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PCB design layout experience summary

Posted:05:14 PM May 29, 2018 writer: G

1. When the high-frequency signal wiring should pay attention to what problems?

Answer: impedance matching of signal lines;

Space isolation from other signal lines;

For digital high-frequency signals, the differential line effect will be better;

2. In the layout, if the line is dense, the vias may be more, of course, will affect the board's electrical performance, how to improve the board's electrical performance?

Answer: For low-frequency signals, vias do not matter, and high-frequency signals minimize vias. If the line can be considered multi-layer board;

3. Isn't the more decoupling capacitors on the board?

A: The decoupling capacitor needs to add the appropriate value in the right place. For example, add in the power port of your analog device, and need to use different capacitance values to filter out spurious signals of different frequencies;

4. What is the standard of a good board?

Answer: The layout is reasonable, power line power redundancy is sufficient, high-frequency impedance impedance, and low-frequency traces are concise.

5. How much difference does the via hole and blind hole have for the signal? What is the principle of application?

A: The use of blind holes or buried holes is an effective way to increase the density of multilayers, reduce the number of layers, and reduce the size of the plated through holes. However, through-holes are better realized in terms of process and lower in cost, so through-holes are used in general designs.

6. When it comes to analog-digital hybrid systems, it was suggested that the electrical layer should be divided and the ground plane should be filled with copper, and it was also proposed that the electrical ground be divided. Different grounds should be connected at the power source terminals, but the return path of the signal would be Far away, how to choose the right method for specific application?

A: If you have high-frequency > 20MHz signal lines, and the length and number are more, then you need at least two layers to give this analog high-frequency signal. A signal line, a large area, and the signal line layer need to play enough vias to the ground. The purpose of this is:

1) For analog signals, this provides a complete transmission medium and impedance matching;

2) The ground plane isolates the analog signal from other digital signals;

3)The ground loop is small enough, because you hit a lot of holes, the ground is a large plane.

7. In the circuit board, the signal input plug is on the leftmost edge of the PCB, and the MCU is on the right side. Then in the layout, the regulated power supply chip is placed close to the connector (the power supply IC output 5V reaches the MCU after a relatively long path. ), or put the power IC to the center to the right (the power supply IC output 5V line to reach the MCU is relatively short, but the input power line through a relatively long period of PCB)? Or have a better layout?

A: First, is your so-called signal input plug-in analog device? If it is an analog device, it is recommended that your power layout should not affect the signal integrity of the analog section as much as possible. Therefore, there are several points to consider:

1) First of all your power supply chip is relatively clean, small ripple power supply. For the analog part of the power supply, the power requirements are relatively high.

2) Is the analog part and your MCU a power supply? In the design of high-precision circuits, it is recommended to supply the analog and digital power supplies.

3) The power supply to the digital part needs to consider minimizing the impact on the analog circuit part.

8. In the application of high-speed signal chain, there are analog grounds and digital grounds for multiple ASICs. Is it split or split? What are the existing criteria? Which effect is better?

A: So far, there is no conclusion. In general, you can consult the chip manual. All of ADI's manuals for hybrid chips recommend a grounding solution. Some are recommended and some are recommended isolation. It depends on the chip design.

9. What are the most important issues to be aware of when designing high-speed multi-layer PCBs? Can you elaborate on the solution to the problem.

A: The most important thing to note is the design of your layer, which is how you divide the signal line, power line, ground, and control line into each layer. The general principle is that at least a single layer must be guaranteed for analog and analog signals. The power supply is also recommended to use a separate layer.

10. When does the specific use of 2-layer board, 4-layer board, and 6-layer board technically have strict restrictions? (The reason for removing the volume) is based on the frequency of the CPU or the frequency of interaction with external device data.

A: The use of multi-layer boards can provide a complete ground plane first, and it can provide more signal layers to facilitate the wiring. For the CPU to control the application of external memory devices, the frequency of interaction should be taken into consideration. If the frequency is high, the complete ground plane must be guaranteed. In addition, the signal lines should preferably be of equal length.

11. How PCB layout influences analog signal transmission How to distinguish between noise introduced during signal transmission is caused by wiring or op amp devices.

A: This is difficult to distinguish. It is only possible to minimize extra wiring noise by PCB layout.

12. I recently learned the design of the PCB, high-speed multi-layer PCB, the power line, ground and signal line width is set to what is appropriate, what are the common settings, can you illustrate? For example, how to set the operating frequency at 300Mhz?

A: The 300MHz signal must do impedance simulation to calculate the linewidth and the distance between the line and the ground; the power line needs to determine the linewidth according to the size of the current line in the mixed signal PCB. Generally, instead of using the “line”, the entire plane is used. This ensures that the loop resistance is minimal and there is a complete plane below the signal line

13. What kind of layout can achieve the best cooling effect?

A: There are three main sources of heat in the PCB: (1) heating of electronic components; (2) heating of the PCB itself; (3) heat from other parts. Among these three heat sources, the maximum heat output of the components is the main source of heat, followed by the heat generated by the PCB, and the external incoming heat depends on the overall thermal design of the system and is not considered for the time being. The purpose of the thermal design is to take appropriate measures and methods to reduce the temperature of the components and the temperature of the PCB, so that the system can work properly at the right temperature. Mainly achieved by reducing heat generation and accelerating heat dissipation

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