Ask a simple but difficult circuit question: What is the nature of the ground in the circuit?
During the PCB Layout process, engineers will face different GND treatments. Why is this?
In the circuit schematic design stage, in order to reduce the mutual interference between circuits, engineers generally introduce different ground as 0V reference points for different functional circuits to form different current loops.
Classification of GND
1. Analog ground, AGND
The analog ground AGND is mainly used in the analog circuit part, such as the ADC acquisition circuit of the analog sensor, the operational amplification circuit and so on.
In these analog circuits, since the signal is an analog signal, it is a weak signal, and it is easily affected by the large current of other circuits. If they are not distinguished, a large current will produce a large voltage drop in the analog circuit, which will cause the analog signal to be distorted, and severely may cause the analog circuit to fail.
2. Digital ground, DGND
The digital ground DGND, obviously opposite to the analog ground AGND, is mainly used for digital circuit parts, such as button detection circuit, USB communication circuit, MCU circuit and so on.
The digital ground DGND is set up because digital circuits have a common feature, they are all discrete signals, and only the digital "0" is distinguished from the digital "1", as shown in the figure below.
In the process of changing from the digital "0" voltage to the digital "1" voltage, or changing from the digital "1" voltage to the digital "0" voltage, the voltage has a change. According to Maxwell's electromagnetic theory, a magnetic field is generated around the changing current, which forms EMC radiation to other circuits.
In order to reduce the EMC radiation effect of the circuit, a separate digital ground DGND must be used to effectively isolate other circuits.
3. Power ground PGND
Whether analog ground AGND or digital ground DGND, they are both low-power circuits. In high-power circuits, such as motor drive circuits, solenoid valve drive circuits, etc., there is also a separate reference ground, which is called the power ground PGND.
High-power circuits, as the name suggests, are circuits with relatively large currents. Obviously, a large current can easily cause ground offsets between circuits with different functions, as shown in the figure below.
Once there is a ground shift in the circuit, the original 5V voltage may not be 5V, but become 4V. Because the 5V voltage is referred to the GND ground 0V, if the ground offset causes the GND ground 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
The analog ground AGND, the digital ground DGND and the power ground PGND are all classified as the DC ground GND. These different types of ground s are finally brought together to serve as the 0V reference ground of the entire circuit. This ground is called the power ground GND.
The power supply is the energy source of all circuits. The voltage and current required by all circuits are from the power supply. Therefore, the ground line GND of the power supply is the 0V voltage reference point for all circuits.
This is why other types of ground, whether it is analog ground AGND, digital ground DGND or power ground PGND, need to be collected together with power ground GND in the end.
5. AC ground CGND
The AC ground CGND generally exists in a circuit project that contains AC power, such as an AC to DC power supply circuit.
The AC-DC power supply circuit is divided into two parts. The front-end circuit in the circuit is the AC part, and the back-end circuit in the circuit is the DC part. This is forced to form two ground s, one is the AC ground and the other is the DC ground.
The AC ground is used as the 0V reference point of the AC circuit part, and the DC ground is used as the 0V reference point of the DC circuit part. Usually, in order to unify a ground GND in the circuit, engineers will connect the AC ground to the DC ground through a coupling capacitor or inductance.
6. Earth ground EGND
The safety voltage of 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 common safety sense that engineers keep in mind when develop and design circuit projects.
In order to enhance the safety factor of the circuit, engineers generally use the EGND in high-voltage and high-current projects, such as in the circuits of household appliances, electric fans, refrigerators, and televisions. The socket with earth ground EGND protection function, as shown in the figure below.
Why the sockets of household appliances are 3 terminals?
220V AC only needs live and neutral wires, two wires are enough. There are three connection terminals of the socket, two of which are used for 220V live and neutral, and the other terminal is the earth ground EGND for protection.
What needs to be pointed out is the earth ground EGND, which is only connected to our earth and plays a high-voltage protection function. It does not participate in the circuit function of the project and has nothing to do with the circuit function. Therefore, the EGND has circuit meanings that are obviously different from other types of GND.
