What is the essence of GND in the circuit
Time:2022-11-22
Views:1543
Ask a simple but difficult circuit question: What is the essence of GND, the ground wire in the circuit?
In the PCB Layout wiring process, engineers will face different GND processing.
Why is this? In the circuit principle design stage, in order to reduce mutual interference between circuits, engineers usually introduce different GND ground wires as 0V reference points of different functional circuits to form different current loops.
01 Classification of GND ground wires
1. Analog ground wire AGND
The analog ground wire AGND is mainly used in the analog circuit, such as the ADC acquisition circuit of analog sensor, the operation amplification proportional circuit, etc.
In these analog circuits, because the signal is analog signal and weak signal, it is easy to be affected by the large current of other circuits. If not distinguished, large current will produce large voltage drop in analog circuit, which will cause analog signal distortion, and may cause functional failure of analog circuit.
2. Digital ground wire DGND
The digital ground wire DGND is obviously compared with the analog ground wire AGND. It is mainly used in the digital circuit, such as key detection circuit, USB communication circuit, microcontroller circuit, etc.
The reason why the digital ground wire DGND is set up is that the digital circuit has a common feature, which belongs to discrete switching signal. Only the digital "0" and the digital "1" are distinguished, as shown in the figure below.
In the process of changing from the digital "0" voltage to the digital "1" voltage, or from the digital "1" voltage to the digital "0" voltage, the voltage has a change. According to Maxwell‘s electromagnetic theory, magnetic fields will be generated around the changed current, which forms EMC radiation to other circuits.
No way. In order to reduce the EMC radiation impact of the circuit, a separate digital ground wire DGND must be used to effectively isolate other circuits.
3. Power ground wire PGND
Both analog ground wire AGND and digital ground wire DGND are low-power circuits. In high-power circuits, such as motor drive circuit, solenoid valve drive circuit, etc., there is also a separate reference ground wire, which is called power ground wire PGND.
High power circuit, as its name implies, is a circuit with relatively large current. It is obvious that large current is easy to cause ground offset between different functional circuits, as shown in the figure below.
Once there is ground offset in the circuit, the original 5V voltage may not be 5V, but become 4V. Because the 5V voltage refers to the GND ground wire 0V, if the ground offset causes the GND ground wire to rise from 0V to 1V, then the previous 5V (5V-0V=5V) voltage will become the current 4V (5V-1V=4V).
4. Power ground wire GND
Analog ground wire AGND, digital ground wire DGND and power ground wire PGND are classified as DC ground wire GND. These different kinds of ground wires must finally be gathered together as the 0V reference ground wire of the whole circuit, which is called the power ground wire GND.
Power supply is the energy source of all circuits. The voltage and current required for the operation of all circuits are from the power supply. Therefore, the ground wire GND of the power supply is the 0V voltage reference point of all circuits.
This is why other types of ground wires, whether analog ground wire AGND, digital ground wire DGND or power ground wire PGND, need to be combined with power ground wire GND at last.
5. AC ground wire CGND
The AC ground wire CGND generally exists in circuit projects containing AC power, such as AC-DC AC to DC power circuit.
AC-DC power circuit is divided into two parts. The former stage of the circuit is the AC part, and the latter stage is the DC part, which forced the formation of two ground wires, one is the AC ground wire, and the other is the DC ground wire.
The AC ground wire is used as the 0V reference point of the AC circuit part, and the DC ground wire is used as the 0V reference point of the DC circuit part. Generally, in order to unify a GND ground wire in the circuit, engineers will connect the AC ground wire with the DC ground wire through a coupling capacitor or inductor.
6. Earthwire EGND
The safe voltage of the human body is below 36V. If the voltage above 36V is applied to the human body, the human body will be damaged. This is a common sense of safety for engineers in developing and designing circuit project schemes.
In order to enhance the safety factor of the circuit, engineers generally use the ground wire EGND of the earth in projects with high voltage and large current, such as in the circuits of household electric fans, refrigerators, televisions, etc. The socket with earth wire EGND protection function is shown in the following figure.
Why are there three terminal blocks for the socket of household appliances? 220V AC only needs two live wires and zero wires. Why is the socket three terminal blocks?
Three connection terminals of the socket, two of which are used for 220V live wire and zero wire, and the other terminal is the ground wire EGND for protection.
The chip brother needs to point out that the earth ground wire EGND is only connected to our earth and plays a high-voltage protection role. It is not involved in the circuit function of the project and has nothing to do with the circuit function.
Therefore, the ground wire EGND is obviously different from other types of ground wire GND in circuit meaning.
02 Study the principle of GND
Engineers may ask, how can a GND ground wire have so many distinctions, and how can a simple circuit problem be so complicated?
Why do we need to introduce so many subdivided GND ground wire functions?
Engineers generally name this kind of GND ground wire design problem simply and uniformly as GND, which is not distinguished in the schematic design process. As a result, it is difficult to effectively identify GND ground wires with different circuit functions when PCB wiring, and directly and simply connect all GND ground wires together.
Although this is easy to operate, it will cause a series of problems:
1. Signal crosstalk
If the ground wire GND with different functions is directly connected together, the high-power circuit will affect the 0V reference point GND of the low-power circuit through the ground wire GND, which will generate crosstalk between the signals of different circuits.
2. Signal accuracy
Analog circuit, the core index of its examination is the signal accuracy. Without precision, analog circuits will lose their original functional significance.
As the ground wire CGND of the AC power supply is a sine wave, it fluctuates up and down periodically. Its voltage also fluctuates up and down. It is not always maintained at a constant 0V like the DC ground wire GND.
Connect the ground wires GND of different circuits together. The periodically changing AC ground wire CGND will drive the ground wire AGND of the analog circuit to change, which affects the voltage accuracy value of the analog signal.
3. EMC experiment
The weaker the signal, the weaker the external electromagnetic radiation EMC; The stronger the signal, the stronger the external electromagnetic radiation EMC.
If the ground wires GND of different circuits are connected together, the ground wire GND of the strong signal circuit directly interferes with the ground wire GND of the weak signal circuit. The consequence is that the original weak signal electromagnetic radiation EMC has also become a signal source of strong external electromagnetic radiation, increasing the difficulty of the circuit processing EMC experiment.
4. Circuit reliability
Between circuit systems, the fewer signal connection parts, the stronger the ability of independent operation of the circuit; The more parts of the signal are connected, the weaker the ability of the circuit to operate independently.
Imagine that if there is no intersection between circuit system A and circuit system B, the function of circuit system A obviously cannot affect the normal operation of circuit system B, and the function of circuit system B cannot affect the normal operation of circuit system A.
This is like a pair of strange men and women. Before they become lovers, the girl‘s emotional changes will not affect the boy‘s mood, because they have no intersection.
If the ground wires of circuits with different functions are connected together in the circuit system, it is equivalent to increasing a link of interference between circuits, that is, reducing the reliability of circuit operation.
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