PCB experience 2

50 [Q] When using the multimeter to measure the analog ground of the chip and the digital ground interface is conductive, so that the analog digital digital is not a multi-point connection? The ground pins inside the chip are connected together. However, a connection is still required on the PCB. The ideal single-point grounding should be to understand the connection point location of the analog and digital parts of the chip, and then design the single-point connection location on the PCB to the analog and digital demarcation points of the chip.

51[Q] Due to the size limitation of the board, my circuit board uses two-sided patch soldering chip. A lot of vias are left on the board, and the signal line is also in the vicinity. Does the line interfere with the signal? A If it is a low-speed digital signal, it should be a small problem. Otherwise it will definitely affect the quality of the signal.

52 [Q] When considering whether to do impedance matching, the digital line is to see if the total time exceeds 20% of the rising edge when the signal is transmitted to the reflection. If it exceeds, impedance matching is required. Does the analog line need impedance matching? How to think about it? Answering the low frequency analog signal does not need to be matched. Of course, the analog signal of the radio frequency should also consider the matching problem.

53 [Q] Regarding the complete ground plane, on the board using the AD/DA chip, if the number of layers is relatively large, a complete analog ground and a complete digital ground can be provided; or both ground planes can be provided. Divide the analog ground and the digital ground separately. What are the advantages of both? In general, the ground level will be paved. Unless it is a special case, such as the analog part of the board and the digital part are clearly separated, it can be easily distinguished.

54 [Q] When using magnetic beads or MECCA to connect digital and analog grounds, the frequency characteristics are used so that the high-frequency components in the digital ground do not affect the analog ground, and the two levels are equal. Then, 0ohm resistors connect digital, analog ground, and sometimes only use a small piece of copper connection, can you analyze it? The equivalent circuit of the magnetic bead is equivalent to the band-stopper, which only significantly suppresses the noise of a certain frequency. When using it, it is necessary to estimate the noise frequency in advance so that the appropriate model can be selected. For frequencies that are uncertain or unpredictable, the beads do not match. The 0 ohm resistor is equivalent to a very narrow current path, which effectively limits the loop current and suppresses noise. The resistor has an attenuation in all frequency bands (0 ohm resistor also has impedance), which is stronger than the magnetic beads. The copper skin is similar to a 0 ohm resistor.

55[Q] How to avoid the noise introduced during wiring? Answer The digital ground and the analog ground should be grounded at a single point, otherwise the digital ground return will cause interference to the analog circuit through the analog ground.

56[Q] How does the PCB prevent the interference of the abrupt signal such as PWM on the analog signal (such as the op amp), and how to test the interference (radiation interference or conducted interference)? In addition to the layout and wiring, it is necessary to pay attention to whether there are other Method to suppress (except for shielding? Answer: Start with several interfaces of the op amp, the input should prevent spatial coupling interference and PCB crosstalk (layout improvement); the power supply needs different capacitance decoupling capacitors. The test can be done with an oscilloscope The probe tests the position mentioned above to determine where the interference comes from. If the PWM signal is converted to DC control voltage by low-pass filtering, you can consider filtering, or connect a small capacitor to the ground to change the waveform of the PWM. Circle, reducing high frequency components

57[Q] Excuse me, in the circuit board, an ARM or FPGA often connects a lot of RAM, FLAH devices, what are the connections between these main chips and these memories, what is the number of vias? Is it restricted? What is the size of the via hole commonly used in digital signals? Does the size of the via aperture have a large effect on the signal? Answer If the speed is greater than 100MHz, it is better not to exceed more than two vias on one signal line. The vias should not be too small. Generally, the aperture of 10 mils can be used.

58 [Q] When the double panel of the cloth (high frequency is), the fewer the vias when the top layer is connected to the bottom layer, the better? So how to put a hole is more reasonable? Answering the hole is for the signal line. If it is the ground via, more appropriate will reduce the ground loop and impedance. The principle of putting it in is to enter the device.

59 [Q] What should I pay attention to LVDS signal wiring? How to route? A parallel isometric

60[Q] Is the data line parallel wiring for mutual interference? Answer and walk the line to pay attention to the line-to-line spacing to prevent crosstalk.

61 [Q] In a 4-layer board, there is an entire acquisition system with analog amplification, digital acquisition, and MCU. After the cloth is laid, how to measure the input impedance of the system, how to match the input impedance of the system and the sensor, how to match, there is no relevant design principle. A don't know how high your analog signal is. If it is not high, you don't need impedance matching. Impedance matching can use some simulation software to calculate the impedance of the PCB. For example APPCAD. The impedance of the device can be queried by the manual.

