System grounding, cabinet grounding, digital grounding and power supply analog grounding in electronic circuits.

These are several different problems: analog ground and digital ground, that is, both analog and digital circuits are grounded.

1. The digital ground and analog ground should be separated;

In demanding circuits, digital ground and analog ground must be separated. Even for A/D and D/A converters, it is best to separate the two "grounds" on the same chip and connect the two "grounds" only at the system point.

2. Floating and grounding;

The floating ground of the system is to float the ground wire of each part of the system circuit without being connected to the ground. This connection has certain anti-interference ability. But the insulation resistance of the system to the ground should not be less than 50MΩ. Once the insulation performance drops, it will bring interference. Usually, the system floats on the ground and the shell is grounded, which can enhance the anti-interference ability and is safe and reliable.

3. One point grounding;

In low frequency circuits, there is not much influence between wiring and components. Usually, the circuit with frequency less than 1MHz is grounded at one point.

4. Multi-point grounding.

In high frequency circuits, parasitic capacitance and inductance have great influence. Usually, circuits with frequencies greater than 10MHz adopt multipoint grounding.

If analog ground and digital ground are directly connected in a large area, it will lead to mutual interference. There are four ways to solve this problem: 1, connected by magnetic beads; 2. Connect with the capacitor; 3. Connect with inductor; 4. Connect a 0 ohm resistor.

The equivalent circuit of magnetic beads is equivalent to a band-stop limiter, which can only significantly suppress the noise of a certain frequency point. When using it, it is necessary to estimate the noise frequency in advance in order to choose a suitable model. Magnetic beads are incompatible when the frequency is uncertain or unpredictable.

The capacitor is separated from the DC current, resulting in floating ground.

Inductor is bulky, with many stray parameters and instability.

0 Ω resistor is equivalent to a very narrow current path, which can effectively limit the loop current and suppress noise. Resistance has attenuation effect in all frequency bands (0 Ω resistance also has impedance), which is stronger than magnetic beads.

Let's talk about the relationship between chassis ground, digital ground and analog ground:

In general, the shell is grounded to the ground wire (not the neutral wire) of the AC power supply to prevent the operator from getting an electric shock (the shell is equipotential with the earth and human body).

Chassis ground can usually be connected to the power ground of the equipment, but:

In principle, the workplaces of digital circuits and analog circuits are strictly forbidden to be directly connected with the power supply ground of the equipment! The reason is that when the equipment itself leaks electricity or encounters strong electromagnetic interference, both digital circuits and analog circuits will be disturbed by this noise, which will lead to misoperation and serious machine damage! ! !

Mainly because the working level of digital circuits and analog circuits is generally 3.3- 15.5V( 15.5V is generally the highest level for 232 interface communication); Usually, the level of power supply loop is generally in the range of commercial power (AC 220 V 10%), which is much higher than the working level of digital circuits and analog circuits. In particular, the commercial power itself may encounter lightning strike, phase mismatch, high voltage breakdown and other faults, and even lead to its instantaneous level being much higher than the normal level (up to 22 times). Therefore, in order to ensure safety, the workplaces of general digital circuits and analog circuits will be isolated from the power supply ground and shell of equipment or adopt different grounding systems.

By the way, the current electromagnetic bomb attack is achieved by radiating high-frequency and high-amplitude electromagnetic fields in the space where the equipment is located, causing damage to the components of the core working circuits such as digital circuits and analog circuits of the equipment!

Connecting several places together is what you call systematization. Generally, it is divided into primary and secondary parts, which can be connected by resistors and capacitors. The requirements for safe spacing between high and low voltage of primary circuit and between primary and secondary circuits are strictly guaranteed. Chassis grounding is primary, and digital grounding is secondary. Eight holes can be distributed on the mounting hole pad, sometimes called the star-moon pad.