Definition: A conductor wrapped with a conductor outside is called a shielded wire. The wrapped conductor is called a shielding layer, which is generally a braided copper mesh or copper foil (aluminum). The shielding layer needs to be grounded, and external interference signals can be introduced into the ground by this layer.

Function: Prevent interference signals from entering the inner layer, and conductor interference while reducing the loss of transmission signals.
Structure: (Ordinary) Insulation layer + shielding layer + conductor
(Advanced) Insulation layer + shielding layer + signal conductor + shielding layer grounding conductor
Note: When selecting a shielded wire, the shielding layer grounding conductor The insulation layer of the shielding layer grounding conductor has a conductive function and can be connected to the shielding layer (with a certain resistance)

The function of shielding wire is to isolate the electromagnetic field noise source from sensitive equipment and cut off the propagation path of the noise source. Shielding is divided into active shielding and passive shielding. The purpose of active shielding is to prevent the noise source from radiating outward, which is a shielding of the noise source; the purpose of passive shielding is to prevent sensitive equipment from being interfered by the noise source, which is a shielding of sensitive equipment. The shielding layer of the shielded cable is mainly made of non-magnetic materials such as copper and aluminum, and the thickness is very thin, which is much smaller than the skin depth of the metal material at the use frequency. The effect of the shielding layer is not mainly due to the reflection and absorption of the electric field and magnetic field by the metal body itself, but due to the grounding of the shielding layer. Different grounding forms will directly affect the shielding effect. The grounding methods for the electric field and magnetic field shielding layers are different. Ungrounded, single-ended grounding or double-ended grounding can be used.

Summary: Single-ended grounding:
1. Single-ended grounding of shielded cables is helpful for avoiding interference from low-frequency electric fields. In other words, it can avoid frequency interference with a wavelength λ much larger than the cable length L. L<λ /20
2. Single-ended grounding of the cable shielding layer can avoid low-frequency current noise on the shielding layer. This current causes common-mode interference voltage internally and has the potential to interfere with analog devices.
3. Single-ended grounding of the shield is desirable for circuits that are sensitive to low-frequency interference (analog circuits).
4. Fluctuations and permanent deviations in continuous measured values ​​indicate low-frequency interference.

Double-ended grounding:
1. Make sure that the connection to the control cabinet or plug (round contact) passes through a large conductive area (low inductance). It is better to choose metal on metal than non-metal on non-metal.
2. Since some analog modules use pulse technology (for example: the processor and A/D converter are integrated in the same module), it is recommended to shield the analog signals from each other to ensure correct equipotential connection. Only in this case, double-ended grounding is performed.
3. The transmission impedance of the metal foil shield is usually much greater than that of the copper braided wire shield, and the effect is 5-10 times different. It cannot be used as a digital signal cable.
4. Occasional functional failure indicates high-frequency interference. This cannot be eliminated by equipotential connection of the wires.
5. It is beneficial to ground the shield at multiple points except at the ends of the cable.
6. Do not connect the shield to the pin to avoid the "pig tail" phenomenon.
7. Always pay attention to the parallel impedance of the shield should be less than 1/10 of its own impedance. Cable trays, mechanical frames, other shields or other parallel cables can make the system equipotential.
8. If the cable shielding layer heats up when both ends of the shielding layer are grounded, or sparks when the shielding layer touches the electrical cabinet casing or the shielding bus, it means that the equipotential connection is unreliable.