Thoroughly Solving the Grounding Problem of EPSON Crystals in Circuit Design

In order to better understand the grounding techniques and methods, the following will no longer pay attention to any textual skills, but it is the essence of the question of grounding. A good circuit requires good components. The Epson original crystal oscillator is one of the best choices.

Epson Crystal FC-135

Grounding mode ← grounding purpose ← grounding function, so what kind of grounding method to take depends on what type of ground is, what is the purpose of this type of ground, these two problems are solved, and the grounding mode can be a matter of course.

The purpose of grounding determines the grounding method. The same circuit, different purposes, may have to adopt different grounding methods. This point must be remembered. For example, the same circuit, used in portable devices, static electricity can not be discharged, the purpose of grounding is ground potential equalization; used in non-removable equipment, there will generally be safety grounding measures, the grounding of electrostatic discharge is to guide The pass impedance is low enough, especially for spike high-frequency on-resistance.

The points of attention in the following explanations are divided into several independent points of view. Although the content of each article is simple, it is recommended that you read N times repeatedly, as if you are faced with a good cup of tea, and your heart beats below 60bpm. Feel the beauty and connotation. Then it is possible to realize the profound truth from simple words.

1) From the performance points, grounding is divided into four categories:

Safety grounding, working ground (digital ground, analog ground, power device ground), surge grounding (surge surge, power surge), and antistatic grounding.

"The purpose of grounding determines the grounding method," and its purpose is to refer to its function. Basically all grounding can be attributed to these four categories. Before each grounding, it must first be clear which one the ground belongs to.

2) The goal of grounding is to achieve low ground impedance, stable ground, and balanced ground.

It is well understood that ground impedance is low, but it is sufficient to use a thick cable, but one problem must not be ignored. For example, if I ground it through a large inductor, if the ground current on the ground fluctuates at a frequency of 0.00000001Hz, this large inductor The perceptual effect is not obvious, and it is equivalent to direct grounding, but if the fluctuating current is 1,000,000Hz, inductive reactance = jωL = j2πfL, it becomes very large. In this case, it is equivalent to poor high-frequency grounding. You may be able to tell you, you come, you who will use a large inductance ground it, the first is in a certain state will have this approach, the second is even if not connected to an inductor, ordinary cable Trace inductance can not be ignored at high frequency. To sum up, the low-frequency grounding is grounded at a high frequency. That is, low-resistance grounding is to analyze whether it is a high-frequency or low-frequency ground.

Ground stability is better understood. In general, if the grounding impedance is low enough, the ground current is easy to drain, and it will not cause a dice pressure drop on the bottom line. It is like an oversized capacitor, the ocean of charge, has an infinite breadth. The mind, how many come in is not stunned.

Ground balance is easily overlooked. For a signal, the useful part is the pressure difference between the two wires. If the ground wire drifts, the differential pressure of the ground wire on both wires 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. It's like the height of the boat, you are 3cm taller than me, standing on the boat, the boat is floating, you are still 3cm higher. This situation is often used during ESD protection. An electrostatic pulse hits the circuit board through the air. For local circuits, the difference in distance will definitely lead to electrostatic induction pressure difference. If a metal plate is used at this time, even if the metal plate floats, the induction circuit will induce a uniform electric field for the circuit board behind the metal plate. Although inductive interference still exists, the circuit is basically balanced. Of course, if this metal plate is grounded, it is better. Of course, the common-mode voltage will generally 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 allow us to face, but when there is no way, such as floating equipment, no Circuit boards that are not subject to electrostatic shock must be balanced when protecting them.

EPSON Jingzhen TG2016SBN

3) Common ground impedance coupling interference

The common ground impedance coupling interference is the core problem that is faced every day in grounding, and it is almost impossible to escape. Just like in the movie theater, you come out from the innermost hall. There are not many people, and it is very smooth. Suddenly the second hall is also disbanded, and the passageway is congested. Then it continues. It is broken. At the third hall, the audience was put into the audience. At once, the flow of people fluctuates. This and ground impedance are a principle. The channel is equivalent to the ground and the human is equivalent to the current. If Halls I, II, and III are similar, there will not be much impact on each other. However, if Hall 3 is a hall, the number of people will be many times that of Halls I and II. Those going in and out of Hall III will be right. The speed of the flow of personnel in Halls I and II has a great influence. The passage that the guests of the boats No. 1, No. 2, and No. 3 have to walk through becomes a total impedance.

