The anti-interference design of the printed circuit board in the PCB design has a close relationship with the specific circuit. Here, only some common measures of the PCB anti-interference design are explained.
- 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 are consistent with the direction of data transmission, which helps to enhance the anti-noise capability.
- Principles of ground wire design
(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 thin line, the ground 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 ring circuit to improve the anti-noise capability.
- Untwisting capacitor configuration
One of the usual practices in PCB design is to configure appropriate decoupling 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 not enough gap in the printed board, a 1~10pF tantalum capacitor 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.
(5) When there are contactors, relays, buttons and other components in the PCB board. A large spark discharge is generated when operating them, and an RC circuit must be used to absorb the discharge current. Generally, R takes 1~2K, and C takes 2.2~47UF.
(6) The input impedance of CMOS is very high and it is susceptible to induction. Therefore, it is necessary to ground or connect the power supply to the unused terminal during use.
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