Grounding
Grounding is a very important issue for electronic devices. There are three grounding purposes:
(1) Grounding ensures that all unit circuits in the entire circuit system have a common reference zero potential, ensuring stable operation of the circuit system.
(2) Prevent interference from external electromagnetic fields. Grounding the casing can release many charges accumulated on the casing due to electrostatic induction through the ground, otherwise the high voltage formed by these charges may cause spark discharge inside the equipment and cause interference. In addition, for the shielding body of the circuit, if a suitable grounding is selected, good shielding effect can also be achieved.
(3) Ensure safe work. When direct lightning electromagnetic induction occurs, it can avoid damage to electronic equipment; When the input voltage of the power frequency AC power supply is directly connected to the casing due to poor insulation or other reasons, it can avoid electric shock accidents for operators. In addition, many medical devices are directly connected to the patient's body, and when the casing carries a voltage of 110V or 220V, there is a fatal danger.
Therefore, grounding is the main method for suppressing noise and preventing 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 the ground potential. To prevent possible damage caused by lightning strikes and the personal safety of workers, the casing of electronic equipment and the metal components of the computer room must be connected to the ground, and the grounding resistance should generally be small and cannot exceed the specified value.
There are basically three types of grounding methods for circuits, namely single point grounding, multi-point grounding, and hybrid grounding. Single point grounding refers to a circuit where only one physical point is defined as the grounding reference point. All other points that require grounding are directly connected to this point. Multipoint grounding refers to the direct connection of each grounding point in a system to the nearest grounding plane, to minimize the length of the grounding lead. The grounding plane can be the bottom plate of the equipment, the grounding wire that runs through the entire system, and in larger systems, it can also be the structural framework of the equipment, etc. Hybrid grounding refers to connecting those high-frequency grounding points that only need to be grounded, using bypass capacitors and grounding planes. However, efforts should be made to prevent resonance caused by bypass capacitance and lead inductance.
Shield
Shielding is the metal isolation between two spatial areas to control the induction and radiation of electric fields, magnetic fields, and electromagnetic waves from one area to another. Specifically, shielding is used to surround the interference sources of components, circuits, assemblies, cables, or the entire system to prevent the spread of interference electromagnetic fields; Enclose the receiving circuit, equipment, or system with shielding to prevent them from being affected by external electromagnetic fields.
Because the shield plays a role in absorbing energy (eddy current loss), reflecting energy (reflection of electromagnetic waves at the interface of the shield), and canceling energy (electromagnetic induction generates reverse electromagnetic fields on the shield layer, which can partially offset interference electromagnetic waves) against external interference electromagnetic waves such as wires, cables, components, circuits, or systems, as well as internal electromagnetic waves, the shield has the function of reducing interference.
The principles for selecting shielding materials are:
(1) When the frequency of the interfering electromagnetic field is high, the eddy current generated in the metal material with low resistivity is used to offset the external electromagnetic waves, thereby achieving a shielding effect.
(2) When the frequency of interfering electromagnetic waves is low, materials with high magnetic permeability should be used to limit the magnetic lines of force inside the shield and prevent them from spreading into the shielded space.
(3) In some situations, if good shielding effects are required for both high-frequency and low-frequency electromagnetic fields, multi-layer shielding bodies are often made of different metal materials.
Filter
Filtering is an important measure to suppress and prevent interference. The filter can significantly reduce the level of conducted interference, because the interference spectrum components do not network with the frequency of the useful signal. The filter has good suppression capabilities for these components that are different from the frequency of the useful signal, thus achieving other interference suppression functions that are difficult to achieve. to the effect. Therefore, the use of filtering networks is a powerful measure whether it is to suppress interference sources and eliminate interference coupling, or to enhance the anti-interference ability of receiving equipment. Using resistor-capacitor and inductor-capacitor decoupling networks can isolate the circuit from the power supply, eliminate coupling between circuits, and prevent interference signals from entering the circuit. For high-frequency circuits, a CLCMπ filter composed of two capacitors and an inductor (high-frequency choke) can be used. There are many types of filters, and selecting the appropriate filter can eliminate unwanted coupling.
Correct selection of passive components
Practical passive components are not "ideal" and their properties differ from ideal properties. Practical components can themselves be a source of interference, so the correct selection of passive components is very important. Sometimes the characteristics of components can also be used to suppress and prevent interference.
Circuit technology
Sometimes the requirements for suppressing and preventing interference cannot be met after using shielding. You can combine shielding and take balancing measures and other circuit technologies. A balanced circuit is one in which the two conductors in a two-wire circuit and all circuits connected to the two conductors have the same impedance to ground or to other conductors. Its purpose is to make the interference signals picked up by the two wires equal. The interference noise at this time is a common mode signal and can disappear on its own at the load. In addition, other circuit technologies can also be used, such as contact networks, shaping circuits, integrating circuits and strobe circuits, etc. In short, the use of circuit technology is also an important measure to suppress and prevent interference.
