The basic task of the anti-interference design is that the system or device does not malfunction or lose its function due to external electromagnetic interference, nor does it send excessive noise interference to the outside world, so as not to affect the normal operation of other systems or devices.
Therefore, improving the anti-interference ability of the system is also an important part of the system design.
Summary of circuit anti-interference design principles:
1, the design of the power cord
(1) Select the appropriate power source;
(2) Try to widen the power cord;
(3) Ensure that the power line and the bottom line are in the same direction as the data transmission direction;
(4) Use anti-interference components;
(5) Add a decoupling capacitor (10~100uf) to the power inlet.
2, the design of the ground wire
(1) Separate analog ground and digital ground;
(2) Try to use a single point grounding;
(3) Try to widen the ground wire;
(4) Connect the sensitive circuit to a stable ground reference source;
(5) Partitioning the pcb board to separate the high-bandwidth noise circuit from the low-frequency circuit;
(6) Minimize the area of ​​the ground loop (the path formed by all devices grounded back to the power ground is called the “ground loopâ€).
3, the configuration of components
(1) Do not have long parallel signal lines;
(2) Ensure that the clock input of the pcb clock generator, crystal oscillator and cpu are as close as possible, while away from other low frequency devices;
(3) Components should be configured around the core device to minimize lead length;
(4) Partition layout of the pcb board;
(5) Consider the position and orientation of the pcb board in the chassis;
(6) Shorten the leads between high frequency components.
4, decoupling capacitor configuration
(1) Add one 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 is placed in each integrated chip;
(4) The anti-noise ability is weak, and the high-power decoupling capacitor should be added to the device with large power supply change during shutdown;
(5) Do not share vias between capacitors;
(6) Decoupling capacitor leads should not be too long.
5, reduce noise and electromagnetic interference principles
(1) Try to use a 45° fold line instead of a 90° fold line (to minimize the external transmission and coupling of high frequency signals);
(2) Use series resistors to reduce the hop rate of the circuit signal edges;
(3) The quartz crystal housing should be grounded;
(4) Do not vacate the unused circuits;
(5) When the clock is perpendicular to the IO line, the interference is small;
(6) Try to make the electromotive force around the clock tend to zero;
(7) The IO driver circuit is as close as possible to the edge of the pcb;
(8) Do not form a loop for any signal;
(9) For high-frequency boards, the distributed inductance of the capacitor cannot be ignored, and the distributed capacitance of the inductor cannot be ignored.
(10) Normally, the power line and AC line should be on a different board than the signal line.
6, other design principles
(1) The unused pins of the CMOS should be grounded or powered by a resistor;
(2) using an RC circuit to absorb the discharge current of an original such as a relay;
(3) Adding a pull-up resistor of about 10k on the bus helps to prevent interference;
(4) It has better anti-interference with full decoding;
(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 try to keep the same length;
(7) The wiring between the two layers is as vertical as possible;
(8) The heating component avoids sensitive components;
(9) The front side of the horizontal line, the reverse side of the line, as long as the space allows, the thicker the better the line (only ground and power lines);
(10) If there is a good stratigraphic line, it should be traced from the front as much as possible, and the reverse side should be used as the stratum line;
(11) Maintain 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;
(12) Long-line plus low-pass filter. The trace should be as short as possible, and the long line that must be taken should be inserted into the C, RC, or LC low-pass filter at a reasonable position;
(13) In addition to the ground wire, do not use thick wires for thin wires.
7, wiring width and current
(1) The general width should not be less than 0.2 mm (8 mil);
(2) On high-density and high-precision pcb, the pitch and line width are generally 0.3mm (12mil);
(3) When the thickness of the copper foil is about 50um, the width of the wire is 1~1.5mm (60mil) = 2A;
(4) The public place is generally 80 mils, and more attention should be paid to applications with microprocessors.
8, power cord
The power cord should be as short as possible and take a straight line. It is best to go tree-shaped and do not take the ring.
9, layout
First, consider the PCB size. When the PCB size is too large, the printed lines are long, the impedance is increased, the anti-noise ability is lowered, and the cost is also increased; if the size is too small, the heat dissipation is not good, and adjacent lines are susceptible to interference.
After determining the PCB size, determine the location of the special component. Finally, according to the functional unit of the circuit, all the components of the circuit are laid out.
Observe the following principles when determining the location of a particular component:
(1) Try to shorten the wiring between high-frequency components as much as possible, and try to reduce their distribution parameters and mutual electromagnetic interference. Components that are susceptible to interference cannot be placed too close together, 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 circuit caused by discharge. Components with high voltage should be placed as far as possible in the hands of the hand when debugging.
(3) Components weighing more than 15g shall be fixed by brackets and then welded. Those components that are large, heavy, and have a lot of heat should not be mounted on the printed board, but should be installed on the chassis of the whole machine, and heat dissipation should be considered. The thermal element should be kept away from the heating element.
(4) For the layout of adjustable components such as potentiometers, adjustable inductors, variable capacitors, microswitches, etc., the structural requirements of the whole machine should be considered. If it is adjusted inside the machine, it should be placed on the printed board to facilitate adjustment; if it is adjusted outside the machine, its position should be compatible with the position of the adjustment knob on the chassis panel.
