PCB Tech - How to carry out the anti-jamming design of the circuit board?

Summary of circuit board anti-jamming design principles:

1. The design of the power cord

(1) Choose a suitable power source;

(2) Widen the power cord as much as possible;

(3) Ensure that the direction of the power cord, the bottom line and the data transmission direction are consistent;

(4) Use anti-interference components;

(5) Add decoupling capacitor (10~100uf) to the power inlet.

2. Design of the ground wire

(1) The analog ground and digital ground are separated;

(2) Try to use single point grounding;

(3) Widen the ground wire as much as possible;

(4) Connect the sensitive circuit to a stable ground reference source;

(5) Partition design of PCB boards to separate high-bandwidth noise circuits from low-frequency circuits;

(6) Minimize the area of the ground loop (the path formed by returning all devices to the power ground after all devices are grounded is called "ground loop").

3. Configuration of components

(1) Do not have too long parallel signal lines;

(2) Ensure that the clock input terminals of the PCB clock generator, crystal oscillator and cpu are as close as possible, while keeping away from other low-frequency devices;

(3) The components should be arranged around the core components, and the lead length should be minimized;

(4) Partition layout of PCB board;

(5) Consider the position and direction of the PCB board in the chassis;

(6) Shorten the leads between high-frequency components.

4. Configuration of decoupling capacitor

(1) Add a charge and discharge capacitor (10uf) for every 10 integrated circuits;

(2) Leaded capacitors are used for low frequencies, and chip capacitors are used for high frequencies;

(3) A 0.1uf ceramic capacitor shall be arranged for each integrated chip;

(4) The anti-noise ability is weak, and high-frequency decoupling capacitors should be added to the devices with large power changes when shutting down;

(5) Do not share vias between capacitors;

(6) The lead of the decoupling capacitor should not be too long.

5. Principles of reducing noise and electromagnetic interference

(1) Try to use a 45° fold line instead of a 90° fold line (to minimize the external emission and coupling of high-frequency signals);

(2) Use series resistance to reduce the jump rate of the circuit signal edge;

(3) The shell of the quartz crystal oscillator should be grounded;

(4) Don't leave the circuits that are not in use;

(5) The interference is small when the clock is perpendicular to the IO line;

(6) Try to make the electromotive force around the clock tend to zero;

(7) The IO drive circuit is as close as possible to the edge of the PCB;

(8) Any signal should not form a loop;

(9) For high-frequency boards, the distributed inductance of the capacitor cannot be ignored, and the distributed capacitance of the inductance cannot be ignored either;

(10) Usually the power line and AC line should be on a different board from the signal line as much as possible.

6. Other design principles

(1) The unused pins of CMOS should be grounded or powered through resistors;

(2) Use RC circuit to absorb the discharge current of relays and other originals;

(3) Adding about 10k pull-up resistor on the bus is helpful for anti-interference;

(4) The use of full decoding has better anti-interference;

(5) The components are connected to the power supply through a 10k resistor without pins;

(6) The bus should be as short as possible and keep the same length as much as possible;

(7) The wiring between the two layers should be as vertical as possible;

(8) Avoid sensitive components with heating components;

(9) Horizontal wiring on the front side and vertical wiring on the reverse side. As long as space permits, the thicker the wiring, the better (only ground wire and power wire);

(10) To have a good ground line, try to route the line from the front side, and use the back side as a ground line;

(11) Keep a sufficient distance, such as the input and output of the filter, the input and output of the optocoupler, the AC power line and the weak signal line, etc.;

(12) Long line plus low-pass filter. The trace should be as short as possible, and the long line that has to be taken should be inserted in a reasonable position with a C, RC, or LC low-pass filter;

(13) Except for the ground wire, do not use thick wires if thin wires can be used.

7. Power cord

The power cord should be as short as possible, in a straight line, preferably in a tree shape, not a loop.

8. Layout

First, consider the PCB size. When the PCB size is too large, the printed lines will be long, the impedance will increase, the anti-noise ability will decrease, and the cost will also increase; if the PCB size is too small, the heat dissipation will not be good, and adjacent lines will be easily disturbed.

After determining the PCB size, determine the location of the special components. Finally, according to the functional units of the circuit, all the components of the circuit are laid out.

The following principles should be observed when determining the location of special components:

(1) Shorten the wiring between high-frequency components as much as possible, try to reduce their distribution parameters and mutual electromagnetic interference. Components that are susceptible to interference should not be too close to each other, and input and output components should be kept as far away as possible.

(2) There may be a high potential difference between some components or wires, and the distance between them should be increased to avoid accidental short circuits caused by discharge. The components with high voltage should be arranged as far as possible in places that are not easily reachable by hands during debugging.

(3) Components weighing more than 15g should be fixed with brackets and then welded. Those components that are large, heavy, and generate a lot of heat should not be installed on the printed circuit board, but should be installed on the chassis bottom plate of the whole machine, and the heat dissipation problem should be considered. Thermal components should be far away from heating components.

(4) For the layout of adjustable components such as potentiometers, adjustable inductors, variable capacitors, and micro switches, the structural requirements of the whole machine should be considered. If it is adjusted inside the machine, it should be placed on the printed circuit board where it is convenient for adjustment; if it is adjusted outside the machine, its position should match the position of the adjustment knob on the chassis panel.

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

9. Wiring.

The principle of wiring is as follows:

(1) The wires used for the input and output terminals should try to avoid being adjacent and parallel. It is best to add ground wires between wires to avoid feedback coupling.

(2) The minimum width of the printed wire is mainly determined by the adhesion strength between the wire and the insulating substrate and the current value flowing through them. When the thickness of the copper foil is 0.05mm and the width is 1 ~ 15mm, the temperature will not be higher than 3°C through the current of 2A, so the wire width of 1.5mm can meet the requirements.

For integrated circuits, especially digital circuits, a wire width of 0.02~0.3mm is usually selected. Of course, as long as possible, use as wide a cable as possible, especially the power cable and the ground cable. The minimum spacing of wires is mainly determined by the worst-case insulation resistance and breakdown voltage between the wires. For integrated circuits, especially digital circuits, as long as the process permits, the spacing can be as small as 5-8mm.

(3) The corners of the printed conductors are generally arc-shaped, and the right angle or the included angle will affect the electrical performance in the high-frequency circuit. In addition, try to avoid using large-area copper foil, otherwise the copper foil will expand and fall off when heated for a long time. When a large-area copper foil is required, it is best to use a grid shape. This will help to eliminate the volatile gas generated by the heating of the adhesive between the copper foil and the substrate.

10. Pad

The center hole of the pad is slightly larger than the diameter of the device lead. If the pad is too large, it is easy to form a false solder. The outer diameter D of the pad is generally not less than (d+1.2) mm, where d is the lead diameter. For high-density digital circuits, the minimum diameter of the pad can be (d+1.0) mm.

11. PCB and circuit anti-interference measures

The anti-jamming design of the printed circuit board has a close relationship with the specific circuit. Here, only a few common measures of PCB anti-jamming design are explained.

12. 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.

13. 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.