The difference and treatment of various grounds in the circuit
The difference between digital ground and analog ground is the
same in essence, that is, both digital ground and analog ground are ground. To understand why we need to separate, listen to a story first; in our company’s business building, the second floor is analog, and the third floor is digital. There is only one elevator in the whole building. It's easy to handle when there are few people. Go to 2. The 3rd floor upstairs does not affect each other, but it is terrible when commuting to work every day. There are so many people. Digital ones have to go up to the 3rd floor. They are always influenced by the people who do analog on the 2nd floor, and those who are analogue on the 2nd floor have to go down. Building, we always have to wait for the elevator to go up to the 3rd floor and then come down. It is very troublesome to influence each other. To solve this problem, the property of the commercial building has proposed two solutions: the first (laughing) elevator expands and can be installed more For people, it’s good if the elevator is bigger, but the company will hire people. If there are too many people, change the elevator and hire people again... There is always an endless loop. There is a way to do it. Everyone simply skips the elevator and just jumps down. Regardless of the second floor and the third floor, the problem will definitely be solved, but there will definitely be problems (the first one was shot and dropped). The second way is to install 2 elevators, one goes up to the 2nd floor and the other goes up to the 3rd floor. Wonderful! It's so witty, so that the staff on the 2nd floor will not affect each other. Do you understand?
Digital ground and analog ground affect each other not because one is called digital and the other is analog, but they use the same elevator: ground, and the hoistway used by this elevator is the ground wire we laid on the PCB. The current in the analog loop goes through this line, and the current in the digital loop also goes through this line. There is nothing wrong with it. The wiring is used to conduct the current, but the problem is that there is resistance on this line! And the most fundamental problem is that the current going through this line has to go to two different circuits. Suppose: There are two currents, digital flow, and analog flow starting from the ground at the same time. There are 2 devices: digital and analog. If the two circuits are not separated, when the digital current and the analog current go to the ground terminal of the digital component, the loss voltage is V=(digital current + analog current) X wiring resistance, which is equivalent to the ground terminal of the digital device relative to the ground The V terminal has increased, and the digital device is not satisfied. I admit that it will increase the voltage a little. I recognize the part of the digital stream, but why should the analog stream be added to my head? Similarly, analog devices will also complain!
Two solutions: The first one: the PCB line you route has no impedance and naturally will not cause interference, just like jumping directly down on the 2nd and 3rd floors, when the hoistway is the widest, that is, you can install an infinite Elevators, naturally, no one affects anyone, but everyone knows that This is mission impossible! The second one: The two circuits are separated, the digital flow and the modular flow are separated, and the digital ground and the modular ground are separated.
In the same way, although it is sometimes in an analog loop, it must be divided into large and small current loops to avoid mutual interference. The so-called interference is: the voltage caused by the currents in two different loops on the PCB traces, and these two voltages are generated by superimposing each other.
The following is a detailed introduction. In simple terms, the digital ground is the common reference terminal of the digital circuit part, that is, the reference terminal of the digital voltage signal; the analog ground is the common reference terminal of the analog circuit part, and the voltage reference terminal (zero potential point) of the analog signal. .
1. The reasons for the division into digital ground and analog ground.
Digital signals are generally rectangular waves with a large number of harmonics. If the digital ground and analog ground in the circuit board are not separated from the access point, the harmonics in the digital signal can easily interfere with the waveform of the analog signal. When the analog signal is a high frequency or strong electric signal, it will also affect the normal operation of the digital circuit. The analog circuit involves weak and small signals, but the threshold level of the digital circuit is higher, and the requirement for the power supply is lower than that of the analog circuit. In a system with both digital and analog circuits, the noise generated by the digital circuit will affect the analog circuit and make the small signal index of the analog circuit worse. The way to overcome it is to separate the analog ground and the digital ground.
The root cause of the problem is that the resistance of the copper foil on the circuit board cannot be guaranteed to be zero. Separating the digital ground from the analog ground at the access point is to minimize the common ground resistance of the digital ground and the analog ground.
2. The basic principles of digital ground and analog ground processing are as follows.
If the analog ground and digital ground are directly connected in a large area, it will cause mutual interference. It is inappropriate if it is not short-circuited. For low-frequency analog circuits, in addition to thickening and shortening the ground wire, the use of one-point grounding for each part of the circuit is the best choice to suppress ground wire interference, mainly to prevent mutual interference between components due to the common impedance of the ground wire.
