PCB and circuit anti-interference measures (II)

3. 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 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 printed 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.