1. What issues should be paid attention to when wiring high-frequency signals?
Impedance matching of signal lines; Spatial isolation from other signal lines; For digital high-frequency signals, differential lines will work better;
2. When laying out the board, if the lines are dense, there may be more via holes, which will of course affect the electrical performance of the board. How can I improve the electrical performance of the board?
For low-frequency signals, via holes do not matter, and for high-frequency signals, minimize via holes. If there are many lines, consider multi-layer boards;
3. Is it better to add more decoupling capacitors to the board?
Decoupling capacitors need to be added to the appropriate value at the appropriate location. For example, add it to the power supply port of your analog device, and you need to use different capacitor values to filter out spurious signals of different frequencies;
4. What are the criteria for a good board?
Reasonable layout, sufficient power line power redundancy, high-frequency impedance impedance, and simple low-frequency wiring.
5. How much difference do through holes and blind holes have on signals? What principles apply?
The use of blind holes or buried holes is an effective method to increase the density of multilayer boards, reduce the number of layers and board size, and greatly reduce the number of plated through holes. However, in comparison, through holes are easier to implement in terms of technology and have lower costs, so through holes are generally used in designs.
6. When it comes to analog and digital hybrid systems, some people suggest that the electrical layer be divided and the ground plane should be covered with copper. Others suggest that the electrical and ground layers be divided and different grounds be connected at the power source end points. However, this will affect the return path of the signal. Far away, how to choose the appropriate method for specific applications?
If you have high-frequency >20MHz signal lines, and the length and quantity are relatively large, then at least two layers are needed to simulate this high-frequency signal. There is one layer of signal lines and a large-area ground layer, and the signal line layer needs to have 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 analog signals from other digital signals; 3.The ground loop is small enough, because you have a lot of Hole, the ground is a large plane.
7. In the circuit board, the signal input plug-in is on the leftmost edge of the PCB, and the MCU is on the right. Then the voltage-stabilized power supply chip is placed close to the connector during layout (the power IC outputs 5V after a relatively long path to reach the MCU, or place the power IC to the middle right (the output 5V line of the power IC is relatively short when it reaches the MCU, but the input power line passes through a relatively long section of PCB board)? Or is there a better layout?
First of all, is your so-called signal input plug-in an analog device? If it is an analog device, it is recommended that your power supply layout should try not to affect the signal integrity of the analog part. Therefore, there are several points to consider: 1) First of all, whether your regulated power supply chip is relatively clean and has small ripples. For the power supply of the analog part, the power requirements are relatively high; 2) Whether the analog part and your MCU are the same power supply. In the design of high-precision circuits, it is recommended to separate the power supplies of the analog part and the digital part; 3) For the digital part The power supply needs to be considered to minimize the impact on the analog circuit part.
8. In the application of high-speed signal chain, there are analog ground and digital ground for many ASICs. Should the ground be divided or not divided? What are the existing guidelines? Which one works better?
So far, the jury is still out. Normally you can consult the chip's manual. The manuals for all ADI hybrid chips recommend a grounding scheme, some recommend a common ground, and some recommend an isolated ground. This depends on the chip design.
9. When should we consider the equal length of lines? If we want to consider using equal-length wires, what is the maximum difference in length between the two signal wires? How to calculate?
Differential line calculation idea: If you transmit a sinusoidal signal, the length difference is equal to half of its transmission wavelength, and the phase difference is 180 degrees. At this time, the two signals will completely cancel out. So the length difference at this time is the maximum value. By analogy, the signal line difference must be smaller than this value.
10. In what situation is snake-shaped wiring suitable for high speed? Are there any disadvantages? For example, for differential wiring, the two sets of signals are required to be orthogonal.
