The basic task of anti-interference planning is to prevent the system or equipment from misoperation or loss of function due to the influence of external electromagnetic interference, and to avoid affecting the normal operation of other systems or equipment. Therefore, it is an important part of the system planning to improve the anti-interference ability of the system.
Summary of circuit immunity planning criteria:
1. Power cord planning
(1) Select appropriate power supply;
(2) Widen the power cord as much as possible;
(3) Ensure the direction of power line, bottom line and data transmission is consistent;
(4) Use of anti-interference element equipment;
(5) Add decoupling capacitance (10 ~ 100uF) at the power inlet.
2. Ground wire planning
(1) Analog and digital separation;
(2) Single point grounding shall be selected as far as possible;
(3) Widen the ground wire as much as possible;
(4) Connect the sensitive circuit to a safe ground reference source;
(5) The PCB is partitioned to separate the high bandwidth noise circuit from the low frequency circuit;
(6) Try to reduce the area of the grounding loop (the path formed by grounding all equipment and returning to the power supply is called "ground loop").
3. Equipment of meta equipment
(1) Do not have too long parallel signal lines;
(2) Make sure that the clock generator, crystal oscillator of PCB and the clock input of CPU are as close as possible and far away from other low-frequency devices together;
(3) The central equipment shall be equipped around the central equipment, and the lead length shall be reduced as far as possible;
(4) Partition the PCB board;
(5) Consider the orientation and direction of PCB board in the case;
(6) Shorten the lead between high-frequency components.
4. Equipment of decoupling capacitor
(1) One charge discharge capacitor (10uF) shall be added for every 10 integrated circuits;
(2) Lead capacitor is used for low frequency and chip capacitor for high frequency;
(3) Each integrated chip shall be equipped with a 0.1uF ceramic capacitor;
(4) Anti noise ability is weak, when the power supply changes greatly, high frequency decoupling capacitor should be added to the equipment;
(5) Do not share vias between capacitors;
(6) Decoupling capacitor leads shall not be too long.
5. Noise reduction and electromagnetic disturbance criteria
(1) Try to choose 45 ° broken line instead of 90 ° broken line (try to reduce the external transmission and coupling of high-frequency signal);
(2) The method of series resistance is used to reduce the jump rate of circuit signal margin;
(3) The shell of quartz crystal oscillator shall be grounded;
(4) Don't leave the unnecessary circuits in the air;
(5) The disturbance is small when the clock is straight to the IO line;
(6) Try to make the electromotive force around the clock tend to zero;
(7) IO driver circuit is close to the edge of PCB as much as possible;
(8) Any signal shall not form a circuit;
(9) For the high frequency board, the distributed inductance of the capacitance cannot be neglected, nor can the distributed capacitance of the inductance be neglected;
(10) Generally, the power line and communication line should be on a board different from the signal line.
6. Other planning criteria
(1) The unused pins of CMOS shall be grounded through resistance or power supply;
(2) RC circuit is used to absorb the discharge current of relay and other components;
(3) The bus with 10K pull-up resistance is helpful to anti disturbance;
(4) Full decoding has better anti-interference;
(5) It is not necessary to connect the pin to the power supply through 10K resistance;
(6) The bus should be as short as possible and keep the same length as possible;
(7) The wiring between the two layers shall be as straight as possible;
(8) Heating element equipment shall avoid sensitive elements;
(9) The front horizontal route and the back vertical route only need space, the thicker the route is, the better (only for ground wire and power line);
(10) In order to have a good stratigraphic line, we should try our best to route from the front and use the reverse as stratigraphic line;
(11) Keep the satisfied interval, such as input and output of filter, input and output of optocoupler, communication power line and weak signal line, etc;
(12) Long line plus low pass filter. The line shall be cut as short as possible, and the long line that has not been run shall be inserted into C, RC or LC low-pass filter in a reasonable direction;
(13) Except for the ground wire, do not use thick wire if you can use thin wire.
7. Wiring width and current
(1) The general width should not be less than 0.2.mm (8mil);
(2) On high-density and high-precision PCB, the spacing and linewidth are generally 0.3mm (12mil);
(3) When the thickness of copper foil is about 50um, the width of conductor is 1 ~ 1.5mm (60mil) = 2A;
(4) Generally, 80mil is used in public, and more attention should be paid to the use of microprocessors.
8. Power cord
The power line should be as short as possible and straight. It's better to take a tree shape instead of a ring shape.
First of all, consider the size of PCB. When the PCB scale is too large, the printed line is long, the impedance is added, the noise resistance can be reduced, and the cost is added; when the PCB scale is too small, the heat dissipation is not good, and the adjacent lines are easy to be disturbed.
After confirming PCB scale, confirm the orientation of special components. Finally, according to the function unit of the circuit, the layout of all elements of the circuit equipment is carried out.
When confirming the orientation of special components, the following criteria shall be observed:
(1) Try to shorten the connection between high-frequency element devices as much as possible, and try to reduce their dispersion parameters and electromagnetic interference between them. The components that are easy to be disturbed shall not be close to each other, and the input and output components shall be kept as far away as possible.
(2) There may be a high potential difference between some components or wires. The interval between them should be increased to avoid accidental short circuit caused by discharge. The equipment with high voltage shall be placed in the place hard to reach by hand during commissioning.
