PCB News - Talking about PCB and Circuit Anti-interference Measures

The anti-interference design of the printed circuit board is closely related to the specific circuit. Here are only a few items of the PCB anti-interference design
Some common measures are explained.

1. Power cord design
According to the current of the printed circuit board, try to increase the width of the power line to reduce the loop resistance. At the same time, make the power cord
, The direction of the ground wire is consistent with the direction of data transmission, which helps to enhance the anti-noise ability.
2. Ground wire design
The principle of ground wire design is:
1. The digital ground is separated from the analog ground. If there are both logic circuits and linear circuits on the PCB circuit board, they should be separated as much as possible
. The ground of the low-frequency circuit should be grounded in parallel at a single point as much as possible. When the actual wiring is difficult, it can be partially connected in series and then grounded in parallel.
The high-frequency PCB circuit should be grounded at multiple points in series, the ground wire should be short and leased, and the grid-like large-area ground foil should be used around the high-frequency component as much as possible.

2. The grounding wire should be as thick as possible. If the ground wire uses a very thin line, the ground potential changes with the change of the current, so that
The anti-noise performance is reduced. 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 above 2~3mm.
3. The grounding wire forms a closed loop. For printed boards composed only of digital circuits, the grounding
High anti-noise ability.
3. Decoupling capacitor configuration
One of the conventional methods of PCB design is to configure appropriate decoupling capacitors on each key part of the printed circuit board.
The general configuration principle of the decoupling capacitor is:
1. Connect a 10 ~ 100uf electrolytic capacitor across the power input. If possible, it is better to connect to 100uF or more.
2. In principle, each integrated circuit chip should be equipped with a 0.01pF ceramic capacitor, if the gap of the printed board is not enough,
A capacitor of 1~10pF can be arranged every 4~8 chips.
3. For devices with weak anti-noise ability and large power changes when shutting down, such as RAM and ROM storage devices, they should be installed in the chip's
Connect the decoupling capacitor directly between the power line and the ground line.
4. Capacitor leads should not be too long, especially for high frequency bypass capacitors. In addition, the following two points should be noted:

A. When there are contactors, relays, buttons and other components in the printed board. Large spark discharges will be generated when operating them
, The RC circuit shown in the figure must be used to absorb the discharge current. Generally, R is 1 ~ 2K, and C is 2.2 ~ 47UF.

B. The input impedance of CMOS is very high and it is susceptible to induction, so the unused terminal should be grounded or connected to a positive power supply when in use.

7. Useful suggestions for using logic circuits: do not use high-speed logic circuits that can be used; add decoupling between the power supply and the ground
Capacitance; pay attention to waveform distortion in long-line transmission; use R-S trigger as a buffer for cooperation between buttons and electronic circuits
Good things to share! Criticism, testimony, and suggestions are welcome!