Ground
Grounding is a very important issue for electronic devices. There are three grounding purposes:
(1) Grounding enables all unit circuits in the entire circuit system to have a common reference zero potential, ensuring that the circuit system can operate stably.
(2) Prevent interference from external electromagnetic fields. The grounding of the casing can cause a large amount of electric charge accumulated on the casing due to electrostatic induction to be discharged through the earth, otherwise the high voltage formed by these charges may cause spark discharge inside the device to cause interference. In addition, for the shield of the circuit, if a proper grounding is selected, a good shielding effect can be obtained.
(3) Ensure safe work. When electromagnetic induction of direct lightning occurs, the destruction of the electronic equipment can be avoided; when the input voltage of the commercial frequency AC power source directly communicates with the casing due to poor insulation or other reasons, the electric shock accident of the operator can be avoided. In addition, many medical devices are directly connected to the patient's human body, and a fatal hazard can occur when the chassis is 110V or 220V.
Therefore, grounding is the main method to suppress noise and prevent interference. Grounding can be understood as an equipotential point or equipotential surface, which is the reference potential of a circuit or system, but not necessarily earth potential. In order to prevent the damage caused by lightning strikes and the personal safety of the workers, the casing of the electronic equipment and the metal components of the equipment room must be connected to the earth, and the grounding resistance is generally small and cannot exceed the specified value.
There are basically three types of circuit grounding methods, namely single-point grounding, multi-point grounding, and hybrid grounding. Single point grounding means that in a line, only one physical point is defined as a ground reference point. All other points that need to be grounded are directly connected to this point. Multi-point grounding means that each grounding point in a system is directly connected to the ground plane closest to it so that the length of the grounding lead is the shortest. The ground plane can be the bottom plate of the device or the ground wire that runs through the whole system. In a relatively large system, it can also be the structural frame of the device. Hybrid grounding is the combination of bypass capacitors and ground planes that require only high frequency grounding points. However, the resonance phenomenon of the bypass capacitor and the lead inductance should be prevented as much as possible.
shield
Shielding is the separation of metal between two spatial regions to control the induction and radiation of electric, magnetic and electromagnetic waves from one region to another. Specifically, the shield is used to surround the components, circuits, assemblies, cables, or interference sources of the entire system to prevent the interference electromagnetic field from spreading outward; the shields are used to surround the receiving circuits, devices, or systems to prevent them from being exposed to the outside world. The influence of electromagnetic fields.
Because the shield absorbs energy (eddy current loss), reflected energy (interface reflection of electromagnetic waves on the shield) and cancels energy (electrical waves) from external electromagnetic waves and internal electromagnetic waves from wires, cables, components, circuits or systems. The magnetic induction generates a reverse electromagnetic field on the shielding layer, which can cancel the effect of partially interfering electromagnetic waves, so the shielding body has the function of reducing interference.
The principle of shielding material selection is:
(1) When the frequency of the interfering electromagnetic field is high, the eddy current generated in the metal material of low resistivity is used to form a canceling effect of the external electromagnetic wave, thereby achieving the shielding effect.
(2) When the frequency of the interfering electromagnetic wave is low, a material with a high magnetic permeability is used, so that the magnetic field line is confined inside the shield to prevent diffusion into the shielded space.
(3) In some cases, if high-frequency and low-frequency electromagnetic fields are required to have good shielding effect, different metal materials are often used to form a multilayer shielding body.
Filter
Filtering is an important measure to suppress and prevent interference. The filter can significantly reduce the level of conducted interference, because the interference spectral components are not net to the frequency of the wanted signal, and the filter has good suppression of these components with different frequency of the useful signal, so that other interference suppression is difficult. The role of the. Therefore, the use of filtering networks is a powerful measure to suppress interference sources and eliminate interference coupling, or to enhance the anti-interference ability of receiving devices. Decoupling and sensing decoupling networks isolate the circuit from the power supply, eliminate coupling between the circuits, and prevent interfering signals from entering the circuit. For the high frequency circuit, a CLCMÏ€ type filter composed of two capacitors and one inductor (high frequency choke) can be used. There are many types of filters, and choosing the right filter eliminates unwanted coupling.
Correct selection of passive components
Practical passive components are not "ideal" and their characteristics are different from the ideal ones. Practical components themselves can be a source of interference, so the correct choice of passive components is very important. Sometimes it is also possible to use the characteristics of the component to suppress and prevent interference.
Circuit technology
Sometimes, after shielding, the requirements for suppression and interference prevention cannot be met, and circuit technology such as balancing measures can be combined with shielding. A balanced circuit is one in which two wires in a two-wire circuit and all circuits connected to the two wires have the same impedance to ground or to other wires. The purpose is to make the interference signals picked up by the two wires equal. The interference noise at this time is a common state signal, which can disappear on the load itself. In addition, other circuit technologies such as contact networks, shaping circuits, integrating circuits, and gating circuits can be used. In short, the use of circuit technology is also an important measure to suppress and prevent interference.
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