Almost every training and exchange will be asked "teacher, is there a general grounding method for reference?" The answer is yes: "No." Well, we can't always use the Chinese chefs to teach the apprentices the ingredients that are used in cooking, such as “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 following explanations are divided into several separate 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, fine products, Feel the beauty and connotation of it. Then it is possible to realize profound truths from simple words.
1, from the performance points, 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.
2. The goal of grounding pursuit is low ground impedance, stable ground, and balanced ground.The ground impedance is very low, and it can be understood by a thick cable. However, there is a problem that must not be ignored. For example, I am grounded through a large inductor. If the ground current of the ground is fluctuating at 0.00000001 Hz, this large inductor The perceptual effect is not obvious, it is equivalent to direct grounding, but if the fluctuating current is 1000000Hz, the inductive reactance = jωL = j2πfL, it is very large, in this case, the 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 the protection of static electricity. An electrostatic pulse is hit on the circuit board through the air. For the local circuit, the difference in distance is certain, 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 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.
3, common ground impedance coupling interferenceCommon ground impedance coupling interference is the core problem that is faced every day in the ground and can hardly escape. 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 the lobby and the staff is many times in Halls 1 and 2, the personnel entering and leaving Hall 3 will be 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 the Id on the other two. The influence of the road is isolated; in Figure 3, the three ground currents are all isolated.
This title uses a "comparative" word for a reason, because the universal grounding method does not exist at all. This is just a basic model. When it is actually used, it needs to be flexible and flexible in combination with the actual situation, just like language. The same sentence, "You hate it," when you speak in a different tone, the information you transmit 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 - the ground yellow-green wire direct connection
This connection is best understood to simply enable reliable low-impedance conduction between the two grounds. 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 ground - wide flat cable between the ground
The flat cable is mainly to solve the problem that the direct connection of the above wires cannot be solved. The grounding cable of the static test workbench does not need a straight line. This is the reason that the ground impedance can be turned on to the ground at a high frequency.
c ground - large resistance connection between the ground
The characteristic of the large resistor is that once the voltage difference across the resistor occurs, a very weak on-current is generated, and after the charge on the ground is discharged, the voltage difference between the two ends is finally =0, which is desirable in charge discharge. But when you don't want to slam it quickly, you will be able to show it to the fullest. 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 - ground magnetic beads connection
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 - ground inductance connection
The inductor has the characteristic of suppressing the change of the state of the circuit. Through the connection of the inductor, the peak and the valley can be filled. For the ground-ground with large current fluctuation, the crossover inductance can solve the problem.
f ground - small resistance connection between the ground
The problem to be solved by the small resistor is to add a damping, which hinders the overshoot of the rapid change of the ground current. When the current changes, the rising edge of the inrush current is slowed down, which is equivalent to the output of the crystal oscillator and the output of the bus to reduce the overshoot and ringing. Matching resistance.
5, safely, lightning protection, surge grounding connectionBecause the lightning surge, the safe current is generally far greater than the signal current is harmful to humans, these two grounding proposals are separately connected to the earth, connected at a single point in the real earth, especially lightning protection grounding.
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