Principles of Electromagnetic Anti-Interference
The principle of electromagnetic anti-interference involves more knowledge points. For example, the corners of copper film lines should be rounded or beveled (because right-angled or sharp-angled corners will affect electrical performance at high frequencies). The wires on both sides of the double-sided board should be perpendicular to each other. , oblique or curved traces, try to avoid parallel traces, reduce parasitic coupling, etc.
一. There are usually various ground wires in an electronic system, such as digital ground, logic ground, system ground, chassis ground, etc. The design principles of the ground wire are as follows;
1. Correct single-point and multi-point grounding
In low-frequency circuits, the operating frequency of the signal is less than 1MHZ, and its wiring and inductance between devices have little influence, while the circulation formed by the ground circuit has a greater impact on interference, so one point grounding should be used. When the signal operating frequency is greater than 10MHZ, if one point grounding is used, the length of the ground wire should not exceed 1/20 of the wavelength, otherwise the multi-point grounding method should be used.
2. Separate the digital ground from the analo ground
If there are both logic circuits and linear circuits on the circuit board, they should be separated as much as possible. Generally, the anti-interference ability of digital circuits is relatively strong. For example, the noise tolerance of TTL circuits is 0.4~0.6V, the noise tolerance of CMOS circuits is 0.3~0.45 times of the power supply voltage, and the analo circuits only need a small noise. It does not work properly, so these two types of circuits should be laid out and routed separately.
3. The ground wire should be as thick as possible
If the grounding wire is made of very thin lines, the grounding 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 above 2~3mm.
4. The ground wire forms a closed loop
For printed boards composed only of digital circuits, the grounding circuit is usually looped to improve noise immunity. Because the ring ground wire can reduce the ground resistance, thereby reducing the ground potential difference.
二. With bitter decoupling capacitor
One of the conventional practices in PCB design is to configure appropriate decoupling capacitors at various key parts of the printed board. The general configuration principles of decoupling capacitors are:
The input terminal of the power supply is connected across a 10~100uf electrolytic capacitor. If the position of the printed circuit board allows, the anti-interference effect of using an electrolytic capacitor above 100uf will be better.
In principle, each integrated circuit chip should be equipped with a 0.01uf~0.1uf ceramic chip capacitor. If there is not enough space in the printed board, a 1~10uf bile capacitor can be arranged for every 4~8 chips (it is best not to use electrolytic capacitors. , The electrolytic capacitor is rolled up by two layers of film. This rolled up structure behaves as an inductance at high frequencies. It is best to use button capacitors or polycarbonate capacitors).
For devices with weak anti-noise ability and large power supply changes when turned off, such as RAM and ROM storage devices, a decoupling capacitor should be directly connected between the power line and ground line of the chip.
The capacitor leads should not be too long, especially the high-frequency bypass capacitors should not have leads.
