grounding reference point, and all other points that need to be grounded are connected to this point.

Single-point grounding is suitable for circuits with lower frequency (below 1MHZ). If the operating frequency of the system is so high that the operating wavelength is comparable to the length of the system grounding lead, single-point grounding is a problem. When the length of the ground wire is close to 1/4 wavelength, it is like a short-circuited transmission line. The current and voltage of the ground wire are distributed in a standing wave, and the ground wire becomes a radiating antenna and cannot function as a "ground". .

In order to reduce ground impedance and avoid radiation, the length of the ground wire should be less than 1/20 wavelength. In the treatment of power circuits, single-point grounding can generally be considered. For PCBs with a large number of digital circuits, it is generally not recommended to use a single-point grounding method due to its rich high-order harmonics.

Multi-point grounding

2. Multi-point grounding: The ground wires of all circuits are grounded nearby, and the ground wires are 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 grounding lead is the shortest.

The multi-point grounding circuit has a simple structure, and the high-frequency standing wave phenomenon that may appear on the grounding line is significantly reduced. It is suitable for occasions with higher operating frequencies (>10MHZ). However, multi-point grounding may cause many ground loops to form inside the device, thereby reducing the device's resistance to external electromagnetic fields. In the case of multi-point grounding, note

Intentionally loop problems, especially when networking between different modules and devices. Electromagnetic interference caused by ground loop:

Ideally, the ground 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 will occur. The ground wire will form a loop with other connections (signal, power cord, etc.). When the time-varying electromagnetic field is coupled to the loop, it will be in the ground loop.

In, induced electromotive force is generated and coupled to the load by the ground loop, which constitutes 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 way, and use a mixed grounding method to complete the connection between the circuit ground wire and the ground plane.

If you do not choose to use the entire plane as a common ground wire, for example, when the module itself has two ground wires, 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 unrelated 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, there will be coupling between layers through the parasitic capacitance of the circuit board;

(3) The signal line between the ground plane (such as the digital ground plane and the analog ground plane) is connected by a ground bridge, 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, which may cause unnecessary radiation.

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

4. Floating ground:

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

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