Detailed analysis of the relationship between power ground and signal ground
Time:2022-02-22
Views:2206
The power ground is mainly for the path of the power circuit current. Generally speaking, the current flowing through the power ground is large, while the signal ground is mainly for the path flowing through the return of the communication signal between two chips or modules. Generally speaking, the current flowing through the signal ground is very small. In fact, both of them are GND. The reason why they are separated is that, The purpose is to let everyone understand that when laying PCB boards, it is necessary to clearly understand the paths of power supply and signal return flow, and then consider how to avoid the common return path of power supply and signal when laying PCB boards. If they are shared, It may cause a large current on the power supply ground to produce a voltage difference on the signal ground (it can be explained that the conductor has impedance, which is only a small resistance value, but if the current flowing is large, it will also produce a potential difference on the conductor, which is also called common impedance interference), so that the real potential of the signal ground is higher than 0V. If the potential of the signal ground is large, It is possible that the signal is originally high level, but it is misjudged as low level.
One、 Different grounding
One、 Different grounding
Of course, the ground of the power supply is not clean, which also avoids misjudgment of the signal due to interference. Therefore, pay a little attention to the two when wiring. Generally speaking, even together, there will be no big problem, because the threshold of digital circuit is high. In addition to the correct grounding design and installation, it is also necessary to correctly handle the grounding of various signals. In the control system, there are roughly the following ground wires:
1. Digitally
Digital ground: also known as logical ground, it is the zero potential of various switching value (digital value) signals.
2 analog ground
3. Signally
4. Exchange place
5. DC ground
6. Shielding ground
Two、 Some views on Grounding
1. The control system should be grounded at one point
2. AC ground and signal ground cannot be shared.
3. Comparison between floating ground and grounding
4. Simulated
5. Shielding ground
2 analog ground
Analog ground: it is the zero potential of various analog signals.
3. Signally
Signal ground: usually the ground of the sensor.
4. Exchange place
AC ground: the ground wire of AC power supply, which is usually the ground that produces noise.
5. DC ground
DC ground: the ground of DC power supply.
6. Shielding ground
Shielding ground: also known as enclosure ground, it is set to prevent electrostatic induction and magnetic field induction.
Two、 Some views on Grounding
The above ground wire treatment is an important problem in system design, installation and commissioning. Here are some views on grounding:
1. The control system should be grounded at one point
Generally, the high-frequency circuit shall be grounded at multiple points nearby, and the low-frequency circuit shall 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 has a great impact. Therefore, one point is often used as the grounding point; However, one point grounding is not suitable for high frequency, because at high frequency, there is inductance on the ground wire, which increases the ground wire impedance and produces inductive coupling between the ground wires. Generally speaking, if the frequency is below 1MHz, one point of grounding can be used; When it is higher than 10MHz, multi-point grounding shall be adopted; One point grounding or multi-point grounding can be used between 1 ~ 10MHz.
2. AC ground and signal ground cannot be shared.
Since there will be several MV or even several V voltages between two points of a section of power supply ground wire, this is a very important interference for low-level signal circuit, so it must be isolated and prevented.
3. Comparison between floating ground and grounding
The whole machine floats, that is, each part of the system floats with the earth. This method is simple, but the insulation resistance between the whole system and the earth shall not be less than 50m Ω. This method has certain anti-interference ability, but once the insulation drops, it will bring interference. There is another way to ground the casing and the rest. This method has strong anti-interference ability, safety and reliability, but its implementation is complex.
4. Simulated
The grounding method of analog grounding is very important. In order to improve the ability of anti common mode interference, shielded floating technology can be used for analog signals. The grounding treatment of specific analog signals shall be designed in strict accordance with the requirements of the operation manual.
5. Shielding ground
In the control system, in order to reduce the capacitive coupling noise in the signal and accurately detect and control, it is very necessary to adopt shielding measures for the signal. According to different shielding purposes, the grounding method of shielding is also different. Electric field shielding solves the problem of distributed capacitance, which is generally grounded; Electromagnetic field shielding mainly avoids the radiation interference of high-frequency electromagnetic fields such as radar and radio. It is made of low resistance metal material with high conductivity and can be grounded. Magnetic field shielding is used to prevent magnetic induction of magnets, motors, transformers and coils. The shielding method is to close the magnetic circuit with high magnetic conductivity materials, which is generally better to be grounded. When the signal circuit is grounded at one point, the shielding layer of low-frequency cable shall also be grounded at one point.
If there is more than one shielding layer of the cable, noise current will be generated to form a noise interference source. When a circuit has an ungrounded signal source connected to the grounded amplifier in the system, the shielding of the input end shall be connected to the common end of the amplifier; On the contrary, when the grounded signal source is connected with the 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 system shall be classified according to the requirements and purposes of grounding. Different types of grounding cannot be simply and arbitrarily connected together, but shall be divided into several independent grounding subsystems. Each subsystem has its common grounding point or grounding trunk line, and finally connected together to implement general grounding.
If there is more than one shielding layer of the cable, noise current will be generated to form a noise interference source. When a circuit has an ungrounded signal source connected to the grounded amplifier in the system, the shielding of the input end shall be connected to the common end of the amplifier; On the contrary, when the grounded signal source is connected with the 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 system shall be classified according to the requirements and purposes of grounding. Different types of grounding cannot be simply and arbitrarily connected together, but shall be divided into several independent grounding subsystems. Each subsystem has its common grounding point or grounding trunk line, and finally connected together to implement general grounding.
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