Snake wiring has different functions depending on the application:
1.If the serpentine trace appears in the computer board, it mainly plays the role of a filter inductor and impedance matching, improving the anti-interference ability of the circuit. The serpentine traces in computer motherboards are mainly used in some clock signals, such as PCI-Clk, AGPCIK, IDE, DIMM and other signal lines;
2.If used in ordinary PCB boards, in addition to serving as a filter inductor, It can also be used as an inductor coil for a radio antenna, etc. For example, it is used as an inductor in 2.4G walkie-talkies;
3.Some signal wiring lengths must be strictly equal. The equal length of high-speed digital PCB boards is to keep the delay difference of each signal within a range and ensure that the system operates in the same Validity of the data read within the cycle (when the delay difference exceeds one clock cycle, the data in the next cycle will be read incorrectly). For example, there are 13 HUBLinks in the INTELHUB architecture, using a frequency of 233MHz. They must be strictly equal in length to eliminate hidden dangers caused by time lag. Wiring is the only solution. It is generally required that the delay difference does not exceed 1/4 clock cycle. The line delay difference per unit length is also fixed. The delay is related to the line width, line length, copper thickness, and board structure. However, if the line is too long, the distributed capacitance and distributed inductance will increase. , causing the signal quality to decrease. Therefore, the clock IC pins are generally connected to " " termination, but the serpentine trace does not function as an inductor. On the contrary, the inductor will phase shift the higher harmonics in the rising edge of the signal, causing the signal quality to deteriorate, so The spacing of the serpentine lines is required to be at least twice the line width. The smaller the rise time of the signal, the more susceptible it is to the influence of distributed capacitance and distributed inductance.
4.The serpentine trace plays a distributed parameter in some special circuits The role of LC filter.
11. When designing PCB, how to consider electromagnetic compatibility EMC/EMI? What specific aspects need to be considered? What measures are taken?
A good EMI/EMC design must consider the location of the device, the arrangement of the PCB stack, the routing of important connections, the selection of devices, etc. at the beginning of the layout. For example, the location of the clock generator should be kept away from the external connector as much as possible. High-speed signals should be routed through the inner layer as much as possible and attention should be paid to the characteristic impedance matching and the continuity of the reference layer to reduce reflection. The slope of the signal pushed by the device should be as small as possible to reduce high-frequency components. Select When decoupling capacitors, pay attention to whether their frequency response meets the requirements to reduce power layer noise. In addition, pay attention to the return path of the high-frequency signal current to keep the loop area as small as possible to reduce radiation. You can also control the range of high-frequency noise by dividing the ground layer. Finally, appropriately select the grounding point of the PCB and the housing.
12. What should we pay attention to in the design of transmission lines of RF broadband circuit PCB? How to set the ground hole of the transmission line appropriately? Do I need to design the impedance matching myself or do I need to cooperate with the PCB processing manufacturer?
There are many factors to consider in this issue. For example, various parameters of PCB materials, the transmission line model finally established based on these parameters, device parameters, etc. Impedance matching is generally designed based on the information provided by the manufacturer.
13. When analog circuits and digital circuits coexist, for example, half is the digital circuit part of FPGA or microcontroller, and the other half is the analog circuit part of DAC and related amplifiers. There are many power supplies with various voltage values. When encountering power supplies with voltage values that are used by both digital and analog circuits, is it possible to use a common power supply? What are the techniques for wiring and magnetic bead arrangement?
It is not recommended to use it this way. This will be more complicated to use and difficult to debug.
14. When designing high-speed multi-layer PCB, what is the main basis for selecting the packaging of resistors, capacitors and other devices?
0402 is commonly used in mobile phones; 0603 is commonly used in general high-speed signal modules; the basis is that the smaller the package, the smaller the parasitic parameters. Of course, the same package from different manufacturers has great differences in high-frequency performance. It is recommended that you use high-frequency dedicated components in key locations.
15. In the design of double-sided panels, should the signal wire or the ground wire be routed first?
With layout first considered, consider routing
16. When designing high-speed multi-layer PCB, what are the issues that should be paid attention to most?
The most important thing to pay attention to is the design of your layers, that is, how you divide signal lines, power lines, ground, and control lines into each layer. The general principle is that analog signals and analog signal grounds must be at least on a separate layer. It is also recommended to use a separate layer for the power supply.
17. Are there any strict technical restrictions on when to use 2-layer boards, 4-layer boards, and 6-layer boards? Is it based on the frequency of the CPU or the frequency of data interaction with external devices?
The use of multi-layer boards can first provide a complete ground plane, and can also provide more signal layers to facilitate wiring. For applications where the CPU wants to control external storage devices, the frequency of interaction should be considered. If the frequency is higher, a complete ground plane must be guaranteed. In addition, it is best to keep the signal lines of the same length.
18. How to analyze the impact of PCB wiring on analog signal transmission, and how to distinguish whether the noise introduced during the signal transmission process is caused by wiring or op amp devices.
