In the design of electronic circuit systems, EMC is about to become "metaphysics" for many novices. The main reason is that it is simple to say but difficult to handle. To put it simply, it is internal and external interference, and it is often difficult to deal with it because it is a mess to find the problem. Whether it is an ordinary circuit system or a high-speed circuit system, it is necessary for us to deal with EMC, so today I will share an article to learn a few key points. In fact, following these principles can greatly reduce the probability of EMC problems.
1. Power system processing:
A principle: To have the concept of active and impedance, the source needs to reach the IC to the maximum and then return from the ground as soon as possible.
This is the design division of the PDN system.
The key point of the design of switching power supply lies in the processing of the switching current loop, the loop should be small!
For copper coating: avoid the RF part from interfering with the digital GND noise digital circuit through coupling, and pay attention to the problem of coupling interference in the radiated area.
Avoid forming a large source loop and bridge the Plane layer.
Pay attention to leaving gaps at the edges of the copper-clad plane layer to reduce the influence of edge effects.
Place decoupling capacitors nearby to reduce parasitic inductance and avoid decoupling failure.
Note that the difference between the AC path and the DC path is coupling.
Regarding the grounding treatment of the signal ground and the support, the purpose of grounding is to provide an interference discharge path, and the purpose of the loop opening is to avoid the formation of a large loop antenna and cause EMI interference.
2. Signal considerations
The frequency spectrum of the high-speed signal is as shown in the figure above. Our goal is to transmit all the energy to the receiving end.
The impedance of the transmission line is equivalent:
The model of the microstrip line:
Coupling model of differential signal:
Impedance of transmission line:
Several termination matches:
Three, layout
Connection:
The principle of wiring is to reduce process interference in the shortest distance and minimum impedance.
The reason for the formation of the eye pattern is the superposition effect of a series of digital signals, which reflects the quality of the transmission signal.
Simple 3W principle.
Special attention to the wiring of the crystal oscillator:
Fourth, the ESD problem
ESD problems are very common and are an important cause of chip damage, especially in the production process, TVS is the best protection device.