For high-frequency circuits and digital circuits, since the inductive effect of the ground wire will have a greater impact at this time, one-point grounding will cause the actual ground wire to be lengthened and adversely affected. At this time, a combination of separate grounding and one-point grounding should be adopted. In addition, for high-frequency circuits, how to suppress high-frequency radiation noise should also be considered. The method is to make the ground wire as thick as possible to reduce the noise-to-ground impedance; full grounding, that is, except for the printed wire that transmits the signal, all other parts are used as ground wires . Don't have useless large areas of copper foil.
The ground wire should form a loop to prevent the generation of high-frequency radiation noise, but the area enclosed by the loop should not be too large to avoid induced current when the instrument is in a strong magnetic field. But if it is only a low-frequency circuit, ground loops should be avoided. The digital power supply and the analog power supply should be isolated, and the ground wires should be arranged separately. If there is an A/D, only a single point should share the ground here. There is not much influence in the low frequency, but it is recommended to ground the analog and digital at one point. At high frequencies, the analog and digital grounds can be shared by a magnetic bead.
Three, four solutions
The series connection between analog ground and digital ground can adopt four methods: 1. Connect with magnetic beads; 2. Connect with capacitor (using the principle of capacitor to separate through and pass through); 3. Connect with inductance ( Generally use a few uH to tens of uH); 4. Use a 0 ohm resistor to connect. The following focuses on the magnetic beads and 0 ohm resistance:
In general, 0 ohm resistance is the best choice. 1. It can ensure equal DC potential; 2. Single-point grounding to limit noise; 3. It has an attenuation effect on noise at all frequencies (0 ohms also have impedance, and the current path Narrow, can limit the noise current to pass); 4. Capacitor (using the principle of capacitor blocking through and passing through).
The magnetic beads are made of sintered ferrite material with good impedance characteristics in the high frequency range. They are specially used to suppress high-frequency noise and spike interference on signal lines and power lines, and also have the ability to absorb electrostatic pulses. Magnetic beads have high resistivity and permeability, which is equivalent to a series connection of resistance and inductance, but the resistance value and inductance value change with frequency. It has better high-frequency filtering characteristics than ordinary inductors, and shows resistance at high frequencies, so it can maintain a higher impedance in a relatively wide frequency range, thereby improving the FM filtering effect. Magnetic beads have a greater hindrance to high-frequency signals. The general specification is 100 ohms/100mMHZ, and its resistance is much smaller than inductance at low frequencies. Ferrite Bead (Ferrite Bead) is a kind of anti-jamming component with rapid application development. It is cheap, easy to use, and has a significant effect on filtering high-frequency noise.
Ferrite beads can be used not only to filter high frequency noise in power circuits (for DC and AC output), but also to be widely used in other circuits, and their volume can be made small. Especially in digital circuits, because the pulse signal contains high-frequency harmonics, it is also the main source of high-frequency radiation in the circuit, so it can play the role of magnetic beads in this kind of occasion. As long as the wire passes through it in the circuit. When the current in the wire passes through, the ferrite has almost no impedance to the low-frequency current, but will have a greater attenuation effect on the higher-frequency current.
4. The difference between inductance and magnetic beads.
A coil with more than one turn is used to be called an inductive coil, and a coil with less than one turn (wire through the magnetic ring) is used to call it a magnetic bead. Inductance is an energy storage element, and magnetic beads are energy conversion (consumption) devices. Inductors are mostly used in power filter circuits, and magnetic beads are mostly used in signal circuits for EMC countermeasures; magnetic beads are mainly used to suppress electromagnetic radiation interference, and inductors are used This aspect focuses on suppressing conducted interference. Both can be used to deal with EMC and EMI problems; inductors are generally used for circuit matching and signal quality control, and magnetic beads are used where analog ground and digital ground are combined.
As a power supply filter, an inductor can be used. The circuit symbol of the magnetic bead is the inductance, but it can be seen from the model that the magnetic bead is used. In the circuit function, the magnetic bead and the inductance have 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. In the composition, the magnetic beads convert the AC signal into heat energy, and the inductor stores the AC and releases it slowly.
Inductance is an energy storage element, and magnetic beads are energy conversion (consumption) devices; inductors are mostly used in power filter circuits, magnetic beads are mostly used in signal circuits for EMC countermeasures; magnetic beads are mainly used to suppress electromagnetic radiation interference, and inductors are used This aspect focuses on suppressing conducted interference. Both can be used to deal with EMC and EMI issues. Magnetic beads are used to absorb ultra-high frequency signals. For example, some RF circuits, PLLs, oscillation circuits, and ultra-high frequency memory circuits (DDR SDRAM, RAMBUS, etc.) need to add magnetic beads to the power input part, and inductance is a kind of storage Energy components, used in LC oscillator circuits, medium and low frequency filter circuits, etc., whose application frequency range rarely exceeds 50MHZ.