This is difficult to distinguish, and additional noise introduced by the wiring can only be minimized through PCB wiring.
19. For high-speed multi-layer PCB, what are the appropriate line width settings for power lines, ground lines and signal lines? What are the commonly used settings? Can you give examples? For example, how to set it when the operating frequency is 300Mhz?
For 300MHz signals, impedance simulation must be done to calculate the line width and the distance between the line and the ground; the power line needs to determine the line width according to the size of the current. When the ground is used in a mixed-signal PCB, the "line" is generally not used, but the entire plane is used, so that Ensure that the loop resistance is minimal and there is a complete plane under the signal line.
20. What kind of layout can achieve the best heat dissipation effect?
There are three main sources of heat in PCB: (1) The heat generated by electronic components; (2) The heat generated by PCB itself; (3) The heat transferred from other parts. Among these three heat sources, the components generate the largest amount of heat and are the main heat source, followed by the heat generated by the PCB board. The heat transferred from the outside depends on the overall thermal design of the system and will not be considered for the time being. Then the purpose of thermal design is to take appropriate measures and methods to reduce the temperature of components and the PCB board so that the system can operate normally at a suitable temperature. This is mainly achieved by reducing heat generation and speeding up heat dissipation.
21. What is the relationship between the line width and the size of the matching via hole?
This is a good question. It is difficult to say that there is a simple proportional relationship because the simulations of the two are different. One is surface transmission and the other is ring transmission. You can find a via-hole impedance calculation software online, and then just keep the via-hole impedance consistent with the transmission line impedance.
22. In an ordinary PCB circuit board controlled by an MCU, but the requirements for high current and high-speed signals are not very high, it would be better to lay a layer of ground wire around the outermost edge of the PCB to wrap the entire circuit board. good?
Generally speaking, just paving a complete floor is enough.
23 The AD conversion chip needs to be connected to a single point of analog ground and digital ground. But what if there are multiple AD conversion chips on the board? In a multi-layer circuit board, when a multi-channel switch switches analog sampling, is it necessary to separate the analog part and the digital part like an AD conversion chip?
Put several ADCs together as much as possible, and connect the analog and digital ground at a single point below the ADC; in a multi-layer circuit board, it depends on the switching speed of the MUX and ADC. Generally, the speed of the ADC will be higher than the MUX, so it is recommended to place it below the ADC. Of course, to be on the safe side, you can also put a magnetic bead package under the MUX.
24. In conventional network circuit design, some use the method of connecting several grounds together. Is this used? Why?
Not very clear about your problem. With a hybrid system there will certainly be several types of grounds, which will ultimately be connected together at one point for the purpose of equalizing potential. Everyone needs a common ground level as a reference.
25. How to effectively deal with the analog part and digital part, analog ground and digital ground in PCB？
Analog circuits and digital circuits should be placed in separate areas, so that the return flow of the analog circuit is in the analog circuit area and the digital circuit is in the digital area, so that the digital will not affect the analog. The starting point of analog ground and digital ground processing is similar, and the return flow of digital signals cannot be allowed to flow to the analog ground.
26. What are the differences in the design of ground wires when designing PCB boards for analog circuits and digital circuits? What issues need to be paid attention to?
The main requirements for analog circuits to ground are completeness, small loops, and impedance matching. If the digital signal has low frequency, there are no special requirements; if the speed is high, impedance matching and ground integrity also need to be considered.
27. There are generally two decoupling capacitors, 0.1 and 10. If the area is tight, how to place the two capacitors? Which one is better to place on the back?
Designed according to specific applications and chips.
28. In radio frequency circuits, two signals, IQ and IQ, often appear. Do the lengths of these two lines need to be the same?
Try to use the same ones in RF circuits.
29. Is the design of high-frequency signal circuits different from the design of ordinary circuits? Can you give a brief explanation using wiring design as an example?
High-frequency circuit design must consider the influence of many parameters. Under high-frequency signals, many parameters that can be ignored in ordinary circuits cannot be ignored, so the transmission line effect may need to be considered.
30. For high-speed PCB, how to avoid via holes during wiring? Do you have any good suggestions?
It is best to drill fewer vias on high-speed PCBs and solve the need for more vias by adding signal layers.
