Grounding is the most basic content in circuit design, but it is almost unclear. Almost every training and communication will be asked "teacher, is there a universal grounding method for reference?" Know the answer to this question, please continue to read the patience. I will give you an answer: "No." Well, we can't always use the Chinese chefs to teach the apprentices the ingredients they use when they are cooking. The ingredients are “a littleâ€, “color yellow†and “micro-focusâ€, of course not.
In order to better understand the grounding skills, the following will not pay attention to any text skills, but the essence of the problem of grounding. Grounding method ↠Grounding purpose ↠Grounding function, so which kind of grounding method is adopted depends on what kind of ground, what is the purpose of this kind of ground, these two problems are solved, and the grounding method can be achieved. The purpose of grounding determines the way to ground. The same circuit, for different purposes, may have to adopt different grounding methods. This point must be remembered. For example, the same circuit is used in portable devices, and the static electricity is accumulated and discharged. The purpose of grounding is ground potential equalization. For non-movable devices, there are generally safety grounding measures. The grounding purpose of static discharge is to guide The pass impedance is low enough, especially for the high frequency on-resistance of the spike. The explanations of the explanations are divided into several independent viewpoints. Although the content of each article is simple, it is recommended to read N times repeatedly, like facing a cup of good tea, letting the heartbeat be below 60bpm, and the fine products. Feel the beauty and connotation of it. Then it is possible to realize profound truths from simple words.
From the performance points, the grounding is divided into four categories: safety grounding, working grounding (digital ground, analog ground, power device ground), anti-surge grounding (lightning surge, power surge), anti-static grounding. As stated in the previous document, “the purpose of grounding determines the way of groundingâ€, and the purpose is to refer to the functions it implements. Basically all grounding can be attributed to these four categories. Before each grounding, it is necessary to know which type of grounding it belongs to. The goal of grounding pursuit is low ground impedance, stable ground, and low ground impedance. It is easy to understand. It can be used with thick cables, but there is a problem that must not be ignored. For example, I grounded through a large inductor, if the ground The fluctuating frequency of the running ground current is 0.00000001 Hz. The inductive effect of this large inductance is not obvious, which is equivalent to direct grounding, but if the fluctuating current is 1,000,000 Hz, the inductive reactance = j ω L = j 2 Ï€ f L, it is very large, in this case, it is equivalent to high frequency grounding is very poor. You guys may say, you come, you will use a large inductor to ground, the first is that in some state there will be this way, the second is that even if you do not connect such an inductor, the ordinary cable Trace inductance is also not negligible at high frequencies.
Summarized in one sentence, low frequency grounding ≠high frequency grounding.
That is, the low-impedance grounding is to analyze whether it is a ground of high frequency or low frequency. The stability of the ground is relatively well understood. Generally speaking, if the grounding impedance is low enough, the ground current is easy to discharge, and the voltage drop of the dice is not generated on the bottom line, just like an oversized capacitor, the ocean of charge, has an infinitely broad The mind, how many come in, are not shocked. The ground balance is easier to ignore. For a signal, the useful part is the voltage difference between the two lines. If the ground line drifts, the voltage difference between the two lines and the ground line rises or falls equally, that is, the differential mode voltage. The value remains unchanged and the common mode voltage changes. In fact, the circuit function is implemented as usual. Just like the water is rising, you are 3cm higher than me, standing on the boat, the boat is floating, you are still 3cm high. This kind of situation is often used in static electricity protection. An electrostatic pulse is hit on the circuit board through the air. For local circuits, the distance between the two is definitely different, which will definitely lead to the pressure difference of static induction. At this time, if a metal plate is used to separate it, even if the metal plate is floating, the circuit board behind the metal plate will induce a uniform electric field. Although the inductive interference still exists, the minimum circuit is basically balanced. Of course, if the metal plate is grounded better.
Of course, the common mode voltage will not generally be maintained. Because the impedance of the transmission line is not uniform, it will often turn into differential mode voltage interference. The problem of ground balance is best not to let us face it, but when there is no way, such as floating equipment, For boards that are not subjected to electrostatic shock, the problem of equalization should be considered when protecting. Common ground impedance coupling interference Common ground impedance coupling interference is the core problem that is faced every day in the grounding, and almost escapes.