Five. Summary of several methods:
Capacitors separate the through-pass and cause floating. If the capacitor is not connected to DC, it will cause pressure difference and static electricity accumulation, which will make your hands numb when touching the case. If the capacitor and the magnetic beads are connected in parallel, it is superfluous, because the magnetic beads will pass through and the capacitor will be invalid. If it is connected in series, it is nondescript.
The inductor has a large volume, many stray parameters, unstable characteristics, poor control of discrete distribution parameters, and large volume. Inductance is also a notch, LC resonance (distributed capacitance), which has special effects on noise.
The equivalent circuit of the magnetic bead is equivalent to a band-rejection trap, which only suppresses the noise at a certain frequency. If the noise cannot be predicted, how to choose the model. Moreover, the frequency of the noise is not necessarily fixed, so the magnetic bead is not a good choice. s Choice.
A resistance of 0 ohm is equivalent to a very narrow current path, which can effectively limit the loop current and suppress noise. Resistance has an attenuation effect in all frequency bands (0 ohm resistance also has impedance), which is stronger than magnetic beads.
In short, the key is that the analog ground and digital ground should be grounded at one point. It is recommended that different types of grounds are connected with 0 ohm resistors; magnetic beads are used when high-frequency devices are introduced into the power supply; small capacitors are used for high-frequency signal line coupling; and inductors are used for high-power and low-frequency.
Grounding treatment of various grounds in the circuit
In addition to the correct grounding design and installation, the grounding of various signals must be handled correctly. In the control system, there are roughly the following types of ground wires:
(1) Digital ground: also called logical ground, which is the zero potential of various switch (digital) signals.
(2) Analog ground: It is the zero potential of various analog signals.
(3) Signal ground: usually the ground of the sensor.
(4) AC ground: the ground wire of the AC power supply, which is usually a noise-generating ground.
(5) DC ground: the ground of the DC power supply.
(6) Shielding ground: also called chassis ground, designed to prevent electrostatic induction and magnetic field induction.
The above ground wire treatment is an important issue in system design, installation, and debugging. Here are some views on the grounding issue:
(1) One point grounding should be adopted for the control system. Generally, the high-frequency circuit should be grounded at multiple points nearby, and the low-frequency circuit should be grounded at one point. In low-frequency circuits, the inductance between wiring and components is not a big problem. However, the interference caused by the loop formed by grounding is great. Therefore, one point is often used as the grounding point; but one point of grounding is not suitable for high-frequency, because high-frequency When the ground wire has inductance, the ground wire impedance is increased, and at the same time, inductive coupling occurs between the ground wires. Generally speaking, if the frequency is below 1MHz, one-point grounding can be used; when it is higher than 10MHz, multiple-point grounding can be used; between 1～10MHz, one-point grounding or multiple grounding can be used.
(2) The AC ground and signal ground cannot be shared. Because there will be several mV or even several V voltage between two points of a section of power ground wire, this is a very important interference for low-level signal circuits, so it must be isolated and prevented.
(3) Comparison of floating and grounding. The whole machine floats, that is, all parts of the system float to the ground. This method is simple, but the insulation resistance between the entire system and the ground cannot be less than 50MΩ. This method has a certain degree of anti-interference ability, but once the insulation drops, it will bring interference. Another method is to ground the chassis and float the rest. This method has strong anti-interference ability and is safe and reliable, but it is more complicated to implement.
(4) Analog ground. The connection of the analog ground is very important. In order to improve the ability to resist common mode interference, shielding floating technology can be used for analog signals. The grounding treatment of specific analog signals must be designed in strict accordance with the requirements of the operation manual.
(5) Shield ground. In the control system, in order to reduce the capacitive coupling noise in the signal, and accurately detect and control it, it is very necessary to shield the signal. According to the shielding purpose, the shielding ground connection method is different. Electric field shielding solves the problem of distributed capacitance and is generally connected to the earth; electromagnetic field shielding mainly avoids the interference of high-frequency electromagnetic field radiation such as radar and radio. It is made of low-resistance metal material with high conductivity and can be connected to the earth. Magnetic field shielding is used to prevent magnetic induction such as magnets, motors, transformers, coils, etc. The shielding method is to close the magnetic circuit with high-permeability materials, and it is generally better to connect to the earth. When the signal circuit is grounded at one point, the shielding layer of the low-frequency cable should also be grounded at one point. If there is more than one location on the shielding layer of the cable, noise current will be generated, which will form a source of noise interference. When a circuit has an ungrounded signal source connected to the grounded amplifier in the system, the shield of the input end should be connected to the common end of the amplifier; on the contrary, when the grounded signal source is connected to the ungrounded amplifier in the system, the input of the amplifier The terminal should also be connected to the common terminal of the signal source.
