PCB Technical - Do you know PCB anti-interference design and layout

The anti-interference problem 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 presence of interference in the PCB board

In actual research, it is found that there are four main interferences in PCB design: power supply noise, transmission line interference, coupling and electromagnetic interference (EMI).

1. Power supply noise

In the high-frequency circuit, the noise of the power supply has a particularly obvious influence on the high-frequency signal. Therefore, the first requirement is that the power supply is low-noise. Here, a clean ground is as important as a clean power source.

Will you be able to design the power characteristics of PCB anti-interference

2. Transmission line

There are only two types of transmission lines possible in PCB: strip line and microwave line. The biggest problem of transmission line is reflection. Reflection will cause many problems. For example, the load signal will be the superposition of the original signal and the echo signal, which increases the difficulty of signal analysis. ; Reflection will cause return loss (return loss), and its impact on the signal is as serious as the impact of additive noise interference.

3. Coupling

The interference signal generated by the interference source causes electromagnetic interference to 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, common lines, etc. After analysis, there are mainly the following types: direct coupling, common impedance coupling, capacitive coupling, electromagnetic induction coupling, radiation coupling, etc.

Common impedance coupling

4. Electromagnetic interference (EMI)

Electromagnetic interference EMI has two types: conducted interference and radiated interference. Conducted interference refers to the coupling (interference) of signals on one electrical network to another electrical network through a conductive medium. Radiated interference refers to the interference source coupling (interference) 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 make some explanations on several common measures of PCB anti-jamming design.

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 ground line consistent with the direction of data transmission, which helps to enhance the anti-noise ability.

2. Ground wire design

The principles of ground wire design are:

(1) The digital ground is separated from the analog ground. If there are both 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 much 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 leased, and the grid-like large-area ground foil should be used around the high-frequency component as much as possible.

(2) The grounding wire should be as thick as possible. If the ground wire uses a very tight line, the ground potential changes with the change of the current, which reduces 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 grounding wire should be 2~3mm or more.

(3) The grounding wire forms a closed loop. For printed boards composed only of digital circuits, most of their grounding circuits are arranged in loops to improve noise resistance.

3. Decoupling capacitor configuration

One of the conventional methods of PCB design is to configure appropriate decoupling capacitors on each key part of the printed board.

The general configuration principles of decoupling capacitors are:

(1) Connect a 10 ~ 100uf electrolytic capacitor across the power input. If possible, it is better to connect to 100uF or more.

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

(3) For devices with weak anti-noise ability and large power changes when shutting down, such as RAM and ROM storage devices, a decoupling capacitor should be directly connected between the power line and the ground line of the chip.

(4) Capacitor leads should not be too long, especially for high-frequency bypass capacitors.

4. Methods to eliminate electromagnetic interference in PCB design

(1) Reduce loops: 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. Ensure that the signal has only one loop path at any two points, avoid artificial loops, and try to use the power layer.

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

Type of filter

(3) Shielding.

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

(5) Increasing the dielectric constant of the PCB board can prevent high-frequency parts such as the transmission line 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 electromagnetic wire from overflowing, and it can also prevent radiation.