Useful Information | Power ground, signal ground, earth - what are the differences between these three types of "ground"?

Latest update time：2020-04-17

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Power ground and signal ground

The power ground mainly refers to the path taken by the power loop current, and the current flowing through the power ground is relatively large. The signal ground mainly refers to the path taken by the return flow of the communication signal between two chips or modules, and the current flowing through the signal ground is very small.

In fact, both are GND. The reason why they are mentioned separately is to make everyone understand: when laying out the PCB board, you must clearly understand the paths that the power and signal return flows through, and then consider how to avoid the power and signal sharing the return path when laying out the board.

If they are shared, it is possible that a large current on the power supply ground will generate a voltage difference on the signal ground. (It can be explained as follows: the wire has impedance, but it is only a very small resistance. However, if the current flowing through it is large, a potential difference will also be generated on this wire. This is also called common impedance interference.)

Make the actual potential of the signal ground higher than 0V. If the potential of the signal ground is large, it is possible that the signal, which is originally at a high level, may be mistakenly judged as a low level.

Of course, the power ground is not clean, so it can also avoid signal misjudgment due to interference. So just pay a little attention to the wiring of the two grounds. Generally speaking, even if they are together, there will be no big problems, because the threshold of digital circuits is higher.

Various "grounds", various "GND"

GND is the abbreviation of the ground terminal of the wire, representing the ground wire or the neutral wire. GND (Ground) on the circuit diagram and the circuit board represents the ground wire or the neutral wire. GND means the common terminal, or the ground, but this ground is not the real ground. It is a ground assumed for application. For the power supply, it is the negative pole of the power supply. It is different from the earth. Sometimes it needs to be connected to the earth, and sometimes it does not, depending on the specific situation.

The signal grounding of a device may be a point or a piece of metal in the device as the ground reference point of the signal, which provides a common reference potential for all signals in the device.

There are single-point grounding, multi-point grounding, floating grounding and mixed grounding:

Single-point grounding: refers to the entire circuit system has only one physical point defined as the ground reference point, and all other points that need to be grounded are directly connected to this point. In low-frequency circuits, there will not be much impact between wiring and components. Usually, circuits with frequencies less than 1MHz use single-point grounding.
Multi-point grounding: refers to that each grounding point in an electronic device is directly connected to the ground plane closest to it (i.e. the metal bottom plate of the device). In high-frequency circuits, parasitic capacitance and inductance have a greater impact. Usually, circuits with frequencies greater than 10MHz often use multi-point grounding.
Floating ground: that is, the ground of the circuit is not connected to the earth by a conductor. Floating ground can make the isolation resistance between the power ground (strong electric ground) and the signal ground (weak electric ground) very large, so it can prevent the electromagnetic interference caused by the coupling of the common ground impedance circuit. The disadvantage is that the circuit is susceptible to parasitic capacitance, which causes the ground potential of the circuit to change and increases the inductive interference to the analog circuit. A compromise is to connect a large-resistance discharge resistor between the floating ground and the common ground to release the accumulated charge. Pay attention to controlling the impedance of the discharge resistor. Too low a resistance will affect the qualification of the equipment leakage current.
Hybrid grounding: Hybrid grounding makes the grounding system present different characteristics at low and high frequencies, which is necessary in broadband sensitive circuits. Capacitors have higher impedance to low frequencies and DC, so they can avoid the formation of ground loops between the two modules. When separating the DC ground and the RF ground, connect the DC ground of each subsystem to the RF ground through a 10-100nF capacitor. The two grounds should be connected at a low impedance at one point, and the connection point should be selected at the point where the highest flip speed (di/dt) signal exists.

"Ground" is a very important concept in electronic technology. Since there are many types and functions of "ground", it is easy to confuse, so let's summarize the concept of "ground". "Grounding" includes signal grounding inside the device and grounding the device. The two concepts are different and the purposes are also different. The classic definition of "ground" is "an equipotential point or plane that serves as a reference for a circuit or system."

Signal "ground", also known as reference "ground", is the reference point of zero potential and is also the common end of the circuit signal loop.

DC ground: DC circuit "ground", zero potential reference point.
AC ground: The neutral wire of AC power should be distinguished from the ground wire.
Power ground: zero potential reference point for high current network devices and power amplifier devices.
Analog ground: The zero potential reference point for amplifiers, sample-and-holds, A/D converters, and comparators.
Digital ground: also called logic ground, is the zero potential reference point of the digital circuit.
"Hot ground": The switching power supply does not require an industrial frequency transformer. The "ground" of its switching circuit is related to the municipal power grid, the so-called "hot ground", which is electrified.
"Cold ground": Since the high-frequency transformer of the switching power supply isolates the input and output ends; and since its feedback circuit often uses optocouplers, it can both transmit feedback signals and isolate the "grounds" of both sides; therefore, the ground at the output end is called "cold ground" and it is not electrified.

