PCB Technical - ​The noise of the circuit board is reduced to a minimum

When we design the circuit board, the circuit principle design is very good, or even very good. However, various noises will appear during the debugging process, and the circuit board cannot achieve the expected purpose. Sometimes, even worse, it has to be re-designed. lay the board. So how can we reduce the noise of the circuit board? Let's analyze it.

A circuit board with good performance can be seen by our electronic engineers at a glance (provided that we know what function the circuit board does). This is the principle of separation of functional modules that we often say. A functional module is a collection of circuits that have some electronic components combined to complete a certain function. In actual design, we need to put these electronic components close to reduce the wiring length between the electronic components in order to increase the role of the circuit module. In fact, this is not difficult to understand. Our common development boards or mobile phones do this, especially mobile phones. If you disassemble the mobile phone, you will find that the separation between the various modules is obvious, and each module is Faraday The electric cage is shielded.

For the circuit board of a development board, it can be seen from the layout that the interface circuits are separated very clearly, and the wiring of SDRAM, DDR and SD card interface circuits will not cause mutual interference. By partitioning the system modules, it helps signal integrity, prevents high-frequency interference between system modules, and improves system stability.

Secondly, it should be noted that when there are analog and digital circuits on a circuit board, they need to be separated. If you have to buckle a hat, there will be a quiet zone. The so-called quiet zone is the area that physically separates analog circuits and digital circuits or various functional modules. In this way, you can prevent other modules from interfering with the module. In the mobile phone circuit board mentioned above, the quiet zone is obvious. Note that the quiet zone and the circuit board ground are not connected.

In actual circuit design, not every circuit board has enough space for us to make a quiet zone. Then, when the space is not allowed, how should we design it?

A. Use transformers or signal isolation components for design. We often use CMOS or transistors and other components to form a circuit separation that is the meaning.

B. The signal passes through the filter circuit before entering the module. This method is a common method to prevent ESD. Putting it here is also considering that this method can play a role in eliminating noise (ESD, high frequency and high voltage noise).

C. Use common mode inductors for signal protection. If you don't know the role of common mode inductance, we will find in the schematic diagram that there are only two coils, which have no effect. In fact, this is not the case, it plays an important role in the stability of the signal and the elimination of noise interference. This also reveals on the other hand that electronic engineers need long-term training to grow.

A method similar to the design of the quiet zone of the PCB circuit board is the trench protection technology. This technique is to remove the divided copper skin in the quiet zone to form a bare circuit board material. The concept of the bridge is also derived from this: the power, ground and signal traces that connect the various sub-zones together are called bridges. The trench protection technology has the ability to withstand the impact of peak voltage and classic discharge protection, which can reduce the noise of the circuit board to a certain extent. In the circuit board design, when the wiring that has nothing to do with the isolation area passes through the protective trench, RF loop current will be generated, which will further affect the performance of the circuit board. This requires attention.

Nowadays, many analog-to-digital or digital-to-analog PCB components have already connected the two parts of the ground together inside the component, typically ADC and DAC components. These components must be divided into a standard reference ground. If the digital signal is If the current cannot return to the source effectively, it will cause noise to generate EMI. When drawing the schematic, we found that the pins with AGND and DGND are a device with superior performance, which will reduce the difficulty of our design.

In general, the circuit is partitioned according to the module, and the obvious quiet zone is set between the partitions to minimize the influence of the power supply and the ground on the signal, and to minimize the noise of the circuit board.