Only those who can connect the ground well are real masters.[Copy link]
Grounding is the most basic content in circuit design, but almost no one can explain it clearly. Almost every time we communicate, someone will ask, "Teacher, is there a universal grounding method that I can refer to?" If you want to know the answer to this question, please continue to read patiently. I will give a definite answer first: "No." What should we do? We can't be like Chinese chefs, teaching apprentices to cook with ingredients that are "a little", "slightly yellow", "slightly burnt" and other perceptual terms, of course not.Part 1 Basic design principles of basic power supply and groundNoise on ground and power linesThe ideal ground resistance is zero, but in reality the ground always has a certain impedance. Especially for high-frequency signals, the impedance change of the ground will increase, thus generating noise.The same problem applies to power lines. When the ground line noise is serious, the power supply noise is usually also serious.(1) Stabilize the potential of the ground wire.(2) For safety.What is the purpose of ground wire and box grounding?Especially when there is a high voltage (more than tens of volts) circuit, the shielding box must be grounded. Otherwise, if leakage occurs, people will get electric shock when they touch it.
High frequency impedance of the conductorThe thicker the wire, the lower the resistance. However, if the length is too long, the impedance will increase due to the inductance effect. The impedance caused by the inductance effect. Especially when the ground wire has a loop, the inductance of the ground wire will increase significantly, and the impedance will also increase significantly, and it will also become a loop antenna. When the ground wire is a long straight line, it will also become an antenna. Especially when the ground wire has a loop, the inductance of the ground wire will increase significantly, and the impedance will also increase significantly, and it will also become a loop antenna. When the ground wire is a long straight line, it will also become an antenna. Therefore, the ground wire should be as "short, thick, and straight" as possible.
What does the earth mean?The ground does not necessarily have to be the earth. A relatively large conductive object can also be the ground, such as the body of an airplane. In the figure below, the solution with lower impedance connected to a relatively large conductive object is better, and b is better than a.
Principle 2: Single-point groundingAll circuit parts should be grounded as close to each other as possible. Even if the common impedance is very small, it still has an effect.
Principle 3: Use ground or stable power supply as much as possible in empty spaceOn the right is the GND copper covering in the non-routing area, which shows obvious improvement.
For double-layer boards, one side is usually blank and all ground lines are used, while the other side is used for power and signal lines. For PCB boards with more than four layers, GND and power are usually made into inner layers to improve loop impedance.Part II Design Requirements for AC Power SupplyThe AC power line is a very large source of noise! Because:1. The AC power line is connected to many instruments and equipment, which are noise sources to each other (switching surges, current change noise, etc.)2. The AC power cord itself is a large antenna that receives and emits noise.3. The AC frequency itself will also become noise.The quality of Japanese AC power supply: Red: highest voltage; Green: average voltage; Black: lowest voltage; Purple: frequency X2Hz.
Part III DC Power Supply and GroundThere are two broad categories of DC power supplies: linear power supplies: simple but inefficient; switching DC power supplies: efficient but noisy. When there are many circuit boards in an instrument, using DC level conversion on each circuit board is beneficial for suppressing the mutual influence of noise between the boards.
The common impedance of the power and ground lines in the circuit board should be as small as possible.For low-frequency analog signal circuits, try to use a single point grounding, and try to do the same for the power supply, but a single point connection on the power supply is not as effective as a ground wire. For high-frequency signal circuits, a large area of ground should be laid (the empty space of the signal line layer should be fully grounded, and the corresponding layer of the signal layer should be fully grounded as much as possible), and the power line should also be done in this way as much as possible. If it is really not possible, a multi-layer board can be used, and the middle layer should be laid as the ground and power supply.Basic Rules for Decoupling CapacitorsThere should be at least two stages of decoupling capacitors. The second stage of decoupling capacitors should be as close to the IC as possible, otherwise the effect will be reduced.
Part 4 Instrument Cabinet and WiringElectromagnetic disturbance propagation or coupling is usually divided into two categories: conducted disturbance propagation and radiated disturbance propagation. Electromagnetic disturbance propagating through conductors is called conducted disturbance, while electromagnetic disturbance propagating through space is called radiated disturbance.
The shorter the trace, the better The two wires of the signal line should be as close as possible. Try to use the right winding wire. Do not create loops. Try to make lines that are likely to affect each other perpendicular to each other.When using cable bundles and cables, keep them as short as possible, keep signal lines that are likely to affect each other away from each other, and keep different cables away from parallel. When using cable bundles and cables, keep them as short as possible, keep signal lines that are likely to affect each other away from each other, and keep different cables away from parallel. Pay special attention not to place the AC line close to other lines.