The purpose of grounding equipment

In engineering practice, in addition to carefully considering the signal grounding inside the equipment, the signal ground of the equipment and the chassis are usually connected to the earth, and the earth is used as the grounding reference point of the equipment. The purpose of grounding the equipment is:

Protective grounding is to make a good electrical connection between the metal shell (or frame) that is not charged during normal operation of the equipment and the grounding device. It is a wiring method set up to protect the safety of personnel. One end of the protective "ground" wire is connected to the housing of the appliance, and the other end is reliably connected to the earth.
Anti-static grounding discharges the charge accumulated on the chassis to prevent the accumulation of charge from increasing the chassis potential and causing unstable circuit operation.
Shielding grounding prevents the device from changing its potential to the earth under the influence of the external electromagnetic environment, causing unstable operation of the device. In addition, there are lightning protection grounding and audio-only grounding in audio equipment, etc.

Grounding related issues

1. The control system should adopt single-point grounding

In general, high-frequency circuits should be grounded at multiple points nearby, and low-frequency circuits should be grounded at one point. In low-frequency circuits, the inductance between wiring and components is not a big problem, but the interference of the loop formed by grounding is very large, so one point is often used as the grounding point. However, single-point grounding is not suitable for high frequencies, because at high frequencies, the ground wire has inductance, which increases the ground wire impedance, and at the same time, inductive coupling is generated between the ground wires. Generally speaking, when the frequency is below 1MHz, single-point grounding can be used; when it is higher than 10MHz, multi-point grounding is used; between 1 and 10MHz, single-point grounding or multi-point grounding can be used.

2. The AC ground and signal ground cannot be shared

Since there will be a voltage of several mV or even several V between two points of a power ground line, this is a very important interference for low-level signal circuits, so it must be isolated and prevented.

3. Comparison between floating ground and grounding

The whole machine is floating, that is, each part of the system is floating on the ground. This method is simple, but the insulation resistance between the whole system and the ground cannot be less than 50MΩ. This method has a certain anti-interference ability, but once the insulation decreases, it will cause interference. Another method is to ground the casing and float the rest of the system. This method has strong anti-interference ability, is safe and reliable, but it is more complicated to implement.

4. Analog ground

The connection method of analog ground is very important. In order to improve the ability to resist common mode interference, shielded floating technology can be used for analog signals. The grounding treatment of specific analog signals must be designed strictly in accordance with the requirements of the operation manual.

5. Shielded ground

In order to reduce the capacitive coupling noise in the signal, accurately detect and control it in the control system, it is necessary to adopt shielding measures for the signal. Depending on the purpose of shielding, the connection method of the shielding ground is also different. Electric field shielding solves the problem of distributed capacitance and is generally connected to the ground; electromagnetic field shielding mainly avoids interference from high-frequency electromagnetic field radiation such as radar and radio stations. It is made of low-resistance metal materials with high conductivity and can be connected to the ground. Magnetic field shielding is used to prevent magnetic induction of magnets, motors, transformers, coils, etc. The shielding method is to close the magnetic circuit with high magnetic permeability materials, and it is generally better to connect to the ground. 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 the shielding layer of the cable has more than one location, noise current will be generated, forming a noise interference source.

When a circuit has an ungrounded signal source connected to a grounded amplifier in the system, the shield of the input end should be connected to the common end of the amplifier; conversely, when a grounded signal source is connected to an ungrounded amplifier in the system, the input end of the amplifier should also be connected to the common end of the signal source.

The grounding of electrical systems must be classified according to the requirements and purposes of grounding. Different types of grounding cannot be simply and arbitrarily connected together. Instead, they must be divided into several independent grounding subsystems. Each subsystem has its own common grounding point or grounding trunk line. Finally, they are connected together to implement total grounding.

Related Questions Answered

1. Why is grounding necessary?

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 into the earth 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 insulation of the wire, aging of the line, etc.) touches the equipment shell, a dangerous voltage will be generated on the equipment shell, and the fault current generated will flow through the PE line to the earth, thereby playing a protective role. With the development of electronic communications and other digital fields, only considering lightning protection and safety in the grounding system is far from meeting the requirements.

