How to Choose Grounding Type for High Speed PCB

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

How to Choose Grounding Type for High Speed PCB

In the design of electronic products, it is not difficult to design a PCB circuit to realize its functions. It is difficult to design a PCB circuit which is not affected by various factors (such as temperature and humidity, air pressure, mechanical impact, corrosion, etc.). In order to maintain normal and stable work, we will adopt various design means 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 safe and stable operation of the system. So Xiaobian today and everyone to talk about high-speed PCB design grounding mode of the relevant knowledge.

PCB grounding design:

The broad sense of grounding includes two meanings, that is, grounding on the ground and grounding on the ground. Connecting to the ground means connecting to the earth; connecting to the virtual ground means connecting to the potential reference point, which is called floating connection when it is insulated from the earth's electricity. 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 the ground loop, which is often called working grounding; the other is to avoid electric shock hazards and ensure the safety of the equipment due to the insulation damage or decline of the equipment, which is called protective grounding.

Grounding selection principle:

For a given device or system, when the maximum frequency (corresponding wavelength) of interest is input, the length of the transmission line L is input, it is regarded as a high frequency circuit, and vice versa, 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 hybrid circuit, hybrid grounding, the applicable operating frequency range is generally 500 kHz - 30 MHz;

PCB grounding mode:

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

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

Single-point grounding is suitable for low frequency circuits (below 1MHZ). If the working frequency of the system is so high that the working wavelength of the system is comparable to the length of the grounding lead, the single-point grounding mode will be problematic. When the length of the local line is close to 1/4 wavelength, it is like a short-circuit transmission line at the terminal. The current and voltage of the ground line are standing wave distribution. The ground line becomes a radiation antenna, but can not play the role of "ground".

In order to reduce grounding impedance and avoid radiation, the length of ground wire should be less than 1/20 wavelength. Generally, single-point grounding can be considered in power circuit processing. For PCBs of digital circuits which are widely used, single-point grounding is generally not recommended because of its rich high-order harmonics.

Multipoint Grounding

2. Multi-point grounding: Ground wires of all circuits are close to ground. Ground wires are very short and suitable for high frequency grounding.

Multipoint grounding means that each grounding point in the equipment is directly connected to the nearest grounding plane to make the length of the grounding lead shortest.

The structure of multi-point grounding circuit is simple, and the high-frequency standing wave phenomenon on the grounding line can be significantly reduced. It is suitable for the occasion of high working frequency (> 10MHZ). However, multi-point grounding may lead to many grounding loops in the equipment, thus reducing the resistance of the equipment to external electromagnetic fields. In the case of multi-point grounding, note

The problem of Yidi Loop, especially when networking between different modules and devices. Electromagnetic interference caused by ground circuit:

The ideal ground wire should be a physical entity with zero potential and impedance. But the actual ground wire itself has both resistance component and reactance component. When there is current passing through the ground wire, voltage drop will occur. The ground wire will form a loop with other connections (signals, power lines, etc.). When the electromagnetic field is coupled to the loop, it will be in the local loop.

Induced electromotive force (EMF) is generated and coupled to the load by the ground loop, which constitutes a potential EMI threat.

3. Hybrid grounding: single-point grounding and multi-point grounding are used together.

Generally, all modules will use two kinds of grounding methods, and use hybrid grounding method to complete the connection between circuit ground wire and ground plane.

If we do not choose to use the whole plane as a common ground line, such as when the module itself has two ground lines, we need to divide the ground plane, which often interacts with the power plane. Attention should be paid to the following principles:

(1) Align all planes to avoid overlap between irrelevant power and ground planes, otherwise all ground planes will be invalidated and interfere with each other.

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

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

(4) Avoid high-frequency routing such as clock lines near the isolated ground surface, causing 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 rule of the loop. The smaller the area of the loop is, the less radiation it emits to the outside world and the less interference it receives from the outside world.