The following figure shows an example. In Fig. 1, the resistance of the RAB segment is the common ground impedance. The three parts of the ground currents Io, Ia, and Id that flow through this segment affect each other; if the three currents are different, the difference is great. In the case of 1-2 orders of magnitude, the influence of each other cannot be ignored, especially when a certain weak current branch is used for quantitative measurement, amplification, or AD conversion; Figure 2 shows Id for the other two The influence of the roads was isolated; in Figure 3, the three ground currents were all isolated.

4) More general grounding method

This title uses a "comparison" for a reason, because the universal grounding method does not exist at all. This is only a basic model. When it is actually used, it needs to be flexibly handled in conjunction with the actual situation, just like language. In the same sentence, "You hate", when you speak in different ways, the information you send is very different. The basic idea is that, in terms of design, safe and protected grounds, digital grounds for work, simulated grounds for work, power for working grounds, lightning surge grounds, and shielded grounds should be ensured to be connected independently and independently, and finally, when the system is linked together, The problem to be solved between various regions is to treat these sites in accordance with the following connection methods according to the purpose of grounding. The connection methods include:

a - Direct connection between yellow and green conductors

The best way to understand this connection is simply to make the two grounds reliably conduct with low impedance. However, keep in mind that this method of connection is limited to the connection between the low- and medium-frequency signal circuits. Because this type of wire has certain trace inductance and trace resistance, it will play a role of large impedance for high-frequency ground current under the action of the inductor, which is equivalent to low-frequency grounding, and high-impedance high-impedance grounding. Basically, reliable conduction at high frequencies cannot be realized.

b. Ground-wide flat cable direct connection

The flat cable is mainly used to solve the problem that the above direct connection of the wires cannot be solved. The reason that the grounding cable of the electrostatic test workbench does not use a straight line is that this method can realize the resistance to ground conduction at a high frequency.

c ground - large resistance connection between ground

The characteristic of a large resistance is that once a pressure difference occurs across the resistance, a very weak on-state current will be generated. After discharging the charge-discharge on the ground line, the pressure difference between the two ends will eventually become 0V. This feature is hopeful that the charge will be discharged. However, when they do not want to expel them quickly, they will do their best. The anti-static pad of the production work site, on-resistance is generally 106-109 ohms, is this purpose. The anti-static mat is equivalent to a large resistance between the ground of the working circuit board and the earth. c. Ground—The capacitance of the capacitor connected between the ground is the DC cut-off and the AC conduction. This method can be considered for applications where it is desired to implement such functions. For example, a switching power supply products, housing and protective ground connection, the circuit board on the ground there is a disturbance of disturbance, but there is nowhere to discharge, in 24V, 12V, 5V and other DC power ground and protective ground across With large capacitors, the fluctuations can be evacuated, but the DC component can be guaranteed to be stable. Note that in this case, the stability of the protective earth and the housing can not be guaranteed, the effect may be anti-European.

d - ground magnetic beads connection

Here, the characteristics of the magnetic beads need to be clarified. Many engineers often equate beads and inductors. This is a fundamental mistake. The bead is equivalent to a resistance that changes with frequency. It shows the resistance characteristic and is the nature of the loss. The inductance is the nature of the energy storage, which is equivalent to the peak filling. Therefore, there is generally a state of rapid and small current fluctuations between the grounds where the magnetic beads are straddled. Since the magnetic beads saturate and the current is too large, it cannot be consumed. It is generally used between weak ground and ground.

e ground - ground inductance connection

Inductors have the property of suppressing changes in the circuit state. Through the connection of the inductors, the valleys can be filled with peaks. For places with large current fluctuations - ground, the inductance can be bridged to solve this problem.

f. ground - small ground resistance connection

The problem to be solved by the small resistor is to increase the damping and hinder the overshoot of the rapid change of the ground current. When the current changes, the rising edge of the inrush current is reduced, which is equivalent to the output of the crystal oscillator and the output of the bus to reduce overshoot ringing. Matching resistance.

5) Safe, lightning surge grounding connection

Because lightning surges and safe currents are generally much greater than the harm of signal currents to humans, these two grounding recommendations are separately received from the earth and are connected at a single point in the real earth, especially against lightning strikes.

This article took about a month or so, and all kinds of ideas have been hovering in the heart, but there is no way to start. In my body, it also confirms the relationship between the problem of grounding and us, the most familiar and most unfamiliar, the most simple and most complex It is the easiest and hardest to reach the end. It is hoped to provide a landing ladder for the grounding design that floats in the clouds through the rough summary, so that the grounding design can be truly connected to the ground.