PCB Tech - Anti-interference PCB circuit board technology

PCB anti-jamming technology design, the primary task of designing printed boards is to analyze the circuit and determine the key circuit. This is to identify which circuits are interference sources and which circuits are sensitive circuits, and find out what paths the interference sources may use to interfere with sensitive circuits. In analog circuits, low-level analog circuits are often sensitive circuits, and power amplifiers are often sources of interference. When the working frequency is low, the interference source mainly interferes with the sensitive circuit through the inter-wire barrel connection; when the working frequency is high, the interference source mainly interferes with the sensitive circuit through electromagnetic radiation. In digital circuits, high-speed repetitive signals, such as clock signals, bus signals, etc., are rich in frequency components, which are the biggest source of interference and often pose a threat to sensitive circuits. Reset circuits, interrupt circuits, etc. are sensitive circuits, which are susceptible to interference from spikes, so digital circuits cannot work normally. The input/output circuit (1/0) is connected to the outside world, and special attention should also be paid. If the UO circuit is close to an interference source such as a clock line, the unnecessary high-frequency energy will be integrated into the output line, and the noise on the line will interfere with the sensitive circuit near the cable through radiation or conduction.

On the basis of fully analyzing the circuit and determining the key circuit, the circuit must be properly arranged on the printed board. For digital circuits, high-speed circuits (such as clock circuits, high-speed logic circuits, etc.), medium and low-speed logic circuits, and UO circuits should be arranged in different areas, and the interference source and sensitive circuits should be separated in space as much as possible, so that the interference source can be separated. Radiation interference to sensitive circuits is greatly reduced.

Printed board anti-jamming design

The purpose of the anti-interference design of the PCB board is to reduce the electromagnetic radiation of the PCB board and the crosstalk between the circuits on the PCB board. In addition, the ground design of the PCB directly affects the common-mode voltage radiation of the 1/0 cable. Therefore, the anti-interference design of the PCB is of great significance for reducing the electromagnetic information radiation of the system.

PCB layout design

The density of the printed circuit board (PCB) is getting higher and higher, and the quality of the PCB design has a great influence on the anti-interference ability, so the layout of the PCB is in a very important position in the design.

Layout requirements for special components:

1. The shorter the wiring between high-frequency components, the better, and minimize the electromagnetic interference between each other; components that are susceptible to interference should not be too close; input and output components should be as far away as possible;

2. Some components have a higher potential difference, so the distance between them should be increased to reduce common mode radiation. Pay special attention to the rationality of the layout of components with high voltage;

3. The thermal element should be far away from the heating element;

4. The solution capacitor should be close to the power pin of the chip;

5. The layout of adjustable components such as potentiometers, adjustable inductance coils, variable capacitors, micro switches, etc. should be placed in an easy-to-adjust position as required;

6. The position occupied by the positioning hole of the printed board and the fixed bracket should be reserved.

Layout requirements for common components:

1. Place the components of each functional circuit unit according to the circuit flow, so that the signal flow direction is as consistent as possible;

2. Take the core components of each functional circuit as the center and lay out around it. The components should be evenly and neatly arranged on the PCB to minimize and shorten the leads and connections between the components;

3. For circuits operating at high frequencies, the interference between components should be considered. In general, the components should be arranged in parallel as far as possible to facilitate wiring;

4. The outplace line of the PCB is generally not less than 80 mils away from the edge of the circuit board. The best shape of the circuit board is rectangular. The aspect ratio is 3:2 or 4:30.

2.2 PCB layout design

The wiring density of PCB is increasing, so PCB wiring design is particularly important.

1. The power line layer of the four-layer board should be as close as possible to the ground line layer to obtain the minimum power impedance. From top to bottom are: signal wire, ground wire, power wire, signal wire. Considering electromagnetic compatibility, the best six-layer boards from top to bottom are: signal wire, ground wire, signal wire, power wire, ground wire, signal wire;

2. The clock line should be adjacent to the ground layer, and the line width should be as large as possible, and the line width of each clock line should be the same;

3. The signal layer adjacent to the ground wire is arranged with high-speed digital signal lines and low-level analog signal lines, and the farther layer is arranged with low-speed signal lines and high-level analog signal lines;

4. The wiring of the input and output terminals should be avoided as far as possible to avoid parallel connection to avoid feedback;

5. The bend of the printed wire is generally 135 degrees obtuse angle;

6. The line width of the power line and the ground line should be increased as much as possible, and the wiring width of the device with a 0.5mm pin pitch should not be less than 12mil;

7. The signal line width of general digital circuit is 8.il-10nul, and the pitch is 6mi1-8mil;

8. Leads of de-radiation capacitors should not be too long, especially for high-frequency bypass capacitors;

9. The digital ground and analog ground on the mixed-signal circuit board are separated. If the wiring crosses the separation gap, electromagnetic radiation and signal interference will increase sharply, causing electromagnetic compatibility problems. Therefore, PCB design generally adopts a unified ground, layout and wiring through digital circuits and analog circuits;

10. For some high-speed signals, differential pair wiring can be used to reduce electromagnetic radiation.