Overview of "location" in a circuit

First, let's get to know several "regions": "hot" refers to areas directly or indirectly connected to the AC power grid; "cold ground" refers to a region that is not connected with the power grid. A signal "Ground", also known as a reference "Ground", is a reference point with zero potential, it is the public end that forms the circuit signal circuit. The "ground" of protection is in Class I electrical equipment (it cannot rely solely on insulation for protection) in order to protect the security of personnel and set a Power wiring, one end of the electrical housing, the other end and the Earth for reliable connection.
I. Signal "location"
The signal "Ground", also known as the reference "Earth", is the reference point of zero potential, and is also the public segment that forms the circuit signal circuit. The graphical symbol "ground ".
1) DC: DC circuit "Ground", zero potential reference point.
2) Communication location: zero line of AC power. Should be different from the ground.
3) power location: zero potential reference point for high current network devices and power amplifier devices.
4) analog location: zero potential reference point of the amplifier, sampling holder, A/D converter, and comparator.
5) Digit location: it is also called logical location, which is the zero potential reference point of a digital circuit.
6) "Hot ground": the switch power supply does not need to use a transformer. The "Ground" of the switch circuit is related to the municipal power grid. The so-called "Hot ground" is charged.
7) "cold ground": the high-frequency transformer of the switching power supply isolates the input and output ends; in addition, the feedback circuit is commonly used for photoelectrical coupling, which can send feedback signals and isolate the "Ground" of both sides. Therefore, the output end is called "cold ground", which is not charged. The graphical symbol is "invalid ".
Ii. "location" in audio"
1) shielding line grounding: In order to prevent interference, the metal casing of the audio system is connected with the signal "Ground", which is called shielding grounding.
2) special audio "location": In order to prevent interference, in addition to shielding the "location", professional audio must be connected to the special "location" of audio. The grounding device shall be specially buried, and shall be connected to the grounding end of the isolating transformer and shielded regulated power supply as the special audio grounding point in the sound control room.
Iii. Differences between "Hot land" and "cold land ":
In the input alternating current, we generally call it a "fire" line (phase line), a "zero" line (center line), which goes through the bridge heap (or diode) after sorting, the DC is obtained. generally, the negative end of the filter capacitor is used as the reference power, that is, "Hot ground ". at this time, the positive end of the filter capacitor is + 300 V. at this time, if the earth is used as a reference to the "Earth" potential to measure the "hot Earth", the "hot Earth" is actually a V alternating current waveform. therefore: 1) if you stand on the ground and contact the "hot ground", you will be affected by an electric shock due to the voltage difference of v ac between the "hot ground" and the earth. 2) if you stand in a wooden chair (completely separated from the Earth) and get in touch with the "hot land", it doesn't matter. It's hard for you to get an electric shock. you have the same potential as "Hot land. this is exactly the same as that of the bird standing on the wire without electric shock. remember to use an isolating transformer (1:1 transformer) when repairing a "live" movement ).
Comparison:
A1 and "cold ground" generally refer to "Ground Wires" that can be directly touched. Their potentials are the same as those of the Earth. because it is not electrified, it becomes "cold ". this is a safe "place ";
The circuit ground line that is completely isolated from the mains supply of V and grounded (it can be grounded or not grounded when connected) is called "cold ground". The cold ground is isolated from the mains supply by capacitor, alternatively, use the voltage relief transformer to decompress the transformer and then transformer the filtered negative grounding (the ground passing through the transformer is cold ground ).
A2 and "Hot ground" are charged "Ground Wires" and cannot be touched directly. Otherwise, an electric shock will occur;
The circuit ground wires connected to V mains and not grounded (ungrounded) are called "Hot ground ";
The hot spot is the grounding of the power supply part that is not separated by capacitor, which is directly rectified by the mains supply (the ground that passes through the rectification bridge is the hot spot ).
4. Handling Methods of different ground wires. 
1. numbers should be separated from analog places;
In high-requirement circuits, numbers must be separated from analog circuits. Even for A/D and D/A converters, it is best to separate the two "locations" on the same chip and connect the two "locations" at the system point.
2. Float and grounding;
The system float is to float the Ground Wires of all parts of the system circuit, not connected to the Earth. This method provides anti-interference capabilities. However, the insulation resistance between the system and the ground cannot be less than 50 MΩ. If the insulation performance declines, interference may occur. Generally, the system float and case grounding are used to enhance anti-interference capability and ensure security and reliability.
3. One point grounding;
In a low-frequency circuit, wiring and components do not have much impact. Generally, a circuit with a frequency less than 1 MHz is grounded at a point.
4. Multi-point grounding.
In high-frequency circuits, parasitic capacitance and inductance are greatly affected. Generally, a circuit with a frequency greater than 10 MHz is grounded at multiple points.