Part 4 The Nature of Grounding ProblemsIn order to better understand the techniques and methods of grounding, the following will no longer emphasize any textual skills, but will directly point out the essence of the grounding problem.Grounding method ← grounding purpose ← grounding function, so the grounding method to be adopted depends on what kind of ground it is and what the purpose of this kind of ground is. Once these two problems are solved, the grounding method can be achieved naturally. The purpose of grounding determines the grounding method. The same circuit, but different purposes, may require different grounding methods. This point must be remembered. For example, the same circuit, when used on a portable device, static electricity accumulation cannot be discharged, and the purpose of grounding is to balance the ground potential; when used on an immovable device, there are generally safety grounding measures, and the purpose of grounding for static electricity discharge is to have a sufficiently low on-impedance, especially for the high-frequency on-impedance of spike pulses.1. In terms of performance, grounding is divided into four categories: safety grounding, working grounding (digital ground, analog ground, power device ground), surge protection grounding (lightning surge, power-on surge), and anti-static grounding. As mentioned in the previous article, "the purpose of grounding determines the grounding method", and the purpose refers to the function it achieves. Basically, all grounding can be attributed to these four categories. Before each grounding, it is necessary to first clarify which type the grounding belongs to.2. The goal of grounding is to achieve low ground impedance, stable ground, and balanced ground.It is easy to understand that the ground impedance is low, and thick cables can be used. However, there is a problem that must not be ignored. For example, if I ground through a large inductor, if the fluctuation frequency of the ground current running on the ground line is 0.00000001Hz, the inductive effect of this large inductor is not obvious, which is equivalent to direct grounding. However, if the fluctuation current is 1000000Hz, the inductive reactance = j ω L = j 2 π f L, which is very large. In this case, it is equivalent to poor high-frequency grounding. You may say, "You are so stupid. Who would use a large inductor for grounding?" First, this method is used in certain conditions. Second, even if an inductor is not connected in this way, the routing inductance of ordinary cables cannot be ignored at high frequencies. In summary, low-frequency grounding ≠ high-frequency grounding. That is, low-impedance grounding should be analyzed to determine whether it is high-frequency or low-frequency grounding.Ground stability is easy to understand. Generally speaking, if the ground impedance is low enough, the ground current can be discharged easily and will not produce any voltage drop on the bottom line, just like a super large capacitor, an ocean of charge, with an infinitely broad mind, no matter how much comes in, it will not be disturbed. Ground balance is easy to be overlooked. 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 will rise or fall equally, that is, the differential mode voltage value remains unchanged, and the common mode voltage changes. In fact, the circuit function is realized as usual. Just like the rising water, you are 3cm taller than me, standing on the boat, the boat floats, and you are still 3cm taller than me. This situation is often used in electrostatic protection. An electrostatic pulse hits the circuit board through the air. For the local circuit, the different distances will definitely cause the electrostatic induced pressure difference. 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 be induced by a uniform electric field. Although the induced interference still exists, at least the circuit is basically balanced. Of course, it would be better if the metal plate is grounded. Of course, the common-mode voltage is generally not maintained because the impedance of the transmission line is uneven, which often turns into differential-mode voltage interference. It is best not to face the problem of ground balance, but when there is no other way, such as floating ground equipment and circuit boards that have to be subjected to static electricity shock, the ground balance problem must be considered when protecting.3. Common ground impedance coupling interferenceCommon ground impedance coupling interference is the core problem that grounding faces every day, and it is almost impossible to avoid it. Just like when the movie is over, you come out of Hall 1 at the back, there are not many people, and the passage is very smooth. Suddenly, Hall 2 is over, and the passage is crowded. If you continue to move forward, it is bad, Hall 3 is letting the audience in, and suddenly, the flow of people fluctuates. This is the same principle as common ground impedance. The passage is equivalent to the ground wire, and people are equivalent to current. If the number of people flowing in Halls 1, 2, and 3 is similar, the impact on each other is not too big, but if Hall 3 is a hall, and the number of people is many times that of Halls 1 and 2, then the people entering and leaving Hall 3 will have a great impact on the flow speed of people in Halls 1 and 2. The section of road that the guests of Halls 1, 2, and 3 have to walk through becomes the common ground impedance.Take the following figure as an example. In Figure 1, the resistance of the RAB segment is the common ground impedance part. The three parts of the ground current Io, Ia, and Id flowing through this segment will affect each other in this segment; if the three currents are very different, with a difference of 1-2 orders of magnitude, the mutual influence cannot be ignored, especially when a weak ground current branch is used for quantitative measurement, amplification or AD conversion circuit; Figure 2 isolates the influence of Id on the other two paths; Figure 3 isolates all three ground currents separately.
