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Anti-interference design is the basic task of the system or device is not due to external electromagnetic interference effect and the misoperation or loss of function, also not to send too much noise outside, so as not to affect other systems or devices work properly. Therefore, improving the anti-jamming capability of the system is also an important part of the system design.
Summary of circuit anti-interference design principles:
1. Design of power cord.
(1) select the appropriate power source;
(2) try to widen the power cord;
(3) ensure that the power line, bottom line direction and data transmission direction are consistent;
(4) use anti-interference components;
(5) add the decoupling capacitor (10~100uf) to the power inlet.
2. Ground wire design.
(1) separation of simulated ground and number;
(2) try to use single point grounding;
(3) as wide as possible;
(4) connect the sensitive circuit to the stable grounding reference source;
(5) partition design of PCB board to separate high-bandwidth noise circuit from low-frequency circuit;
(6) try to minimize the area of the ground loop (the path formed after grounding of all devices is called "ground loop").
3. Configuration of components.
(1) do not have long parallel signal lines;
(2) ensure that the clock generator, crystal oscillator and CPU clock input end of PCB are as close as possible while away from other low-frequency devices;
(3) components shall be configured around the core devices to minimize lead length;
(4) partition layout of PCB board;
(5) considering the position and direction of PCB board in the chassis;
(6) shorten the lead between high frequency components.
4. Configuration of decoupling capacitor.
(1) a charge and discharge capacitance (10uf) is added for every 10 integrated circuits;
(2) the lead type capacitor is used for low frequency, and the chip capacitor is used for high frequency;
(3) each chip should be equipped with a ceramic capacitor of 0.1uf;
(4) in the case of weak resistance to noise, high frequency decoupling capacitors should be added to the devices with large power changes during the shutdown;
(5) do not share holes between capacitors;
(6) the decoupling capacitor leads cannot be too long.
5. Reduce noise and electromagnetic interference principle.
(1) try to use 45 lines instead of 90 lines (minimize the emission and coupling of high frequency signals);
(2) using series resistance method to reduce the jump speed of the edge of the circuit signal;
(3) quartz crystal vibration enclosure shall be grounded;
(4) do not suspend the circuits that are idle;
(5) when the clock is perpendicular to the IO line, the interference is small;
(6) try to keep the electromotive force near the clock to zero;
(7) IO drive circuit is as close to the edge of PCB as possible;
(8) do not form a loop in any signal;
(9) for the high frequency board, the distribution inductance of capacitance cannot be ignored, and the distribution capacitance of inductance cannot be ignored;
(10) usually power lines and ac lines are as much as possible on boards with different signal lines.
6. Other design principles.
(1) the unused pins of CMOS should be grounded or powered by resistance;
(2) use RC circuit to absorb the discharge current of the original such as relay;
(3) adding 10k on the bus can help to resist interference;
(4) better anti-interference with full decoding;
(5) the component does not use the pin to connect the power with 10k resistance;
(6) the bus is as short as possible and as long as possible;
(7) the wiring between the two layers is as vertical as possible;
(8) the heating element avoids sensitive components;
(9) the front side of the line, the reverse side of the line, as long as the space allows, the line is as thick as possible (only the ground wire and power line);
(10) in order to have a good stratigraphic line, we should try to take the front line as far as possible, and use the reverse as the stratigraphic line.
(11) maintain sufficient distance, such as input and output of the filter, input and output of optical coupling, ac power line and weak signal line, etc.;
(12) long line and low pass filter. The line should be as short as possible, and the long line should be inserted into C, RC, or LC low-pass filter in a reasonable position;
In addition to the ground line, use thin lines rather than thick lines.
7. Wiring width and current.
(1) general width should not be less than 0.2.mm (8mil);
(2) in high-density high-precision PCB, the spacing and line width are generally 0.3mm(12mil);
(3) when the thickness of copper foil is around 50um, the wire width is 1~1.5mm(60mil) = 2A;
(4) common ground 80mil, more attention should be paid to applications with microprocessors.
8. Power cord.
The power line is as short as possible, walking in a straight line, it is better to walk in a tree rather than ring.
First, consider the PCB size. When PCB is too large, the printing line is long, the impedance is increased, the anti-noise ability is decreased, and the cost is increased. Too small, the heat dissipation is not good, and the adjacent lines are easily disturbed.
After determining the PCB size, determine the location of the special element. Finally, according to the functional unit of the circuit, all components of the circuit are laid out.
The following principles should be observed in determining the location of special components:
(1) reduce the connection between high frequency components as much as possible, and try to reduce their distribution parameters and electromagnetic interference between them. Vulnerable components cannot get too close to each other, and input and output components should be kept away from each other.
(2) there may be a high potential difference between some components or wires, and the distance between them should be increased so as to avoid an unexpected short circuit. The components with high voltage should be arranged as far as possible when the hand is not easy to reach.