Just like when you are in the movie theater, you come out from the innermost hall. There are not many people. It is very smooth. Suddenly, the second hall is also scattered. The passage is crowded and you continue to move forward. It is broken. Hall 3 is putting audiences in, and all of a sudden, the flow of people fluctuates. This and the common ground impedance are a principle, the channel is equivalent to the ground line, and the person is equivalent to the current. If the people in Halls 1, 2, and 3 are similar, the influence between them is not too big, but if Hall 3 is a hall and the number of staff is many times in Halls 1 and 2, the personnel entering and leaving Hall 3 will The flow rate of personnel in Halls I and II has a great impact. The road that the guests of the first, second and third boats have to walk through has become a common impedance.
The following figure is an example. In Figure 1, the resistance of the RAB section is the common ground impedance part. The ground currents Io, Ia, and Id flowing through this section will affect each other during this period; if the three currents are different, the difference will be With 1-2 orders of magnitude, the effects of each other cannot be ignored, especially when a weak ground current branch is used for quantitative measurement, amplification or AD conversion circuits; Figure 2 puts Id on the other two. The influence of the road is isolated; in Figure 3, the three ground currents are all isolated.
The more general type of grounding method uses a "comparative" word for a reason, because the general grounding method does not exist at all. This is only a basic model. When it is actually used, it needs to be flexible with the actual situation. Processing, just like language, the same sentence "you hate", when you speak in different voices, the information conveyed is very different. The basic idea is that, in the design, the safety protection ground, the work digital ground, the work simulation ground, the working power ground, the lightning surge, the shielding ground first ensure the separate independent connection, and finally, when the system is coordinated, according to The problem to be solved between different places, that is, according to the purpose of grounding, the several places are treated according to the following connection methods, and the connection modes include: a ground--ground yellow-green wire direct connection It is easy to understand that it is simple to make two grounds with reliable low impedance conduction. But remember, this connection is limited to the connection between the low and medium frequency signal circuits.
Because there are certain trace inductances and trace resistances on such conductors, the current is fluctuated by high frequency. Under the action of inductance, the cable acts as a large impedance, which is equivalent to low frequency grounding, and high impedance is grounded at high frequency. Basically, reliable conduction at high frequencies cannot be achieved. b---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- through. c---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- This feature is vividly expressed when you want the charge to bleed, but you don't want to slam it quickly. The anti-static mat on the production work site, the on-resistance is generally 106-109 ohms, which is the purpose.
The anti-static mat is equivalent to a large resistance between the ground of the working circuit board and the protective earth. The ground-capacitor connection capacitor is characterized by DC cut-off and AC conduction. This method can be considered for situations where it is desirable to implement such functions. For example, a switching power supply product, the outer casing and the protective ground connection, the ground on the circuit board inside has disorderly fluctuations, but there is nowhere to lie, in the 24V, 12V, 5V, etc. between the DC power ground and the protective ground With large capacitors, the ripples can be vented, but the DC component can be guaranteed to be stable; note that in this case, the stability of the protective ground and the outer casing cannot be guaranteed, and the effect may be counter-European. d ground - the ground magnetic beads are connected here, the characteristics of the magnetic beads need to be clear, many engineers often equate the magnetic beads with the inductance, which is a fundamental error.
A magnetic bead is equivalent to a frequency-dependent resistance. It exhibits a resistance characteristic and is a lossy property; an inductance is a storage energy property, which is equivalent to a pin-to-peak fill. Therefore, there is generally a state of rapid small current fluctuation between the grounds of the magnetic beads, because the magnetic beads will be saturated, the current is too large, and it cannot be consumed. Generally used between the ground of the weak signal - the ground. E-ground-inductance connection inductance has the characteristic of suppressing the state change of the circuit. Through the connection of the inductor, the pin can be filled with the peak. For the ground-ground with large current fluctuation, the crossover inductance can solve this problem. f ground - the small resistance between the ground and the small resistance to solve the problem is to increase a damping, hinder the overshoot of the ground current rapid change, when the current changes, the rising edge of the inrush current becomes slower, equivalent to the crystal output end, the bus The output is a matching resistor that reduces overshoot ringing.
The grounding method of safely and lightning-proof surge grounding is generally because the lightning surge and the safe current are generally far greater than the harm of the signal current to the human. These two grounding suggestions are separately connected to the earth and are single point in the real earth. Docking, especially lightning protection grounding. This article took about a month, and all kinds of ideas have been hovering around, but there is no way to start. In my body, it also proves the relationship between grounding and our relationship, the most familiar and the most strange, the simplest and most complicated. The most easy to get on the road and the hardest to reach the end. I hope that through a rough summary, we will provide a landing ladder for the grounding design that floats in the cloud, so that the grounded design can really be connected to the ground.
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