(3) The component equipment exceeding 15g shall be fixed with bracket and then welded. Those large, heavy and heat generating equipment should not be installed on the printed board, but on the chassis floor of the whole machine, and the heat dissipation should be considered. The thermistor shall be kept away from the heating element.
(4) For the layout of adjustable elements such as potentiometer, adjustable inductance coil, variable capacitor and microswitch, the structural requirements of the whole machine shall be considered. In case of internal conditioning, it shall be placed above the printed board for convenient conditioning; in case of external conditioning, its orientation shall be suitable for the orientation of conditioning knob on the cabinet panel.
(5) The orientation occupied by the positioning hole of the printing plate and the fixing bracket shall be reserved.
When the layout of all elements of the circuit is based on the functional unit of the circuit, the following criteria shall be met:
(1) According to the flow of the circuit, the orientation of each functional circuit unit is organized to make the layout convenient for signal flow and keep the signal in the same direction as much as possible.
(2) Take the central component of each functional circuit as the center, and make layout around it. The components shall be evenly, neatly and compactly placed on the PCB. The lead and connection between components shall be reduced and shortened as far as possible.
(3) For the circuit operating at high frequency, the scatter parameters between the components should be considered. General circuits should be placed in parallel as far as possible. In this way, it is not only beautiful, but also easy to assemble and weld, and easy to mass produce.
(4) The component equipment located at the edge of the circuit board is generally no less than 2mm away from the edge of the circuit board. The best shape of circuit board is rectangle. The ratio of length to width is 3:2 to 4:3. When the circuit board surface dimension is greater than 200x150mm, the mechanical strength of the circuit board shall be considered.
The guidelines for wiring are as follows:
(1) The conductors used at the input and output terminals shall be as parallel as possible. It is better to add the ground wire between wires to avoid reaction coupling.
(2) The minimum width of printed wire is mainly determined by the adhesion strength between the wire and the insulating base plate and the current flowing through them. When the thickness of copper foil is 0.05mm and the width is 1 ~ 15mm, the temperature will not be higher than 3 ℃ after 2A current, so the conductor width is 1.5mm, which can meet the requirements.
For integrated circuits, especially digital circuits, 0.02-0.3mm wire width is usually selected. Of course, just promise to use as wide a line as possible, especially the power line and ground line. The minimum spacing of conductor is mainly determined by the insulation resistance and breakdown voltage between wires in the worst case. With regard to integrated circuits, especially digital circuits, the gap can be as small as 5-8mm only if the process permits.
(3) The corner of printed wire is generally circular arc, and the right angle or included angle will affect the electrical function in high frequency circuit. In addition, try to prevent the use of large areas of copper foil, otherwise, when heated for a long time, the phenomenon of copper foil swelling and falling easily occurs. When a large area of copper foil must be used, it is better to use the grid shape. This is conducive to eliminate the volatile gas generated by the heating of the adhesive between the copper foil and the substrate.
The center hole of the pad is slightly larger than the lead wire diameter of the equipment. The pad is too large to form a faulty weld. The outer diameter D of the pad is generally not less than (D + 1.2) mm, in which D is the lead hole diameter. For high density digital circuit, the minimum diameter of pad can be (D + 1.0) mm.
12. PCB and circuit anti disturbance method
The anti-interference planning of PCB is closely related to the specific circuit. Here, only a few common methods of PCB anti-interference planning are explained.
13. Power line planning
According to the size of PCB current, try to rent the width of power line and reduce the loop resistance. Together, make the direction of power line and ground line consistent with the direction of data transmission, which helps to enhance anti noise ability.
14. Ground wire planning
The ground wire planning criteria are:
(1) Digital and analog separation. If there are both logic circuits and linear circuits on the circuit board, they should be separated as far as possible. Single point parallel connection should be selected as far as possible for the grounding of low-frequency circuit. When the actual wiring is difficult, part of the circuit can be connected in series and then connected to the ground. The high frequency circuit should be grounded in series with multiple points, the ground wire should be short and rented, and the grid like large area ground foil should be used as far as possible around the high frequency components.
(2) The earth wire should be as bold as possible. If the ground wire is made of very stitched lines, the ground potential will change with the change of current, so that the anti noise function will be reduced. Therefore, the grounding wire should be thickened so that it can pass through three times the allowable current on the printed board. If possible, the ground wire should be more than 2-3mm.
(3) The ground wire forms a closed loop. For PCB composed of digital circuit only, the ability of anti noise can be improved when the ground circuit is arranged into a group loop.
15. Decoupling capacitor equipment
One of the conventional methods of PCB planning is to equip proper decoupling capacitors at each key part of PCB.
The general equipment criteria for decoupling capacitors are:
(1) The input end of the power supply is connected with 10 ~ 100uF electrolytic capacitor. If possible, it is better to connect 100uF or above.
(2) In principle, each IC chip should be equipped with a 0.01pf ceramic chip capacitor. If it is not possible to use a printed circuit board in an empty space, a 1-10pf capacitor can be installed for every 4-8 chips.
(3) As for the equipment with weak anti noise ability and big power change when switching off, such as RAM and ROM storage equipment, the decoupling capacitor should be directly connected between the power line and the ground wire of the chip.
(4) The capacitance leads shall not be too long, especially the high-frequency bypass capacitance shall not have leads.
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