Anti-interference in PCB design

The problem of anti-interference in PCB factory is a very important link in modern circuit design, which directly reflects the performance and reliability of the entire system. For PCB engineers, anti-interference design is the key and difficult point that everyone must master.

The existence of interference in PCB board It is found in practical research that there are four main interference aspects in the design of PCB board: power supply noise, transmission line interference, coupling and electromagnetic interference (EMI).

1. Power supply noise In high-frequency circuits, the noise carried by the power supply has a particularly significant impact on high-frequency signals. Therefore, firstly, the power supply is required to be low-noise. Here, clean ground is as important as clean power.

2. Transmission line There are only two kinds of transmission lines possible in PCB: strip line and microwave line. The biggest problem of transmission lines is reflection, which will cause many problems. For example, the load signal will be the superposition of the original signal and the echo signal, which will increase the difficulty of signal analysis; The impact is as severe as that produced by additive noise interference.

3. The interference signal generated by the coupling interference source has the effect of electromagnetic interference on the electronic control system through a certain coupling channel. The coupling method of interference is nothing more than acting on the electronic control system through wires, spaces, public lines, etc. There are mainly the following types of analysis: direct coupling, common impedance coupling, capacitive coupling, electromagnetic induction coupling, radiation coupling and so on.

4. Electromagnetic Interference (EMI)

Electromagnetic interference EMI has two kinds of conduction interference and radiation interference. Conducted interference refers to the coupling (interference) of a signal on one electrical network to another electrical network through a conductive medium. Radiated interference means that the source of interference couples (interferes) its signal to another electrical network through space. In high-speed PCB and system design, high-frequency signal lines, integrated circuit pins, various connectors, etc. may become radiation interference sources with antenna characteristics, which can emit electromagnetic waves and affect other systems or other subsystems in the system. normal work.

PCB and circuit anti-interference measures

The anti-jamming design of the printed circuit board is closely related to the specific circuit. Next, we will only explain some common measures for the anti-jamming design of the PCB.

1. Power cord design

According to the size of the printed circuit board current, try to increase the width of the power line to reduce the loop resistance. At the same time, make the direction of the power line and the ground line consistent with the direction of data transmission, which will help to enhance the anti-noise capability.

2. Ground wire design

The digital ground is separated from the analog ground. If there are logic circuits and linear circuits on the circuit board, they should be separated as much as possible. The ground of the low-frequency circuit should be grounded in parallel at a single point as far as possible. When the actual wiring is difficult, it can be partially connected in series and then grounded in parallel. The high-frequency circuit should be grounded at multiple points in series, the ground wire should be short and thick, and the large-area grid-shaped ground foil should be used around the high-frequency components as much as possible.

The ground wire should be as thick as possible. If the ground wire is very thin, the ground potential will change with the change of the current, which will reduce the anti-noise performance. Therefore, the ground wire should be thickened so that it can pass three times the allowable current on the printed board. If possible, the ground wire should be more than 2~3mm.

The ground wire forms a closed loop. For printed boards composed only of digital circuits, most of the grounding circuits are arranged in a loop, which can improve the anti-noise ability.

3. Decoupling capacitor configuration

One of the common practices in PCB design is to configure appropriate decoupling capacitors in various key parts of the printed board.

The general configuration principles of decoupling capacitors are:

①The input terminal of the power supply is connected to an electrolytic capacitor of 10~100uf. If possible, it is better to connect with more than 100uF.

②In principle, each integrated circuit chip should be arranged with a 0.01pF ceramic capacitor. If the printed board space is not enough, a 1~10pF capacitor can be arranged every 4~8 chips.

③ For devices with weak anti-noise ability and large power changes when turned off, such as RAM and ROM storage devices, decoupling capacitors should be directly connected between the power line and the ground line of the chip.

④The lead wire of the capacitor should not be too long, especially the high frequency bypass capacitor should not have lead wire.

4. Methods of Eliminating Electromagnetic Interference in PCB Design

①Reduce the loop: Each loop is equivalent to an antenna, so we need to minimize the number of loops, the area of the loop and the antenna effect of the loop. Make sure that the signal has only one loop path at any two points, avoid artificial loops, and try to use the power plane.

②Filtering: Filtering can be used on both power lines and signal lines to reduce EMI. There are three methods: decoupling capacitors, EMI filters, and magnetic components.

③ Shield.

④ Try to reduce the speed of high-frequency devices.

⑤ Increasing the dielectric constant of the PCB board can prevent the high-frequency parts such as transmission lines close to the board from radiating outward; increasing the thickness of the PCB board and minimizing the thickness of the microstrip line can prevent the overflow of the electromagnetic wire and also prevent radiation.

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