62[Q] I often see a lot of holes on the PCB. The more holes, the better? Is there any rules? A: No. To minimize the use of vias, consider reducing the effects of vias on the circuit when you have to use vias.

63 [Q] There is inevitably a cross-plane phenomenon when wiring a multi-layer board. Our current cooking is to give priority to the differential line without crossing the plane when cutting the plane. But once I thought the teacher’s statement was that the single-ended could not cross, and the difference was not so strict. Ask the teacher about this. Answer that the single-ended and differential signals have to flow back after crossing the ground plane. If the reflow returns around a large circle, it will induce more interference. If the noise on the difference line is the same, it will cancel each other, so there is Certainly reasonable.

64 [Q] How to distinguish between digital ground and analog ground in high-speed multilayer PCB design? Is it connected as described in the device data sheet? A high-speed design does not need to be divided into digital ground and analog ground.

65 [Q] How to consider the fuse current of the PCB trace? How much is the fuse when the PCB traces, what factors are related? Answer 0.15× line width (mm)=A, then the maximum current. It is not possible to use the blown current to make a budget when designing. This is the cross-sectional area of the copper wire.

66[Q] Please ask what protection is needed in the signal input and output interface and power input interface? When the power supply is 220V input to DC, what protective measures are needed in actual application? Answer TVS tube, fuses are necessary on the power supply. If the signal is used, it is necessary to add a TVS tube and a diode to protect the analog circuit from the presence of a large voltage.

67[Q] See the 45-degree angle and the arc when the PCB board is bent. What are the advantages and disadvantages? How to choose? A. From the perspective of impedance matching, both lines can be made to match the corners. However, rounded corners may not be well processed.

68[Q] If the size is limited in the high-frequency trace, what are the most commonly used routing methods or reasonable routing methods? For example, snake-shaped lines, can you? Bad answer, will introduce more parasitic parameters

69[Q] I ask the key input model when using the instrumentation amplifier. I still need copper over the device layer. I have already covered the copper on the bottom layer of the device. There is also the feedback resistance of the instrumentation amplifier. I use the in-line, the lead wire is long, and the temperature drift and accuracy of the patch are not up to the requirements. How to deal with it. A general method of placing the chip data will have a recommended layout method and diagram, you can refer to. It is necessary to ensure that the leads are short and thick. It is better to choose the low-precision resistor of the chip or the high-precision resistor. It depends on the result of the specific debugging.

70[Q] The PCB software can be automatically routed, but is the position layout of the device to be manually placed? A best place and route are done manually.

71[Q] When doing PCB board making, is there any special regulation for PCB material selection or how to choose materials? I am currently working on a high-frequency signal board. What is the best choice for a PCB board? A. The high-frequency circuit board substrate currently used is a fluorine-fluoride dielectric substrate, such as polytetrafluoroethylene (PTFE), which is usually called Teflon, and is usually applied at 5 GHz or higher. When you are doing the board, you can explain it with the PCB manufacturer.

72[Q] I am a beginner in PCB design. I want to know what is the selection rule for the coupling capacitor? How is the size of the value calculated? A general situation, for the power generation part, to use 10u and 0.1u capacitor decoupling, we must consider both high frequency and low frequency decoupling; for other originals are generally 0.1u capacitor decoupling in the power supply part.

73[Q] What is the attenuation of a 5khz pulse signal after walking 20cm long and 10mil wide on the board? A. Different materials have different parasitic parameters of the PCB, which can be calculated according to the parasitic parameters you use.

74[Q] What are the requirements for the distance between the microstrip line and the ground plane in the high frequency? For example, greater than 1mm. Still not much demand, as long as it is almost enough? Still have to calculate by coplanar waveguide? Answer must be calculated using the impedance simulation of the coplanar waveguide or microstrip line.

75 [Q] How to route to minimize the crosstalk of high frequency signals between lines? Answering high frequency signals will reduce reflections and also reduce radiation.

76[Q] I would like to ask DC-DCConvertIC, need to connect to the ground plane under the IC, connect to the ground plane through Via, what is the number of Via holes and how much influence? . The answer can generally be designed according to the reference design. Due to the large current, a certain number of Vias may be required.

77[Q] When the impedance is matched, if the impedance value given by the pin is complex, that is, both the impedance part and the reactance part, what is the impedance matching? Does light take into account the resistance part? A. Consider conjugate matching and offset the imaginary part of the impedance.

