Engineers in the design of the circuit, in order to prevent various circuits in the normal operation of the circuit interfere with each other so that they can work effectively compatible with each other. According to the nature of the circuit, the circuit "zero potential" - "ground" is divided into different types.
First, let's know several kinds of "ground": "hot ground" means the area directly or indirectly connected to the AC power grid; "cold ground" means the area that is not connected to the grid; the signal "ground" is also called Reference "ground" is the zero-potential reference point which is the common terminal that forms the circuit's signal circuit; protection "ground" is provided for the protection of personnel in Class I electrical equipment ( equipment that can not be protected solely by insulation) A power wiring, one end of the shell with electrical appliances, the other end of the earth for a reliable connection.
First, the signal "to"
Signal "ground", also known as reference "ground", is the reference point of the zero potential, but also constitute the common end of the circuit signal circuit, the symbol "⊥".
1) DC ground: DC circuit "ground", zero potential reference point.
2) AC ground: AC zero line, should be distinguished from the ground.
3) Power: high current network devices, power amplifier zero potential reference point.
4) Analog Ground: Zero-Potential Reference Point for Amplifier, Sample-and-Hold, A / D Converter and Comparator.
5) Digitally: Also called logically, is the zero-potential reference point of a digital circuit.
6) "hot": switching power supply without the use of transformers , the switching circuit of the "ground" and the electricity grid, both the so-called "hot", it is charged.
7) "Cold": As the high-frequency switching power supply transformer input and output isolation; and because of its feedback circuit commonly used optocoupler, both to send feedback signal and the two sides of the "ground" isolation, so the output of the ground It is "cold ground" and it is not live. The graphic symbol is "⊥".
Second, the sound of "land"
1) Shielded cable grounding: sound system to prevent interference, the metal chassis wire with the signal "ground" phase, called the shield ground.
2) audio dedicated "ground": professional audio In order to prevent interference, in addition to shielding the "ground", but also with the audio dedicated "ground" connected. The grounding device should be buried specifically, and should be isolated transformer, shielded power supply connected to the corresponding ground after the audio control room as a dedicated audio ground point.
Third, the difference between "hot" and "cold"
In the input AC, we generally call a "fire" line (phase), a "zero" line (center line), after the bridge pile (or diode) to get the DC. In general, the negative side of the filter capacitor for the reference potential, that is, "hot." The positive side of the filter capacitor is + 300V at this time. At this time, if the earth is referenced to the "ground" potential, the "hot ground" and the "hot ground" are actually 220V AC waveforms. So: 1) If you stand on the ground and come into contact with "hot land", you will be struck by the voltage difference of 220V AC between "hot land" and the earth; 2) if you stand on a wooden chair (Completely separated from the earth) to touch the "heat", it does not matter, you will not be electric shock. You yourself have the same "hot" potential. This is exactly the same as the bird standing on the wire will not be shocked. Remember, the isolation transformer (1: 1 transformer) must be used when repairing a "live" movement.
Contrast to explain: 1) "Cold" generally refers to the direct touch of "ground", the same potential and the earth. As uncharged, thus becoming "cold ground." This is a safe "ground";
The circuit ground that is completely isolated from 220V mains and grounded (the circuit connection can be grounded or ungrounded) is called "cold ground"; the cold ground is rectified and filtered by the capacitor isolated power supply or stepped down by the step-down transformer The negative ground (through the transformer is cold ground).
2) "hot" refers to the charged "ground", must not be directly touched, otherwise it will be electric shock;
The circuit ground connected to 220V mains and ungrounded (can not be grounded) is called "hot ground"; hot ground is the mains direct rectified output grounded (the ground through the rectifier bridge is hot) without the capacitive isolation.
Fourth, different ways to deal with
1. Digital and analog should be separated. In demanding circuits, digital and analog must be separated. Even for A / D, D / A converter on the same chip on the two "ground" should also be separated, only the system at a little two "ground" connected.
Floating and grounding. Floating system, the system circuit is to float all parts of the ground, not connected with the earth. This connection, there is a certain anti-interference ability. However, the insulation resistance of the system and the ground can not be less than 50MΩ, once the insulation performance decreases, it will bring interference. Floating system is usually used, the chassis ground, anti-interference ability can be enhanced, safe and reliable.
3. A little grounding. In low-frequency circuits, wiring and components will not have much impact. Usually less than 1MHz frequency circuit, using a little grounding.
4 more ground. In high-frequency circuits, parasitic capacitance and inductance have a greater impact. The circuit is usually more than 10MHz frequency, multi-point grounding.
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