PCB and circuit anti-interference measures (II)

3. Demodulation of capacitance:

One of the common practices of PCB design is to configure proper uncoupling capacitance in each key part of the PCB. The general configuration principle of uncoupling capacitance is as follows:

(1) the power input terminal is bonded to the electrolytic capacitor of 10-100uf. If possible, connect above 100uF.

(2) in principle, each IC chip should be equipped with a 0.01pF porcelain chip capacitor. If there is insufficient space on the printing plate, one 1-10pf tantalum capacitor can be arranged for each 4-8 chips.

(3) for devices with weak anti-noise capability and large power changes when switching off, such as RAM and ROM memory devices, the unloaded capacitance should be directly connected between the power cord and ground wire of the chip.

(4) the capacitance leads should not be too long, especially the high-frequency by-pass capacitors should not have leads.

(5) when there are contactor, relay, button and other elements in the board. When they are operated, large spark discharge will occur, and RC circuit must be used to absorb discharge current. Generally, R is 1-2k and C is 2.2-47uf.

(6) the input impedance of CMOS is very high and easy to be induced, so it needs to be grounded or connected with positive power when using.