For example, in a communication system, the interconnection of signals between a large number of devices requires that each device must have a reference "ground" as the reference ground for the signal. Moreover, as electronic equipment becomes more complex, signal frequencies are getting higher and higher. Therefore, in grounding design, electromagnetic compatibility issues such as mutual interference between signals must be given special attention. Otherwise, improper grounding will seriously affect the reliability and stability of system operation. Recently, the concept of "ground" has also been introduced in the signal return technology of high-speed signals.

2. Definition of grounding

In the modern concept of grounding, for line engineers, the term usually means 'the reference point of the line voltage'; for system designers, it is often a cabinet or rack; for electrical engineers, it means the green safety ground wire or the ground. A more common definition is "grounding is a low impedance path for current to return to its source." Note that the requirements are "low impedance" and "pathway".

3. Common grounding symbols

PE, PGND, FG - protective ground or casing; BGND or DC-RETURN - DC - 48V (+24V) power supply (battery) return; GND - working ground; DGND - digital ground; AGND - analog ground; LGND - lightning protection ground; GND is often set as the voltage reference point in the circuit.

In electrical terms, 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 for the convenience of circuit analysis. Furthermore, there are two kinds of "ground" that must be distinguished due to different circuit forms: digital ground and analog ground. Both digital ground and analog ground have two situations: signal ground and power ground. Between digital ground and analog ground, some circuits can be directly connected, some circuits must be connected with a reactor, and some circuits cannot be connected.

4. Appropriate grounding method

There are many ways to ground, including single-point grounding, multi-point grounding and mixed types of grounding. 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 distinction between different functional modules, and low-frequency (f<1MHz) electronic circuits. When designing high-frequency (f>10MHz) circuits, multi-point grounding or multi-layer boards (complete ground plane layers) must be used.

5. Introduction to signal return and cross-segmentation

For an electronic signal, it needs to find a path with the lowest impedance for the current to flow back to the ground, so how to deal with this signal return becomes very critical.

First, according to the formula, the radiation intensity is proportional to the loop area, that is, the longer the path the return current needs to take, the larger the loop formed, and the greater the interference to external radiation. Therefore, when laying out the PCB, the area of the power supply loop and the signal loop should be reduced 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 with the ground layer (or power layer) as a reference. If there is a continuous ground plane near the high-speed line, the impedance of this line can remain continuous. If there is no ground reference near a section of the line, the impedance will change, and the discontinuous impedance will affect the integrity of the signal. Therefore, when routing, the high-speed line should be allocated to the layer close to the ground plane, or one or two ground lines should be run in parallel next to the high-speed line to provide shielding and nearby return.

Third, why do we say that when wiring, try not to cross the power split? This is because after the signal crosses different power layers, its return path will be very long and easily interfered with. Of course, it is not strictly required not to cross the power split. For low-speed signals, it is okay because the interference generated can be ignored compared to the signal. For high-speed signals, you must carefully check and try not to cross. You can adjust the routing of the power supply part. (This is for the situation of multiple power supplies on multi-layer boards)

6. Why should analog ground and digital ground be separated, and how to separate them?

Both analog and digital signals need to return to the ground, because digital signals change quickly, which will cause a lot of noise on the digital ground, and analog signals need a clean ground reference to work. If the analog and digital grounds are mixed together, the noise will affect the analog signal.

Generally speaking, analog ground and digital ground should be processed separately, and then connected together through thin traces, or connected at a single point. The general idea is to try to block the noise on the digital ground from reaching 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, they can be combined.

7.How to ground the signals on the board?

For general devices, it is best to ground them nearby. 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 routing, reduce the number of vias, and be close to the ground plane or power plane, etc.

8.How to ground the interface devices of the board?

Some boards have external input and output interfaces, such as serial port connectors, network port RJ45 connectors, etc. If their grounding design is not good, it will affect normal operation, such as bit errors and packet loss in network port interconnection, and will become an external electromagnetic interference source, sending the noise inside the board to the outside. Generally speaking, an independent interface ground will be separated out, and the connection with the signal ground is connected by a thin trace, which can be connected in series with a zero-ohm or small-resistance resistor. The thin trace can be used to block the noise on the signal ground from passing to the interface ground. Similarly, the filtering of the interface ground and interface power supply should also be carefully considered.

9. How to ground the shield of a shielded cable?

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 to interfere outward along the shielding layer. Therefore, poorly designed cables are generally the largest noise output source of electromagnetic interference. Of course, the premise is that the interface ground must also be very clean.

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