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Interference sources may exist both inside and outside the instrument. If outside the digital display meter, high-power electrical equipment and the power grid may become interference sources. In the digital display meter, transformers, coils, relays, power lines, etc. may become interference sources.
The wires and internal wiring between the signal source and the instrument form interference in the circuit through magnetic coupling. There is a strong AC magnetic field in the surrounding space of the high-power transformer, AC motor, and power line, and the closed circuit will generate electromotive force in this changing magnetic field. This induced potential is connected in series with the useful signal. When the sensor is far away from the digital display meter, this series mode interference is particularly prominent.
The interference source forms interference in the circuit through capacitive coupling, which is the result of the interaction of two electric fields. Through electrostatic coupling, the common voltage to the ground can be induced at two input terminals, which appears in the form of common mode interference. Since common mode interference does not overlap with the signal, it does not directly affect the instrument. But it can form the leakage current to the ground through the measurement system, and the leakage current can directly act on the instrument through the resistance coupling to generate interference. Electromagnetic induction and electrostatic induction are mostly power frequency interference voltage, but frequency converter and motor with commutator will produce harmonic interference.
In some temperature measurement situations, the thermocouple electrode is directly welded to the electrically heated metal piece. Because the metal piece has potential differences at each point parallel to the current direction, the interference voltage introduced is also large. In the high-temperature state, the insulation resistance of the refractory material drops sharply, and the insulation performance of the thermocouple protective tube also drops, so the power supply voltage leaks to the thermocouple wire through the refractory brick, thermowell, etc., which generates interference voltage between the thermocouple electrode and the ground.
There is often a potential difference between different points on the earth, especially near high-power electrical equipment. When the insulation performance of these pieces of equipment decreases, the potential difference is greater. However, in the use of field instruments, sometimes it is not noticed that there are more than two grounding points in the circuit, and the potential difference of different grounding points will be introduced into the digital display meter to form common mode interference.
When the bridge power supply of the instrument is grounded, in addition to the unbalanced signal voltage output by the bridge, the signal line has a common voltage to the ground. The common voltage is not the signal voltage to be measured, but a manifestation of common mode interference.
The signal transmission wire uses twisted pair, which can make the distance between the two signal wires and the interference source approximately equal, and the distributed capacitance is approximately the same, so the serial mode interference of the incoming number display can be greatly reduced.
In order to prevent the interference of the electric field, the signal wire can be threaded into the iron pipe, or the shielded wire can be used, and the shielding layer shall be grounded at one point. For the DC signal, a filter circuit can be added at the input end of the digital display meter to minimize stray signal interference. The signal line should be far away from the power line. The signal line and power line should not enter the instrument through the same hole. The signal line should be connected to the adjacent position of the signal terminal with the shortest possible stranded wire.
The enclosures of the digital display meter and transmitter shall be grounded to maintain zero potential. In order to improve the anti-interference ability of the instrument, the amplifier of the instrument can be "floating to the ground", that is, the amplifier can be insulated from the instrument enclosure to cut off the leakage path of common mode interference voltage.