31. How to choose the thickness of power traces in PCB board design? Are there any rules?
0.15 × line width (mm)=A, copper thickness also needs to be considered
32. When digital circuits and analog circuits are on the same multi-layer board, should the analog ground and digital ground be arranged on different layers?
This is not required, but the analog and digital circuits should be kept separate.
33. In general, how many vias are suitable for digital signal transmission? (Signals below 120Mhz)
It's best to have no more than two vias.
34. In a circuit with both analog and digital circuits, how to avoid mutual interference problems during PCB board design?
If the analog circuit is properly matched and the radiation is very small, it is usually interfered with. Interference sources come from devices, power supplies, space and PCB; digital circuits must be sources of interference because they have many frequency components. The solution is generally reasonable device layout, power supply decoupling, and PCB layering. If the interference characteristics are large or the analog part is very sensitive, you can consider using a shielding cover.
35. For high-speed circuit boards, parasitic parameters may exist everywhere. Faced with these parasitic parameters, should we accurately measure various parameters and then eliminate them, or should we use empirical methods to solve them? How should we balance this issue of efficiency and performance?
Analyze the impact of parasitic parameters on circuit performance. If the impact cannot be ignored, it must be addressed and eliminated.
36. What should we pay attention to when laying out multi-layer boards?
When laying out a multilayer board, because the power supply and ground layers are on the inner layer, be careful not to have a floating ground plane or power plane. In addition, make sure that the via holes drilled to the ground are indeed connected to the ground plane. Finally, some important signals must be provided. Add some test points to facilitate measurement during debugging.
37. How to avoid crosstalk of high-speed signals?
You can keep the signal lines farther away, avoid parallel lines, and provide shielding by laying the ground or adding protection.
38. Power planes are often used in multi-layer board design, but is it necessary to design power planes in double-layer boards?
It’s difficult, because your various signal lines are already almost the same in a double-layer layout.
39. Does the thickness of the PCB board have any impact on the circuit? How is it generally selected?
Thickness is more important when doing impedance matching. The PCB manufacturer will ask what the board thickness is when the impedance matching is calculated, and the PCB manufacturer will make it according to your requirements.
40. The ground plane can minimize the signal loop, but it will also produce parasitic capacitance with the signal line. What should be the choice?
It depends on whether the parasitic capacitance has a non-negligible impact on the signal. If it cannot be ignored, then reconsider.
41. Should the LDO output be used as a digital power supply or an analog power supply? Which one is better to connect the digital or analog power supply first?
If you want to use an LDO to provide power for digital and analog, it is recommended to connect the analog power supply first. After the analog power supply is filtered by LC, it becomes the digital power supply.
42. Should I use magnetic beads between analog Vcc and digital Vcc, or should I use magnetic beads between analog ground and digital ground?
The analog VCC is filtered by LC to obtain the digital VCC, and magnetic beads are used between the analog ground and the digital ground.
43. How to route differential signal lines such as LVDS?
All wiring, including surrounding device placement and ground planes, need to be symmetrical.
44. A good PCB design needs to emit as little electromagnetic radiation as possible, and also prevent external electromagnetic radiation from interfering with itself. What measures need to be taken to prevent external electromagnetic interference in the circuit?
The best way is to shield to prevent external interference from entering. On the circuit, for example, when there is an INA, an RFI filter needs to be added before the INA to filter out RF interference.
45. Using fast integrated circuit chip circuits with high clock frequency, how to solve the problem of transmission line effect in PCB board design?
What is this fast integrated circuit chip? If it is a digital chip, generally there is no need to consider it. If it is an analog chip, it depends on whether the transmission line effect is large enough to affect the performance of the chip.
46. In a multi-layer PCB design, is copper pouring still required? If copper is poured, which layer should it be connected to?
If there are complete ground planes and power planes inside, the top and bottom layers can be copper-free.
47. When designing high-speed multi-layer PCB, how is impedance simulation generally performed and what software is used? Are there any issues that require special attention?
Multisim software is used to simulate the resistor-capacitor effect.
48. Some devices have thinner pins, but the PCB traces are thicker. Will it cause impedance mismatch after connection? If so, how to solve it?
There are different analyzes for specific devices, and the impedance of the device is generally given in the data sheet, and generally has little to do with the pin thickness.
49. Differential lines generally need to be of equal length. If it is difficult to implement in LAYOUT, are there any other remedies?