4. More general grounding methodThere is a reason why the word "relatively" is used in this title, because there is no universal grounding method at all. This is just a basic model. When it is actually used, it needs to be handled flexibly according to the actual situation, just like language. The same sentence "you hate" can convey very different information when spoken in different tones. The basic idea is that in design, the safety protection ground, working digital ground, working analog ground, working power ground, lightning surge ground, and shielding ground should be connected independently. Finally, when the system is debugged, according to the problems to be solved between the various places, that is, according to the purpose of grounding, these grounds should be connected according to the following methods. The connection methods include:a. Direct connection of yellow-green wire between ground and groundThis connection method is easiest to understand, which is to simply make the two grounds reliably connected with low impedance. But remember, this connection method is limited to the connection between medium and low frequency signal circuits. Because this type of wire has a certain routing inductance and routing resistance, for high frequency fluctuating current, under the action of inductance, the cable plays a role of large impedance, which is equivalent to low frequency grounding. At high frequency, the high impedance grounding basically cannot achieve reliable conduction at high frequency.b. Direct connection between ground and ground with wide flat cableFlat cables are mainly used to solve problems that cannot be solved by direct connection of the above wires. This is why the grounding cable of the electrostatic test workbench does not need to be a straight line. It can achieve ground impedance conduction to the ground at high frequencies.c. Large resistance connection between ground and groundThe characteristic of a large resistor is that once a voltage difference appears across the resistor, a very weak conduction current will be generated. After the charge on the ground line is discharged, the voltage difference between the two ends will eventually be 0V. This characteristic will be fully demonstrated when the charge is expected to be discharged but not quickly. The anti-static mat at the production site generally has an on-resistance of 10^6-10^9 ohms, which is for this purpose. The anti-static mat is equivalent to a large resistor between the ground of the working circuit board and the protective earth. c Ground-ground capacitor connection capacitor has the characteristic of DC cutoff and AC conduction. This method can be considered for occasions where such functions are desired. For example, a product powered by a switching power supply, the shell and the protective ground are connected, and the ground on the circuit board inside has chaotic fluctuations, but there is no place to discharge it. A large capacitor is connected between the DC power supply ground of 24V, 12V, 5V, etc. and the protective ground. The fluctuation can be discharged, but the DC component can be guaranteed to be relatively stable; note that in this case, if the stability of the protective ground and the shell ground cannot be guaranteed, the effect may be counterproductive.d. Ground-to-ground magnetic bead connectionThe characteristics of magnetic beads need to be clarified. Many engineers often equate magnetic beads with inductors, which is a fundamental mistake. Magnetic beads are equivalent to a resistor that changes with frequency. It shows resistance characteristics and is lossy in nature; inductors are energy storage properties, which is equivalent to peak shaving and valley filling. Therefore, there is generally a state of rapid small current fluctuations between the grounds that bridge the magnetic beads, because the magnetic beads will saturate and the current is too large for it to consume. It is generally used between the grounds of weak signals.e. Ground-to-ground inductance connectionInductors have the characteristic of suppressing changes in circuit states. By connecting inductors, peaks can be smoothed and valleys can be filled. For ground-to-ground circuits with large current fluctuations, a jumper inductor can solve this problem.f. Small resistance connection between ground and groundThe problem that small resistors need to solve is to add a damping to hinder the overshoot of rapid changes in ground current. When the current changes, the rising edge of the impact current slows down, which is equivalent to the matching resistors at the output end of the crystal oscillator and the output end of the bus to reduce overshoot ringing.5. Safe grounding and lightning surge protection groundingBecause the current of lightning surge and safety ground is generally much more harmful to people than the signal current, it is recommended that these two grounds be connected to the earth separately and connected at a single point on the real earth, especially for lightning protection grounding.
The relationship between grounding and us is the most familiar and the most unfamiliar, the simplest and the most complex, the easiest to start and the most difficult to reach the end. I hope that through a superficial summary, we can provide a ladder to the ground for our grounding design that is floating in the clouds and fog, so that the grounding design can truly connect to the earth.
If you are not satisfied after reading the article, we will provide a wave of design tips for those who are learning about power supply~~
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