(5) The position occupied by the printing plate positioning hole and the fixing bracket should be left.
When laying out all the components of the circuit according to the functional unit of the circuit, the following principles must be met:
(1) Arrange the position of each functional circuit unit according to the flow of the circuit, so that the layout facilitates signal circulation and keeps the signal as consistent as possible.
(2) Center around the core components of each functional circuit and arrange it around it. Components should be evenly, neatly and compactly arranged on the PCB. Minimize and shorten leads and connections between components.
(3) For circuits operating at high frequencies, the distribution parameters between components should be considered. In general, the circuit should be arranged in parallel as much as possible. In this way, it is not only beautiful, but also easy to load and weld. It is easy to mass produce.
(4) The components located at the edge of the board are generally not less than 2 mm from the edge of the board. The optimal shape of the board is a rectangle. The aspect ratio is 3:2 to 4:3. When the board surface size is larger than 200x150mm, the mechanical strength of the board should be considered.
10, wiring
The principles of wiring are as follows:
(1) The wires used at the input and output terminals should be avoided as far as possible. It is best to add the ground wire between the wires to avoid feedback.
(2) The minimum width of the printed photoconductive wire is mainly determined by the adhesion strength between the wire and the insulating base plate 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 exceed 3 °C through the current of 2A. Therefore, the wire width of 1.5mm can meet the requirements.
For integrated circuits, especially digital circuits, a wire width of 0.02 to 0.3 mm is usually selected. Of course, as far as possible, use wide lines as much as possible, especially power and ground. The minimum spacing of the wires is primarily determined by the worst case interline insulation resistance and breakdown voltage. For integrated circuits, especially digital circuits, the pitch can be as small as 5~8mm as long as the process allows.
(3) The curved corner of the printed conductor generally takes a circular arc shape, and the right angle or angle affects the electrical performance in the high frequency circuit. In addition, try to avoid the use of large areas of copper foil, otherwise, when heated for a long time, copper foil expansion and shedding are prone to occur. When a large area of ​​copper foil is used, it is preferable to use a grid shape. This is advantageous in eliminating the volatile gas generated by the heat of the adhesive between the copper foil and the substrate.
11, the pad
The center hole of the pad is slightly larger than the diameter of the device lead. The pad is too large to form a solder joint. The pad outer diameter D is generally not less than (d + 1.2) mm, where d is the lead aperture. For high-density digital circuits, the minimum pad diameter can be (d + 1.0) mm.
12, PCB and circuit anti-interference measures
The anti-jamming design of printed circuit boards is closely related to the specific circuit. Here, only some common measures of PCB anti-interference design are explained.
13, power cord design
According to the current of the printed circuit board, try to increase the width of the power line and reduce the loop resistance. At the same time, the direction of the power line and the ground line is consistent with the direction of data transmission, which helps to enhance the anti-noise capability.
14, ground design
The principle of ground design is:
(1) The digital ground is separated from the simulated ground. If there are both logic and linear circuits on the board, they should be separated as much as possible. The ground of the low-frequency circuit should be grounded in parallel with a single point. If the actual wiring is difficult, it can be partially connected and then grounded in parallel. The high-frequency circuit should adopt multi-point series grounding, the ground wire should be short and rented, and the grid-like large-area foil should be used as much as possible around the high-frequency components.
(2) The grounding wire should be as thick as possible. If the grounding wire uses a very strong line, the grounding potential changes with the change of the current, which reduces the noise immunity. Therefore, the ground wire should be thickened so that it can pass three times the allowable current on the printed board. If possible, the grounding wire should be 2~3mm or more.
(3) The grounding wire constitutes a closed loop. In a printed circuit board composed only of digital circuits, the grounding circuit is mostly formed into a loop, which can improve the anti-noise capability.
15, untwisting capacitor configuration
One of the usual practices in PCB design is to place appropriate untwisting capacitors at various critical points in the printed board.
The general configuration principle for the untwisting capacitor is:
(1) The power input terminal is connected to an electrolytic capacitor of 10~100uf. If possible, it is better to pick up 100uF or more.
(2) In principle, each integrated circuit chip should be equipped with a 0.01pF ceramic capacitor. If there is insufficient gap in the printed board, a capacitor of 1 ~ 10pF can be arranged every 4~8 chips.
(3) For devices with weak anti-noise capability and large power supply changes during shutdown, such as RAM and ROM storage devices, the decoupling capacitor should be directly connected between the power cable and the ground of the chip.
(4) The capacitor leads should not be too long, especially the high-frequency bypass capacitors must not have leads.
Two - way trigger diode, also known as two - end ac device (DIAC), and two - way thyristor at the same time.Because of its simple structure and low price, it is often used to trigger the bidirectional thyristor, and can also constitute overvoltage protection circuits.Construction, symbol and equivalent circuit of a bidirectional trigger diode.
Also commonly used in the overvoltage protection, timing, shift equal circuit, Figure 2 is the overvoltage protection circuit composed of bidirectional trigger diode and bidirectional thyristor.When the transient voltage exceeds the DIAC and Ubo, the DIAC quickly conducts and triggers the bidirectional thyristor to also conduct, so that the following load is protected from overvoltage damage.
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