78[Q] Is it better to concentrate parameters and distribution parameters in high frequencies? How to choose these two methods is more appropriate? Thank you! The answer distribution method has higher precision, but it is more complicated; the lumping method is relatively simplified, but there is a certain error.

79[Q] What are the requirements for the distribution of the vias connecting the upper and lower copper plates? Generally speaking, in order to improve connectivity, there should be no requirements for each.

80[Q] How to balance the parasitic inductance and parasitic capacitance at the input of the amplifier in IF applications? A. Generally speaking, parasitic inductance and capacitance have little effect on the IF circuit and can be ignored. Just make sure you don't introduce large parasitic capacitance and inductance values.

81[Q] How can I effectively reduce the interference effects between circuit components and how the amplifier is laid out to minimize the introduction of ripple? The principle of reducing interference is: 1. Reduce the radiation end; 2. Strengthen the isolation, shielding and decoupling of the interference; The principle of ripple reduction is also, 1. Reduce the ripple output of the switching power supply; 2. Sufficient decoupling filter ;

82 [Q] When the 6-layer design, the layer distribution skills, those lines have to go to the middle layer? Answer your design. The principle is to ensure that the analog signal line and the analog ground have two separate layers.

83 [Q] When the analog ground is connected to the digital ground, is the method used to connect a suitable magnetic bead to the analog ground in the digital ground? How to choose this magnetic bead? Thank you! The magnetic beads mainly play the role of isolating high-frequency noise. Different bead filter frequencies are different, so it is necessary to select the appropriate device according to the noise on the board.

84[Q] What should I pay attention to for signal layout above 5G? Answers should consider the transmission line effect, consider parasitic effects, and EMI.

85 [Q] When there is a high-speed logic device in the circuit, what is the maximum wiring length? Answer wiring is not afraid of long, afraid of asymmetry or a relatively large difference, so it is easy to cause wrong logic due to delay

86[Q] In a high-speed digital circuit board, there are multiple power supplies with different voltage values. When laying the power plane, should you use multi-layer power planes or separate layouts on the same power plane? A can be multiple voltages on one plane, pay attention to the isolation. It is also possible to separate the most important power supply by one layer, so that it is not interfered by other power sources.

87[Q] When the difference line is taken, due to space constraints, it cannot be completely equidistant. Is it equidistant or equal? Answer length can guarantee impedance matching, but unequal distance actually affects differential matching and requires simulation test.

88[Q] How to reduce electromagnetic interference in PCB layout? Which other modules should be closer to the main control chip? Thank you! Answer For the main controller, the main signal is transmitted, so the analog and power parts should be away from the controller; for reducing electromagnetic interference, attention should be paid to matching, decoupling, place and route, layering, etc. It is recommended to refer to some information.

89 [Q] When considering signal integrity, if you only know that the frequency of the digital chip is 1 GHz, it is generally estimated that his rise time is 1/10 of the period, that is, 0.1 ns. What is the basis? A This is a general principle and the speed of the edge depends on the speed of the device's output. If it is too slow, it will affect the judgment. Faster chip technology can't be achieved.

90[Q] Hello, may I ask the ARM chip to improve the anti-interference of the power supply, in addition to the TVS tube at the input of the power supply, the input pin of the power input terminal should be connected to the inductor better, or the magnetic beads are better, the magnetic beads are generally used. .

91[Q] Hello, can pcb board be simulated online, that is, how to verify whether there is any problem with the board, thank you? A. Some PCB software can do some trace inspection and integrity analysis, such as CADENCE

92 [Q] Some people in the pcb wiring when the signal input and output end string of a resistor to terminate, this role is large? How to choose this resistor? Which places need to do this? Thank you! Answer this depends on the role of series resistors, some of which are used to limit current, and some may be impedance matching.

93[Q] Is there any good suppression scheme for high-speed pulse trains that affect the power supply or a comparison system? Answer your so-called high-speed pulse train, nothing more than interference signals of different frequencies, using different values of capacitance decoupling.

94 [Q] What are the special requirements for high-speed PCB for sheet metal? A high frequency circuit has requirements for PCB materials. Consider the transmission line effect at high frequencies

95[Q] About the impedance matching of the signal line, please introduce and practice? In the case of low frequency, it is necessary to consider the relationship between the width of the signal line and the carrying capacity of the current. At high frequencies, it is necessary to consider the problem of matching equal length.

96[Q] What are the anti-interference measures for high-frequency signal lines? What should you pay attention to when wiring? Answering this question is quite broad and it is difficult to make it clear in one or two sentences. There is a lot of relevant information to refer to.

