PCB Layout anti-interference design principle

1. Ground wire layout: 1. Digital ground is separated from analog ground. 2. The grounding wire should be as thick as possible, so that it can pass at least 3 times the allowable current on the printed circuit board, generally 2~3mm. 3. The ground wire should constitute an infinite loop as far as possible, so as to reduce the potential difference of the ground wire.
Second, the power cord layout:
1, according to the current size, try to widen the wiring.
2. The direction of power cord and ground wire should be consistent with the direction of data transmission.
3. A 10~100μF decoupling capacitor should be connected to the power input of the printed circuit board.
3、 Decoupling capacitor configuration:
1. The lead of decoupling capacitor should not be too long, especially for high frequency bypass capacitor.
2. Power input terminal of printed circuit board jumper 10 ~ 100μIf the electrolytic capacitance of F is greater than 100μF is better.3. One 0.01 ~ 0.1 is bridged between VCC and GND of each integrated chipμF ceramic capacitor.If space is not allowed, 1-10 chips can be configured for every 4-10 chipsμF tantalum capacitor.4. In addition, the ability to resist the change of VCC, VCC and VCC, and the ability to resist the change of VCC, VCC, and low capacitance.5. 0.01 is set on the reset terminal of MCUμThe decoupling capacitance of F.
4、 Device configuration: 1. Clock input terminals of clock generator, crystal oscillator and CPU should be close to and away from other low-frequency devices.2. Small current circuit and high current circuit should be far away from logic circuit.3. The position and direction of the printed board in the case shall ensure that the device with high heat output is at the top.
V. Separate wiring of power line, AC line and signal line. The power line and AC line should be arranged on a board different from the signal line as far as possible, otherwise they should be routed separately from the signal line.
VI. Other principles: 1. When wiring, all address lines should be as long as possible and as short as possible.
2. Add 10K pull-up resistance to the bus, which is beneficial to anti-interference.
3. Lines on both sides of PCB board should be arranged vertically as far as possible to prevent mutual interference. 4. Generally, the size of decoupling capacitor is C=1/F, and f is the data transmission frequency. 5. The unused pins are connected to Vcc through a pull-up resistor (about 10K), or connected in parallel with the used pins. 6. Heated components (such as high-power resistors, etc.) should avoid those that are easily affected by temperature (such as electrolytic capacitors, etc.). 7. Compared with line decoding, full decoding has stronger anti-interference performance.
In order to curb the interference of high-power devices to some digital circuits of microcontrollers and the interference of digital circuits to analog circuits, high-frequency choke rings should be used when connecting digital ground and analog ground to common grounding points. It is a cylindrical ferrite magnetic material with several holes in the axial direction. A thicker copper wire passes through the holes and winds one or two times. This device can be regarded as zero impedance for low-frequency signals, and can be regarded as an inductor for interference with high-frequency signals .. (Because of its large DC resistance, the inductor cannot be used as a high-frequency choke coil).
When signal lines outside the printed circuit board are connected, shielded cables are usually used. For high-frequency signals and digital signals, both ends of the shielded cable are grounded, and one end of the shielded cable for low-frequency analog signals is better grounded. Circuits that are very sensitive to noise and interference, or circuits with particularly serious high-frequency noise, should be shielded with metal covers. Ferromagnetic shielding has no obvious effect on 500KHz high frequency noise, but thin copper skin has better shielding effect. When using screw bolts to fix the shield, attention should be paid to the corrosion caused by potential difference caused by contact of different materials.
Seven, make good use of decoupling capacitor
The decoupling capacitor between the integrated circuit power supply and the ground has two functions: on the one hand, it is the energy storage capacitor of the integrated circuit, and on the other hand, it bypasses the high-frequency noise of the device. A typical decoupling capacitor value in digital circuits is 0.1 μ F The typical value of the distributed inductance of this capacitor is 5 μ h. A decoupling capacitor of 0.1μF has a distributed inductance of 5μH, and its parallel resonance frequency is about 7MHz. That is to say, it has a good decoupling effect for noises below 10MHz, but hardly works for noises above 40MHz.
1μF, 10μF capacitor, parallel resonance frequency above 20MHz, the effect of removing high-frequency noise is better.
Every 10 or so integrated circuits should be equipped with a charging and discharging capacitor or an energy storage capacitor, which can be about 10 μ F. It’s better not to use electrolytic capacitor, which is rolled up by two thin films. This rolled-up structure shows inductance at high frequency. Use tantalum capacitors or polycarbonate capacitors.
The choice of decoupling capacitor is not strict, and it can be 0.1 μ f for 10MHz and 0.01μF for 100 MHz according to C = 1/f.
When welding, the pins of decoupling capacitor should be as short as possible, and long pins will make the decoupling capacitor self-resonate. For example, the frequency of self-resonance of 1000pF ceramic capacitor is about 35MHz when the pin length is 6.3mm, and 32MHz when the pin length is 12.6 mm.
VIII. Experience of reducing noise and electromagnetic interference. Anti-interference design principles of printed circuit boards: 1. You can reduce the jumping rate of the upper and lower edges of the control circuit by stringing resistors. 2. Try to keep the potential around the clock signal circuit close to 0, circle the clock area with a ground line, and keep the clock line as short as possible.
3. The interference of the clock line perpendicular to the I/O line is less than that parallel to the I/O line.
4. I/O drive circuit should be as close as possible to the edge of PCB. 5. The output of the unused gate circuit should not be suspended, the positive input of the unused operational amplifier should be grounded, and the negative input should be connected to the output. 6. Try to use the 45 fold line instead of the 90 fold line for wiring to reduce the external transmission and coupling of high-frequency signals. 7. The pins of components should be as short as possible. 8. Do not run the wires under the Shi Ying crystal oscillator and the components that are particularly sensitive to noise. 9. Don’t form a current loop around the ground wire of weak signal circuit and low frequency circuit.
10. When necessary, add ferrite high-frequency choke coil in the line to separate signal, noise, power supply and ground.
A via on the printed board causes a capacitance of about 0.6pF; The packaging material of an integrated circuit itself causes a distributed capacitance of 2pF~10pF; A connector on a circuit board with a distributed inductance of 520μH; A dual in-line 24-pin integrated circuit socket with a distributed inductance of 4 μ h ~ 18 μ h.