The problem of equal lengths can be solved by running serpentine lines. Nowadays, most PCB software can automatically run equal length lines, which is very convenient.
50. When using a multimeter to measure the analog ground and digital ground interfaces of the chip, they are connected. In this way, isn’t the analog and digital ground connected at multiple points?
The ground pins inside the chip are all connected together. But connections still need to be made on the PCB board. The ideal single-point grounding should be to understand the connection point location of the analog and digital parts inside the chip, and then design the single-point connection location on the PCB board at the analog and digital dividing point of the chip.
51. Due to the limitation of the size of the board, my circuit board uses two-sided chip welding. There are many via holes on the board and the signal lines are also nearby. Will this wiring cause interference to the signal?
If it is a low-speed digital signal, it should not be a big problem. Otherwise it will definitely affect the quality of the signal.
52. When considering whether to perform impedance matching on digital lines, it depends on whether the total time from when the signal is transmitted to when it is reflected back exceeds 20% of the rising edge. If it exceeds, impedance matching is required. Does the analog line require impedance matching? How to consider it?
Low-frequency analog signals do not need to be matched, and radio-frequency analog signals must also consider matching issues.
53. Regarding the complete ground plane, on a board using AD/DA chips, if the number of layers is relatively large, a complete analog ground and a complete digital ground can be provided; it can also be divided into two separate ground planes. Analog ground, digital ground. Which one is better?
Under normal circumstances, a complete ground plane will be paved. Unless there are some special circumstances, such as the analog and digital parts of the board being clearly separated, they can be easily distinguished.
54. When using magnetic beads or MECCA to connect digital and analog grounds, its frequency characteristics are used to prevent the high-frequency components of the digital ground from affecting the analog ground, while ensuring that the levels of the two are equal. So, what is the function of connecting the 0 ohm resistor to the digital and analog ground? Sometimes only a small piece of copper is used to connect it. Can you analyze it?
The equivalent circuit of the magnetic bead is equivalent to a band-stop limiter, which only has a significant suppression effect on the noise at a certain frequency point. When using it, you need to estimate the noise frequency in advance to select an appropriate model. For situations where the frequency is uncertain or unpredictable, magnetic beads are not suitable. A 0 ohm resistor is equivalent to a very narrow current path, which can effectively limit the loop current and suppress noise. Resistors have an attenuation effect in all frequency bands (0 ohm resistors also have impedance), which is stronger than magnetic beads. Copper is similar to a 0ohm resistor.
55. How to avoid noise introduced during wiring?
The digital ground and analog ground must be grounded at a single point, otherwise the digital ground return flow will flow through the analog ground and cause interference to the analog circuit.
56. How does PCB prevent the interference caused by mutation signals such as PWM to analog signals (such as op amps), and how to test the size of this interference (radiation interference or conduction interference)? In addition to paying attention to layout and wiring, are there any other methods? Means of suppression (removal of shielding?
We need to start with several interfaces of the op amp. The input end must prevent spatial coupling interference and PCB crosstalk (layout improvement); the power supply requires decoupling capacitors of different values. For testing, you can use an oscilloscope probe to test the above-mentioned positions to determine where the interference comes from. If the PWM signal is converted into a DC control voltage through low-pass filtering, you can consider filtering, or connect a small capacitor in parallel to ground to round the PWM waveform and reduce high-frequency components.
57. In a circuit board, an ARM or FPGA is often connected to a lot of RAM, FLAH and other devices. What should I pay attention to when connecting the main chips to these memories? Is there any limit on the number of vias? What is the commonly used via hole size in digital signals? Does the size of the via hole have a big impact on the signal?
If the speed is greater than 100MHz, it is best not to have more than two vias on one signal line, and the vias should not be too small. Generally, an aperture of 10 mils is enough.
58. When laying out a double-sided board, is it better to have as few vias as possible when connecting the top ground to the bottom ground? So what is the most reasonable way to release the via holes?
Fewer vias are for signal lines. If they are ground vias, more appropriately will reduce ground loops and impedance. The principle of putting is to put in the device.
59. What should you pay attention to when LVDS signal wiring? How to route the wiring?
Parallel and equal length
60. Is the parallel wiring of data lines intended to interfere with each other?
When running parallel lines, pay attention to the spacing between lines to prevent crosstalk.