97[Q] Why do high-speed signals do not need to be divided into numbers and analog grounds? A: Because the driver can adjust the output phase difference, it is very difficult to adjust the PCB layout. The receiver directly inputs it and cannot adjust it.

98[Q] Regarding the equal length compensation of differential lines, why do you directly recommend compensation at the driver end? Could you explain that? EricBogatin's book only gives conclusions, but no explanation. A. Some chips have an adjustment function on the driver side. The PCB line design is not easy to change. The direct input of the receiving end generally has no delay adjustment function.

99[Q] When the high-frequency board material is used, is the dielectric constant as small as possible? Thank you! A means that the parasitic capacitance is small. However, the design of the signal line characteristic impedance is required for the dielectric constant and cannot be generalized.

100[Q] How many crystal oscillators should consider the way of routing between MCU and crystal oscillator? A. The crystal oscillator and the MCU should be as close as possible, connected by the shortest straight line.

101 [Q] The DC power coming from the switching power supply has a noise of about 100mv. How to effectively filter it out? Answer You can consider adding a regulator of the LDO product to stabilize the power supply, or consider the appropriate decoupling capacitor to filter the ripple.

102[Q] Can the analog power supply also be laid flat, is it the same as the ground? A power supply can of course be flat. If you can't lay the plane, the power cord should be as thick as possible.

103[Q] Ask the experts, the copper of the two-layer circuit board, when do you choose to cover both sides, when do you choose only one side of copper? A. If you can guarantee that the ground is full, you can only lay one layer.

104[Q] What are the requirements for the size of the vias and the via pitch in the design of high frequency (1 GHz or higher) boards? What are the factors to consider when impedance matching? Need to pay attention to the sheet? What are the precautions for the distance between the differential trace and the ground plane? Answer How to comprehensively consider the above indicators, it is recommended to do the overall circuit simulation and debugging, parasitic effects will affect the simulation effect, need to be repeatedly verified and tried.

105 [Q] 9 points of attention for copper. The so-called copper coating is to use the unused space on the PCB as the reference surface, and then fill it with solid copper. These copper areas are also called copper filling. The significance of copper coating is to reduce the ground line impedance, improve the anti-interference ability, reduce the voltage drop, improve the power supply efficiency, and connect with the ground wire to reduce the loop area. There are some issues to be aware of when applying copper:

1. If the PCB has more ground, there are SGND, AGND, GND, etc., according to the position of the PCB surface, the main "ground" as the reference reference to separate copper, digital ground and analog ground separately Copper is not enough to say, at the same time, before the copper coating, first increase the corresponding power connection: 5.0V, 3.3V, etc., thus forming a variety of different shapes of multi-deformation structure.

2. For a single point connection in different places, the method is to have a high resistivity and magnetic permeability through a 0 ohm resistor bead. It is equivalent to a series connection of a resistor and an inductor, but both the resistance value and the inductance value vary with frequency. He has better high-frequency filtering characteristics than ordinary inductors and resistive at high frequencies, so it can maintain high impedance over a wide frequency range, thus improving the frequency modulation filtering effect. As a power supply filter, an inductor can be used. The circuit symbol of the magnetic bead is the inductance. However, it can be seen that the magnetic bead is used in the circuit function. The magnetic bead and the inductor are the same principle, but the frequency characteristics are different. The magnetic bead is composed of an oxygen magnet, and the inductance is composed of a magnetic core and a coil. Composition, the magnetic beads convert the AC signal into heat energy, and the inductor stores the AC and slowly releases it. Magnetic beads have a great hindrance to high-frequency signals. The general specification is 100 ohms/100mMHZ, which is much smaller than the inductor at low frequencies.

3. Crystal oscillator: The crystal oscillator in the circuit is a high-frequency emission source. The method is to apply copper to the surrounding crystal oscillator, and then ground the crystal casing separately.

4. The problem of an isolated island (dead zone), if it feels great, it would not be a big deal to define a place to add it.

5. When starting the wiring, the ground line should be treated equally. When the line is routed, the ground line should be taken well. You cannot rely on the copper to remove the ground pin by adding a via hole. This effect is very bad.

6. It is best not to have sharp corners on the board ("180 degrees"), because from the electromagnetic point of view, this constitutes a transmitting antenna! For other things that always have an effect, it's just big or small. I recommend using the edge of the arc.

7. Do not apply copper to the wiring area of the middle layer of the multilayer board. Because it is very difficult for you to make this copper "good grounding"

8. Metals inside the equipment, such as metal radiators, metal reinforcement strips, etc., must be “good grounded”.

9. The reflow area of the three-terminal regulator reduces the external electromagnetic interference of the signal.