First, the ground line design In electronic equipment, grounding is an important method of suppressing noise. If the grounding and shielding are properly combined, most noise problems can be solved. The ground line structure in the electronic device is roughly divided into a system ground, a chassis ground (shielded ground), a digital ground (logical ground), and a power supply analog ground. Pay attention to the following points in the ground line design: A. Correct selection of single point grounding and multi-point grounding In the low-frequency circuit, the operating frequency of the signal is less than 1MHz, and the influence of the inductance between the wiring and the component is small, and the circulating current formed by the grounding circuit has a great influence on the noise, so a grounding should be adopted. When the signal operating frequency is greater than 10MHz, the ground line impedance becomes very large. At this time, the ground line impedance should be reduced as much as possible. When the operating frequency is between 1 and 10 MHz, if a grounding is used, the grounding length should not exceed 1/20 of the wavelength. Otherwise, the multi-point grounding method should be used. B. Separate the digital circuit from the power analog circuit If there are high-speed logic circuits and linear analog circuits on the board, they should be separated as much as possible, and the ground wires of the two should not be mixed, and they are connected to the ground of the power supply (???). Try to increase the grounding area of ​​the linear circuit. C. Try to thicken the ground wire If the grounding wire is very thin, the grounding potential changes with the change of the current, causing the timing signal level of the electronic device to be unstable and the anti-noise performance to deteriorate. Therefore, the ground wire should be as thick as possible so that it can pass three of the allowable currents on the printed circuit board. If possible, the width of the grounding wire should be greater than 3mm. D. Make the ground wire constitute an infinite loop When designing the grounding system of a printed circuit board consisting only of digital circuits, making the grounding line into an infinite loop can significantly improve the noise immunity. The reason is that there are many integrated circuit components on the printed circuit board, especially when there are components with high power consumption, due to the limitation of the thickness of the grounding wire, a large potential difference will be generated on the ground junction, causing the noise resistance to decrease. If the ground structure is looped, the potential difference will be reduced to improve the noise immunity of the electronic device. This is a few different issues: analog ground and digital ground, as the name suggests is the grounding of analog and digital circuits. 1. The digital ground and the simulated ground should be separated; In high demand circuits, the digital ground must be separated from the analog ground. Even for the A/D, D/A converter, the two "grounds" on the same chip should be separated, and only the two "grounds" are connected at one point in the system. 2. Floating and grounding; The floating system of the system floats the ground of each part of the system circuit and is not connected to the earth. This connection has a certain anti-interference ability. However, the insulation resistance between the system and the ground should not be less than 50 MΩ. Once the insulation performance is degraded, it will cause interference. The system is usually floating and the chassis is grounded to enhance the anti-interference ability, which is safe and reliable. 3. One point of grounding; In low frequency circuits, there is not much influence between the wiring and the components. Generally, circuits with a frequency less than 1MHz are grounded at one point. 4. Multi-point grounding. In high-frequency circuits, the effects of parasitic capacitance and inductance are large. Generally, circuits with a frequency greater than 10 MHz are grounded at multiple points. If the analog ground and the digital ground are directly connected to each other, it will cause mutual interference. It is not short-circuited and not appropriate. There are four ways to solve this problem as above: 1. Connected with magnetic beads; 2. Connect with a capacitor; 3. Connect with an inductor; 4. Connect with a 0 ohm resistor. The equivalent circuit of the magnetic bead is equivalent to the band-stopper, which only significantly suppresses the noise of a certain frequency point. When using it, it is necessary to estimate the noise frequency in advance so that the appropriate model can be selected. For frequencies that are uncertain or unpredictable, the beads do not match. The capacitor is connected straight to the ground, causing floating. The inductor is bulky, has many stray parameters, and is unstable. The 0 ohm resistor is equivalent to a very narrow current path, which effectively limits the loop current and suppresses noise. The resistor has an attenuation in all frequency bands (0 ohm resistor also has impedance), which is stronger than the magnetic beads. In addition, the analog ground and digital ground are connected, and the 0 ohm resistor belongs to a single point connection, which is suitable for low frequency weak current. Poor capacitance, isolated DC, easy to cause floating, generating static high voltage. Let's talk about the relationship between the chassis ground and the digital ground and analog ground: The ground of the general chassis is connected to the grounding wire of the AC power supply (not the neutral line), in order to prevent the operator from getting an electric shock (the casing and the earth, the human body, etc.). The casing ground can generally be connected to the power ground of the equipment, but: The working ground of the digital circuit and analog circuit is strictly prohibited from directly connecting with the power supply ground of the equipment! The reason is that the device itself is leaking. When electricity or strong electromagnetic field interference occurs, digital circuits and analog circuits will be affected by this noise, causing malfunctions, which will cause damage to the machine! ! ! Mainly because the working level of digital circuit and analog circuit is generally 3.3-15.5V (15.5V is generally used for the highest level of communication of 232 interface); usually the level of power supply circuit is generally in the commercial range (AC220V±10%) , far greater than the working level of digital circuits and analog circuits. In particular, the mains itself can encounter lightning, phase failure, high voltage breakdown and other faults, which can cause its instantaneous level to be much larger than its normal level (up to 22 times). Therefore, in order to ensure safety, the working places of digital circuits and analog circuits are isolated from the power supply ground, chassis, etc. of the equipment or different grounding systems are used. By the way, the current electromagnetic ** attack is achieved by divulating high-frequency, high-amplitude electromagnetic fields in the space where the device is located, causing damage to components such as digital circuits and analog circuits of the device. Putting together a few grounds is the systematic place you said. Generally, the ground is divided into two places: primary and secondary. The ground can be connected by resistors and capacitors. The high and low voltages of the primary circuit and the primary and secondary circuits are strictly guaranteed. The chassis is primary and the digital ground is secondary. There are 8 small holes on the mounting hole pad, sometimes called the Xingyue pad. If there are network ports, households, safety regulations and EMC requirements are strict, generally add protective circuits. It is best to divide the primary and secondary grounds. The ground can be connected between the high and low voltages of the primary circuit and the primary and secondary circuits through resistors and capacitors to ensure strict safety spacing requirements. The chassis is primary and the digital ground is secondary. There are 8 small holes on the mounting hole pad, sometimes called the Xingyue pad. Short all the grounds together to make the center hole non-metallized (otherwise it becomes a large-area connection because the center hole is large); connect the digital ground with 8 metallized holes evenly distributed on the pad to limit noise Provides a large current path; the pads can be made into a horseshoe shape to allow for expansion space.
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