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To solve EMC, how to solve the ground first?
Posted:10:40 AM June 05, 2018 writer: G

In fact, the problem of segmentation should not be a problem for EMC design. It should not be the requirement of EMC design. However, I believe we must have read a lot of books about EMC design. It can be said that some EMC books talk about the importance of partitioning the analog, digital, and power supply parts in EMC design, and how to perform EMC design for partitioning and zoning of analog, digital, and power supplies. In a sense, it is actually a cart before the horse. It does not really understand the essence of grounding and EMC design.

In the case of connection to the output port, the probability of an EMI/ESD problem occurring at the time of initial design, such as splitting the analog audio ground to the system ground, is almost 100%. (This sentence is really very real)

In theory, ground partitioning violates the most basic principles of EMC design. But in some cases, the partition of different circuits and ground lines is necessary. Why is this?

First of all, we should first understand why the ground partition violates the most basic principles of EMC design.

The so-called "land" is mainly divided into two categories: safe ground and signal ground.

Safely means generally that the equipment is connected via a low-resistance path to the earth ground that we speak of. The metal enclosure in a Class I device is usually connected to a safe ground and then connected to ground through a power ground wire. Safely, as the name suggests, is usually connected with the accessible metal part of the device to provide security protection for the device to prevent electric shock accidents. In Class II equipment, all grounds can be considered as signal ground because it is not safe. For the signal ground, it is usually divided into the digital ground, analog ground, and power ground. When there is safety, it is usually short-circuited with the signal ground.

Please pay special attention to: EMC EMI noise and ESD discharge point charge are referenced to the earth (Earth)

In general, grounding is a design requirement to meet and guarantee the performance of the circuit. For example, in the audio circuit, in order to ensure the low noise of the audio signal to improve the fidelity, the analog signal is separated from the digital signal and grounded at a single point on the power supply terminal to prevent the switching signal from interfering with the analog circuit. For example, in some high-precision data acquisition circuits, if the analog signal collected is very weak and the analog ground is not separated from the digital ground, the digital switch noise will even submerge the weak analog signal and cannot distinguish the acquisition amplification. Therefore, ground division is the basic requirement for circuit anti-jamming design.

However, for EMC design, a very important design principle is to provide a low-resistance, continuous return path that ensures high-frequency signals mirror the return current. Otherwise, the high-frequency current can only find its own path around the return. This will increase the return current density, and secondly increase the loop area of the current. As a result, the EMI radiation intensity of the circuit will increase and the anti-interference performance of the circuit will decrease.

Therefore, the smaller the number of circuit boards, the less complete the ground layer, and the more difficult the design of EMC. Due to cost constraints, I have seen two-story version of the set-top box design. Although the component density is very high, it is not a big problem for simple layout wiring. However, for EMC design engineers, it will face great challenges.

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