Xiaozhi Science Popularization丨How many types of ground are there in circuit design? The meaning and essence of various GNDs

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Xiaozhi Science Popularization丨How many types of ground are there in circuit design? The meaning and essence of various GNDs [Copy link]

Introduction: GND (Ground) on circuit diagrams and circuit boards represents the ground line or zero line. GND means the common terminal, or the ground, but this ground is not the real ground, but a ground assumed for application. 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.

During the PCB Layout process, engineers will face different GND ground wire treatments. Why is this? During the circuit principle design stage, in order to reduce mutual interference between circuits, engineers generally introduce different GND ground wires as the 0V reference point of different functional circuits to form different current loops.

Classification of GND ground wires

1. Analog ground wire AGND

Analog ground wire AGND is mainly used in analog circuit parts, such as ADC acquisition circuits of analog sensors, operational amplifier proportional circuits, etc. In these analog circuits, since the signal is an analog signal and a weak signal, it is easily affected by the large current of other circuits.

If it is not distinguished, the large current will produce a large voltage drop in the analog circuit, which will distort the analog signal and may cause the analog circuit to fail in serious cases.

2. Digital ground wire DGND

Digital ground wire DGND is obviously relative to the analog ground wire AGND, and is mainly used in digital circuit parts, such as key detection circuits, USB communication circuits, single-chip microcomputer circuits, etc. The

reason for setting up the digital ground wire DGND is that digital circuits have a common feature, which are all discrete switching signals, and only digital "0" and digital "1" are distinguished.

In the process of jumping from digital "0" voltage to digital "1" voltage, or from digital "1" voltage to digital "0" voltage, the voltage changes. According to Maxwell's electromagnetic theory, a magnetic field will be generated around the changing current, which will form EMC radiation to other circuits.

There is no other way. In order to reduce the impact of EMC radiation on the circuit, a separate digital ground line DGND must be used to effectively isolate other circuits.

3. Power ground line PGND

Whether it is analog ground AGND or digital ground DGND, they are all low-power circuits. In high-power circuits, such as motor drive circuits, solenoid valve drive circuits, etc., there is also a separate reference ground wire, which is called power ground PGND.

As the name implies, a high-power circuit is a circuit with relatively large current. Obviously, large current can easily cause ground offset between circuits with different functions.

Once there is a ground offset in the circuit, the original 5V voltage may not be 5V, but become 4V. Because the 5V voltage is referenced to the GND ground line 0V, if the ground offset causes the GND ground line to rise from 0V to 1V, then the previous 5V (5V-0V=5V) voltage becomes the current 4V (5V-1V=4V).

4. Power ground GND

Analog ground AGND, digital ground DGND and power ground PGND are all classified as DC ground GND. These different types of ground wires will eventually be gathered together as the 0V reference ground wire of the entire circuit, which is called the power ground wire GND.

The power supply is the energy source for all circuits. The voltage and current required for all circuits to work come from the power supply. Therefore, the ground wire GND of the power supply is the 0V voltage reference point for all circuits.

This is why other types of ground lines, whether it is analog ground AGND, digital ground DGND or power ground PGND, need to be brought together with the power ground GND in the end.

5. AC ground wire CGND

The AC ground wire CGND generally exists in circuit projects containing AC power supply, such as AC-DC AC to DC power supply circuit.

The AC-DC power supply circuit is divided into two parts. The front stage of the circuit is the AC part, and the back stage of the circuit is the DC part, which is forced to form 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, and the DC ground wire is used as the 0V reference point of the DC circuit. Usually, in order to unify a ground wire GND in the circuit, engineers will connect the AC ground wire to the DC ground wire through a coupling capacitor or inductor.

6. Earth ground wire EGND

The safe voltage for the human body is below 36V. If a voltage exceeding 36V is applied to the human body, it will cause damage to the human body. This is a safety common sense for engineers when developing and designing circuit project solutions.

In order to enhance the safety factor of the circuit, engineers generally use the earth ground wire EGND in high voltage and high current projects, such as in the circuits of household appliances such as electric fans, refrigerators, and televisions.

Why do sockets for household appliances have three terminals? 220V AC only needs two wires, the live wire and the neutral wire, so why do sockets have three terminals?

The socket has three wiring terminals, two of which are used for the 220V live wire and neutral wire, and the other terminal is the protective ground wire EGND.

It should be pointed out that the earth ground wire EGND is only connected to our earth and plays a role in high voltage protection. It does not participate in the project circuit function and has nothing to do with the circuit function.

Therefore, the earth ground wire EGND has obvious circuit differences from other types of ground wires GND.

丨A closer look at the principles of GND

Engineers may ask, why is there so much differentiation for a ground wire GND, and why a simple circuit problem is made so complicated?

Why is it necessary to introduce so many subdivided GND ground line functions?

Engineers usually simply name this type of GND wire design problem as GND, and do not distinguish between them during the schematic design process. This makes it difficult to effectively identify the GND wires of different circuit functions during PCB wiring, and they simply connect all the GND wires together.

Although this is easy to operate, it will lead to a series of problems:

1. Signal crosstalk

If the ground wires GND of different functions are directly connected together, the high-power circuit will affect the 0V reference point GND of the low-power circuit through the ground wire GND, thus generating crosstalk between different circuit signals.

2. Signal accuracy

For analog circuits, the core indicator of its assessment is the accuracy of the signal. Without accuracy, the analog circuit will lose its original functional significance.

The ground wire CGND of the AC power supply is a sine wave, which fluctuates up and down periodically. Its voltage also fluctuates up and down, and it is not like the DC ground wire GND that always remains at 0V.

When the ground wires GND of different circuits are connected together, the periodically changing AC ground wire CGND will drive the ground wire AGND of the analog circuit to change, thus affecting 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 circuit with strong signal directly interferes with the ground wire GND of the circuit with weak signal. As a result, the electromagnetic radiation EMC with weak signal originally becomes a signal source with strong external electromagnetic radiation, which increases the difficulty of circuit processing EMC experiment.

4. Circuit reliability

The fewer signal connections between circuit systems, the stronger the circuit's ability to operate independently; the more signal connections, the weaker the circuit's ability to operate independently.

Imagine that if two circuit systems A and B have no intersection, the function of circuit system A will obviously not affect the normal operation of circuit system B, and similarly, the function of circuit system B will not affect the normal operation of circuit system A.

*Some of the information and pictures in this article are from the Internet. The copyright belongs to the original author. If there is any infringement, please contact me to delete it. Thank you!

maychang

"For a power source, it is the negative pole of the power source."

That's not necessarily the case.

Don't mislead beginners.

NGI123

maychang posted on 2022-12-2 10:44 "For the power supply, it is the negative pole of the power supply." That's not necessarily the case. Don't mislead beginners.

OK, thanks for the correction.

led2015

I don't understand so many places. Although I have been doing this for several years, I have always done the wiring blindly. Maybe it is my negligence.

秦天qintian0303

led2015 posted on 2022-12-3 21:17 I don’t understand so many places. Although I have been doing this for several years, my wiring is blind. Maybe it’s my negligence

There is no high requirement for analog signals, basically the same ground is enough