1. Electromagnetic induction (refers to magnetic coupling). In the surrounding space of high-power transformers, AC motors, high-current power grids, etc., there is a strong alternating magnetic field, and the control system (detection, transmission, conversion, adjustment, calculation, execution, auxiliary, display, etc.) lines are formed. In the closed loop, the magnetic field will be induced in this changing magnetic field, so that the connecting wire between the signal source and the instrument and the wiring inside the instrument are magnetically coupled to form interference in the circuit. This electromagnetic induction potential is connected in series with the useful signal. When the signal source is far away from the display instrument, the interference is more prominent. In addition, high-frequency generators, motors with commutators, etc., also generate high-frequency interference.
2. Electrostatic induction (referring to the coupling of electricity). Static induction is the result of the interaction of two electric fields. In the opposite two wires, if the potential of one of them changes, the potential of the other wire also changes due to the change in capacitance between the wires, and the interference source forms a disturbance in the loop by capacitive coupling.
3. Additional thermoelectric potential and chemical potential. DC electrical interference is formed in the circuit loop due to the thermoelectric potential generated by different metals and the chemical potential generated by metal corrosion.
4. Vibration. In a highly vibrating environment, the conductor generates an induced potential due to its motion in the magnetic field. This interference is in series with the signal and enters the instrumentation in the form of series mode interference.
5. Interference introduced by different ground potentials. In the vicinity of high-powered electrical equipment, when the insulation performance of the equipment is poor, the introduction of the potential difference of different ground potentials forms interference, and in the use of the instrument, there are often more than two connection points on the input end. This will introduce the potential difference of different grounding points into the instrumentation in the form of common mode interference, which occurs simultaneously on the two signal lines.
6. The signal source is an unbalanced bridge. When the bridge power supply is grounded, except for the unbalanced voltage (ie, signal voltage) of the diagonal of the bridge, both signal lines have a common common mode interference voltage to ground. Although the common mode interference does not overlap with the signal and does not directly affect the meter, it can form a leakage current to the ground through the measurement system. The coupling of the resistor can directly act on the meter (or amplifier) ​​to generate interference.
7. Some pulse-shaped interference voltage de-energization acts outside the analog circuit, and sometimes it can directly enter the digital circuit to give interference. The sources of these interference voltages are inductive loads such as switches, motors, and relays, and machines that generate discharges.
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