For the grounding of the electrical system, it should be classified according to the requirements and purpose of grounding. Different types of grounding cannot be simply and arbitrarily connected together. Instead, it must be divided into several independent grounding subsystems, each of which has its common grounding point. Or grounding trunks, which are connected together at the end to implement total grounding.
Q1: Why is it grounded?
Answer: The introduction of grounding technology was originally a protective measure to prevent electrical or electronic equipment from being struck by lightning. The purpose was to introduce the lightning current generated by lightning to the ground through the lightning rod, thereby protecting the building. At the same time, grounding is also an effective means to protect personal safety. When the phase line caused by some reason (such as poor wire insulation, line aging, etc.) touches the equipment shell, the equipment shell will have dangerous voltages. The generated fault current will flow through the PE line to the ground, thereby playing a protective role. With the development of electronic communication and other digital fields, it is no longer sufficient to consider only lightning protection and safety in the grounding system. For example, in a communication system, the interconnection of signals between a large number of devices requires each device to have a reference ground as the signal reference ground. And with the complexity of electronic equipment, the signal frequency is getting higher and higher. Therefore, in the grounding design, special attention must be paid to electromagnetic compatibility issues such as mutual interference between signals. Otherwise, improper grounding will seriously affect the reliability of system operation. Sex and stability. Recently, the concept of "ground" has also been introduced in the signal return technology of high-speed signals.
Q2: Definition of grounding
Answer: In modern grounding concepts, for line engineers, the meaning of this term is usually'reference point of line voltage'; for system designers, it is often a cabinet or rack; for electrical engineers, it is It means green safety ground wire or connected to the earth. A more general definition is "grounding is a low-impedance path for current to return to its source". Note that the requirements are "low impedance" and "path".
Q3: Common grounding symbols
Answer: PE, PGND, FG-protective ground or chassis; BGND or DC-RETURN-DC-48V (+24V) power (battery) return; GND-working ground; DGND-digital ground; AGND-analog ground; LGND- Lightning protection ground. GND is often set as the voltage reference base point in the circuit. In the electrical sense, GND is divided into power ground and signal ground. PG is the abbreviation of Power Ground. The other is Signal Ground. In fact, they may be connected together (not necessarily mixed together!). The two names are mainly to facilitate the analysis of the circuit. Furthermore, there are two kinds of "grounds" that must be distinguished due to different circuit forms: digital ground and analog ground. Both digital ground and analog ground have signal ground and power ground. Between digital ground and analog ground, some circuits can be connected directly, some circuits need to be connected with reactors, and some circuits cannot be connected.
Q4: Appropriate grounding method
Answer: There are many grounding methods, including single-point grounding, multi-point grounding and mixed types of grounding. The single-point grounding is divided into series single-point grounding and parallel single-point grounding. Generally speaking, single-point grounding is used for simple circuits, grounding distinctions between different functional modules, and low-frequency (f10MHz) circuits, it is necessary to use multi-point grounding or multilayer boards (complete ground plane layers).
Q5: Introduction to signal reflow and cross-segmentation
Answer: For an electronic signal, it needs to find a path for the lowest impedance current to return to the ground, so how to deal with this signal return becomes very critical.
First, according to the formula, it can be known that the radiation intensity is proportional to the loop area, that is, the longer the path that reflow needs to take, the larger the loop formed, and the greater the interference with external radiation. Therefore, when the PCB is laid out It is necessary to reduce the area of the power circuit and signal circuit as much as possible.
Second, for a high-speed signal, providing good signal return can ensure its signal quality. This is because the characteristic impedance of the transmission line on the PCB is generally calculated based on the ground layer (or power layer). There is a continuous ground plane nearby so that the impedance of this line can remain continuous. If there is no ground reference near a segment of the line, the impedance will change. Discontinuous impedance will affect the integrity of the signal. Therefore, when wiring, the high-speed line should be allocated to the layer close to the ground plane, or one or two ground lines should be walked beside the high-speed line to play the function of shielding and providing nearby return.
