The basic task of anti-interference design is that the system or device will not malfunction or lose function due to the influence of external electromagnetic interference, and will not 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.
Design of power line
1) Choose the right power supply;
2) Widen the power cord as much as possible;
3) Ensure that the direction of power line, bottom line and data transmission is consistent;
4) Use anti-interference components;
5) A decoupling capacitor (10-100uf) is added to the power inlet.
Design of ground wire
1) Analog and digital separation;
2) Single point grounding shall be adopted as far as possible;
3) Widen the ground wire as much as possible;
4) Connect the sensitive circuit to a stable grounding reference source;
5) The PCB board is partitioned to separate the high bandwidth noise circuit from the low frequency circuit;
6) Minimize the area of the ground loop (the path formed by the grounding of all devices back to the power ground is called “ground loop”).
Configuration of components
1) Do not have too long parallel signal line;
2) The clock generator of PCB, crystal oscillator and clock input of CPU should be close to each other as far as possible, and away from other low-frequency devices;
3) The components should be arranged around the components to minimize the lead length;
4) The PCB board is partitioned;
5) The position and direction of PCB board in the chassis are considered;
6) Shorten the lead between high frequency components.
Configuration of decoupling capacitor
1) One charge discharge capacitor (10uF) is added for every 10 integrated circuits;
2) The lead capacitor is used for low frequency, and the chip capacitor is used for high frequency;
3) Each IC should be equipped with a 0.1uF ceramic capacitor;
4) High frequency decoupling capacitors should be added to the devices with weak anti noise ability and large power supply change when they are turned off;
5) Do not share vias between capacitors;
6) The lead of decoupling capacitor should not be too long.
Principle of reducing noise and EMI
1) Try to use 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 the circuit signal edge;
3) The shell of quartz crystal oscillator should be grounded;
4) Don’t let the unused circuits hang in the air;
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 should be close to the edge of PCB as far as possible;
8) Any signal should not form a loop;
9) For the high frequency board, the distributed inductance of capacitance can not be ignored, nor can the distributed capacitance of inductance;
10) Usually, the power line and AC line should be on the board different from the signal line.
Other design principles
1) Unused pins of CMOS should be grounded or powered by resistance;
2) RC circuit is used to absorb the discharge current of relay and other components;
3) Adding about 10K pull-up resistor on the bus is helpful to anti-interference;
4) Using full decoding has better anti-interference performance;
5) The components are connected to the power supply through 10K resistor without pins;
6) The bus should be as short as possible and keep the same length as possible;
7) The wiring between the two layers should be vertical as far as possible;
8) Heating components should avoid sensitive components;
9) Horizontal wiring on the front and vertical wiring on the back, as long as the space allows, the thicker the wiring, the better (only limited to ground wire and power line);
10) In order to have a good stratigraphic line, we should try to route it from the front and use it as stratigraphic line from the back;
11) Keep enough distance, such as input and output of filter, input and output of optocoupler, AC power line and weak signal line, etc;
12) Long line plus low pass filter. The route should be as short as possible, and C, RC, or LC low-pass filters should be inserted in a reasonable position for the long route that must be taken;
13) In addition to the ground wire, can use thin wire do not use thick wire.