Nowadays, all kinds of electronic products have penetrated into all corners of people's life, leading to the fast development of PCBs that are the core of electronic devices. Whether electronic devices are capable of working normally, safely and stably depends on PCB design to a great extent. In the process of PCB design, the most important link is the design in terms of grounding and anti-interference for electronic products. Up to now, designers for specific PCBs hold their own opinions towards grounding and anti-interference and both methods and technologies concerning grounding and anti-interference are progressing from time to time, which will provide significant insurance to constantly stable security operation to electronic devices. This article discusses the strategies of anti interference and grounding for PCBs.
Grounding of Digital Signals and Analog Signals
In the process of PCB design, we fail to strictly distinguish digital signal areas or analog signal areas. Another example, in a circuit, as a public section, it's difficult to judge which section power belongs to. The common method of anti interference is to distinguish digital circuits from analog circuits and they should be drawn in different areas. But how to design the section that can't be strictly distinguished, such as power section mentioned above? The essentiality of distinguishing analog signals from digital signals lies in the property of the concerning chip, that is, whether the chip is analog or digital. Power section supplies power to analog circuits when it belongs to analog section while it belongs to digital section when supplying power to digital chip. When both of sections, however, apply the same power simultaneously, the method of bridge will be applied to lead power from another part. This anti-interference system mentioned above is a relatively common method right now. In reality, this method only works in some small systems or PCBs. Nevertheless, in large circuit systems, lots of potential problems are usually caused with the application of this method, especially in complicated system where these problems are so protruding that EMI issues are therefore led in routing bypassing distribution spacing. For example, when a typical A/D converter is being applied, PCB Fab Houses will suggest that AGND and DGND on A/D converter be connected to the ground with low impedance through the shortest lead. Therefore, with the method mentioned above applied, two grounds are connected through the connection bridge that has equivalent width with IC under A/D converter.
However, for systems with a lot of A/D converters, if each was processed according to the method mentioned above, multi-point connections would be generated. It would be no significance for the isolation between digital ground and analog ground. To solve this problem, earth ground should be applied with earth ground divided into digital ground and analog ground, which is both capable of meeting the requirement of manufacturers and reducing EMI issues as much as possible.
Analysis on High-Frequency Signal Anti Interference
In the process of designing PCBs with high-frequency signals, any metal or lead can be regarded as a component consisting of resistor, inductor and capacitor. A printed lead with a length of 25mm on a PCB is capable of generating inductance of 15nH to 20Nh. Therefore, multi-point grounding strategy should be applied to make each circuit system assessed into adjacent grounding line with the lowest impedance. Moreover, ground impedance and inductance between ground lines should be reduced as much as possible and mutual coupling between circuits caused by distributed capacitance should be decreased as well. The simplest method of multi-point grounding lies in complete copper coating. Grounding points of components are connected to the coating copper and the ground plane covering most of PCB provides a reference plane with extremely low impedance. Then, unnecessary high-frequency coupling can be avoided between each component and unit circuit.
Digital ground and analog ground are required to be processed independently in high-frequency PCBs. Ground levels of high-frequency digital signal lines are usually different from each other and voltage deviation often occurs between them. Furthermore, high-frequency digital signal ground lines always hold quite rich harmonic component of high-frequency signals. When digital signal ground lines are directly connected with analog signal ground lines, harmonic wave of high-frequency signals will interfere with analog signals in the way of ground line coupling. Commonly, high-frequency digital signal ground lines should be separated from analog signal ground lines either in the method of single-point interconnection at the suitable position or in the method of high-frequency choking magnetic bead interconnection.
Analysis on High-Frequency Signal Anti Interference
In PCB design, component layout and thickness of leads are greatly related with interference, which requires professional technology and full recognition capability of designers. Anti interference of PCB design is related with application performance of electronic products. The list of rules introduced in this article is the summary of practical design experience from designers, which is definitely useful to PCB designers.