(3) components weighing more than 15g shall be fixed with brackets and then welded. Those large, heavy and calorific components should not be installed on the printed board, but should be installed on the chassis of the machine, and the heat dissipation should be considered. The thermal element shall be removed from the heating element.
(4) the layout of adjustable elements such as potentiometers, adjustable inductors, variable capacitors, and micro switches should be considered as the structural requirements of the whole machine. If the adjustment in the machine, should be placed in the printing plate to facilitate the adjustment of the place; In case of external adjustment, the position should be adjusted to the position of the adjusting knob on the cabinet panel.
(5) the positioning holes and the position of fixed brackets shall be set aside.
According to the function unit of the circuit, the following principles should be applied to the layout of all components of the circuit:
(1) arrange the location of each functional circuit unit according to the process of the circuit, so as to facilitate the distribution of signals and keep the signals as consistent as possible.
(2) center around the core components of each functional circuit and arrange the layout around it. The components shall be arranged in a uniform, neat and compact manner on the PCB. Minimize and shorten the leads and connections between the components.
(3) the circuit that works at high frequency should consider the distribution parameters between components. General circuits should be arranged in parallel as far as possible. In this way, it is not only beautiful, but also easy to weld. Easy to mass production.
(4) the component on the edge of the circuit board is not less than 2mm from the edge of the circuit board. The best shape of the circuit board is the rectangle. The ratio of length to width is 3:2 to 4:3. When the circuit board size is greater than 200x150mm, the mechanical strength of the circuit board should be considered.
10 and wiring
The principles of wiring are as follows:
(1) the wires used in the input and output terminals should be avoided in parallel. It is better to add line between lines to avoid feedback coupling.
(2) the minimum width of the printed conductor 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 0.05 mm, width is 1 ~ 15 mm. By 2 a of current, temperature is not higher than 3 ℃, so. Wire width of 1.5 mm can meet the requirements.
For the integrated circuit, especially the digital circuit, usually choose 0.02~0.3mm wire width. Of course, use wide lines whenever possible, especially power lines and ground wires. The minimum spacing of the wires is determined by the insulation resistance and breakdown voltage between the lines in the worst case. For the integrated circuit, especially the digital circuit, as long as the process permits, the spacing can be small to 5~8mm.
(3) the curve of the printed wire is generally rounded, while the right Angle or Angle will affect the electrical performance in the high frequency circuit. In addition, avoid using large area copper foil, otherwise, copper foil expansion and shedding can occur during prolonged heating. A large area of copper foil must be used for the best use of a raster. This facilitates the removal of volatile gases produced by the heat between copper foil and substrate.
11, welding plate
The center hole of the welding disc is slightly larger than the diameter of the device lead. Welding disc is too large and easy to form virtual welding. D is generally not less than (D +1.2)mm, where D is the lead aperture. For high density digital circuit, the minimum diameter of the welding disc is recommended (d+1.0)mm.
12. PCB and circuit anti-interference measures.
The anti-interference design of printed circuit board is closely related to the specific circuit, and there are several common measures for PCB anti-interference design.
13. Power cord design.
According to the printed circuit board current size, as far as possible to rent the power line width, reduce the loop resistance. At the same time, the direction of the power line, ground wire and the direction of data transmission are consistent, which will help to enhance the anti-noise ability.
14. Ground wire design.
The principle of ground line design is:
(1) the digital ground is separated from the simulation. If there are logic circuits and linear circuits on the circuit board, they should be separated as far as possible. The low frequency circuit should be connected with single point in parallel, and the actual wiring can be connected in series and connected in parallel. The high frequency circuit should adopt multi-point series grounding, the ground wire should be short and rent, and the high frequency element should be surrounded by a grid large area.
(2) the ground wire should be as thick as possible. If the ground wire is used in a very tight line, the ground potential changes with the current and reduces the anti-noise performance. The grounding line should therefore be thickened to allow it to pass three times the allowable current on the printed board. If possible, the grounding line should be above 2~3mm.
(3) the ground wire forms a closed loop. Only the printed plate made up of digital circuit, the ground circuit of the ground circuit can improve the anti-noise ability.
15. Drain capacity configuration.
One of the conventional methods of PCB design is to configure the appropriate fallback capacitance in each key part of the printed board.
The general configuration principle of decoupling capacitor is:
(1) electrolytic capacitor with 10~100uf on the input end of the power supply. If possible, connect 100uF or above.
(2) in principle, each IC chip should be equipped with a chip capacitor of 0.01pF. If there is not enough space in the printed plate, a 1-10pf capacitor can be arranged for every 4~8 chips.
(3) for devices with weak anti-noise capability and power supply changes when the power is switched off, such as RAM and ROM storage devices, it should be directly connected to the back coupling capacitance between the power line and ground wire of the chip.
(4) capacitance leads cannot be too long, especially for high frequency bypass capacitors.