Third, why do we try not to split across the power supply when wiring? This is also because after the signal crosses different power layers, its return path will be very long and it will be easily interfered. Of course, it is not strictly required that the power supply cannot be divided. It is possible for low-speed signals, because the interference generated can be ignored compared to the signal. For high-speed signals, check carefully and try not to cross over. You can adjust the wiring of the power supply part. (This is for the multi-layer board multiple power supply situation)
Q6: Why should I separate the analog ground from the digital ground, and how to separate it?
Answer: Both analog and digital signals return to the ground. Because the digital signal changes quickly, the noise caused on the digital ground will be very large. The analog signal requires a clean ground reference. If the analog ground and digital ground are mixed together, noise will affect the analog signal. Generally speaking, the analog ground and digital ground should be treated separately, and then connected together through a thin trace, or connected together at a single point. The general idea is to try to prevent the noise on the digital ground from fleeing to the analog ground. Of course, this is not a very strict requirement that the analog ground and digital ground must be separated. If the digital ground near the analog part is still very clean, it can be combined.
Q7: How to ground the signal on the board?
Answer: For general devices, the nearest grounding is the best. After adopting a multi-layer board design with a complete ground plane, it is very easy to ground general signals. The basic principle is to ensure the continuity of the wiring and reduce the excess Number of holes; close to the ground plane or power plane, etc.
Q8: How to ground the interface components of the board?
Answer: Some boards have external input and output interfaces, such as serial port connectors, network port RJ45 connectors, etc. If their grounding is not designed well, it will also affect normal operation, such as network port interconnection errors. , Packet loss, etc., and will become an external electromagnetic interference source, sending the noise in the board to the outside. Generally speaking, an independent interface ground will be separated separately, and the connection with the signal ground will be connected by a thin trace, and a 0 ohm or small resistance resistor can be connected in series. Thin traces can be used to block signal ground noise from passing to the interface ground. Similarly, the filtering of the interface ground and interface power should be carefully considered.
Q9: How to ground the shielding layer of the cable with shielding layer?
Answer: The shielding layer of the shielded cable should be connected to the interface ground of the board instead of the signal ground. This is because there are various noises on the signal ground. If the shielding layer is connected to the signal ground, the noise voltage will drive the common mode current along the shielding layer. External interference, so poorly designed cables are generally the largest noise output source of electromagnetic interference. Of course, the premise is that the interface ground should also be used for marking in the very clean hybrid circuit. VCC represents the analog signal power supply, GND represents the analog signal ground, VDD represents the digital signal power supply, and VSS represents the digital power supply ground. VCC mainly refers to the power supply of the Bipolar circuit, and C refers to the collector of the collector. The power supply is generally connected to the collector of the NPN (or the emitter of the PNP). When the integrated circuit first appeared, only the NPN tube was used, and the PNP tube was integrated later. VDD/VSS generally represents the power supply and "ground" of the MOS circuit, and D/S respectively represents the Drain/Source of the MOS tube.
1. Explanation
VCC: C=circuit means the meaning of the circuit, that is, the voltage connected to the circuit;
VDD: D=device means the device, that is, the working voltage inside the device;
VSS: S=series means common connection, usually refers to the voltage of the common ground terminal of the circuit.
2. Description
1. For digital circuits, VCC is the supply voltage of the circuit, VDD is the working voltage of the chip (usually Vcc>Vdd), and VSS is the grounding point.
2. Some ICs have both VDD pin and VCC pin, indicating that this device has its own voltage conversion function.
3. In the field effect transistor (or CMOS device), VDD is the drain and VSS is the source. VDD and VSS refer to component pins, not the supply voltage.
VDD: power supply voltage (unipolar device); power supply voltage (4000 series digital circuit); drain voltage (field effect tube)
VCC: Power supply voltage (bipolar device); Power supply voltage (74 series digital circuit); Voice Controlled Carrier (Voice ControlledCarrier)
VSS: Ground or negative power supply
VEE: Negative voltage power supply; the source of the field effect tube (S)
VPP: Program/Erase voltage.
Detailed explanation:
In electronic circuits, VCC is the supply voltage of the circuit, and VDD is the working voltage of the chip:
VCC: C=circuit indicates the meaning of the circuit, that is, the voltage connected to the circuit, D=device indicates the meaning of the device, that is, the working voltage inside the device, in ordinary electronic circuits, generally Vcc>Vdd!
VSS: S=series means common connection, which is negative.
Some ICs have both VCC and VDD, and this device has a voltage conversion function.
In the "field effect" or CMOS component, VDD is the drain pin of CMOS, and VSS is the source pin of CMOS. This is the component pin symbol. It does not have the name "VCC". Your question contains 3 symbols. , VCC / VDD /VSS, this is obviously a circuit symbol
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