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1. The interference generated by the interference from the source of interference may exist inside and outside the instrument. Outside the instrument, some high-powered electrical equipment and power equipment may become sources of interference, and power transformers, relays, switches, and power lines inside the instrument may also become sources of interference. The introduction of interference is mainly as follows.
1.1 Series Mode Interference E
n It is the interference superimposed on the signal under test, which is mainly generated in the following manner.
1.1.1 Electromagnetic induction electromagnetic induction, that is, magnetic coupling. There are strong alternating magnetic fields in the surrounding space of high-power transformers, AC motors, high-voltage power grids, etc. used in the project. The connecting wires between the signal source and the secondary instrument and the wiring inside the secondary instrument pass the alternating magnetic field. The magnetic coupling creates an interference in the circuit, and the closed circuit of the secondary meter will generate an induced potential in the varying magnetic field, and the induced potential can be expressed by the equation. This induced potential is connected in series with the useful signal. When the signal source is far away from the secondary meter, the interference is more prominent. In order to reduce the induced potential, items such as B, A or cos must be minimized, so it is necessary to keep the wires away from these strong electrical equipment and power grids, to adjust the direction of the wiring and to reduce the loop area of ​​the conductor. Only by twisting the two signal lines at a short pitch, the magnetic induction potential can be reduced to the original 110.
1.1.2 Static induction electrostatic induction is the coupling of electricity. In the opposite two objects, if the potential of one of them changes, the potential of the other object also changes due to the capacitance between the objects. The source of interference is the formation of interference in the loop through capacitive coupling, which is the result of the interaction of the two electric fields.
In the middle, the potential of the wire 1 induces a voltage to the ground on the wire 2. When the two signal wires are laid in parallel with the power line, the distributed capacitance is not equal because the distance between the power line and the two signal lines is not equal. A potential difference will be generated on the two signal lines, sometimes up to tens of millivolts or more. When the signal line is twisted, the potential difference generated by the electric field on the two signal lines is greatly reduced. After using electrostatic shielding, the induced potential can be reduced to 11.
1.1.3 Additional metal potentials with different thermoelectric potentials and chemical potentials, thermoelectric potentials generated by friction, and chemical potentials caused by corrosion of metals, etc., may also become interferences in the electrical circuit. Such interferences mostly occur in the form of direct current. The thermoelectric potential is easily generated at the terminal block or the reed relay.
1.1.4 When the vibration wire moves in a magnetic field, an induced electromotive force is generated. Therefore, it is necessary to fix the signal wires in a vibrating environment.
1.2 Common mode interference E
cEc is the interference superimposed between any input of the secondary meter and the ground, and is mainly generated in the following manner.
1.2.1 Different ground potentials In the earth, there is often a potential difference between different points, especially in the vicinity of high-power electrical equipment. When the insulation performance of these equipments is poor, this potential difference is larger. In the use of the instrument, the input loop often has multiple grounding points intentionally or unintentionally, so that the potential difference of different grounding points is introduced into the meter, and the ground potential difference can sometimes reach 110V or more, and simultaneously appears in 2 The root signal line is as shown.
1.2.2 Common mode interference between the signal source and the secondary instrument. By means of electrostatic coupling, the common voltage Ec to ground can be induced at the two input terminals, which appears in the form of common mode interference.
122 signal source is unbalanced bridge
3a) is a schematic diagram of the connection between the secondary instrument when the signal source is an unbalanced bridge. When the bridge power supply is grounded, except for the unbalanced voltage signal of the diagonal of the bridge, that is, the signal source voltage Ea, the two signal wires have a common voltage Ec to the ground, when the secondary instrument input terminal has leakage impedance Z3 and Z4 to the ground. Ec generates leakage currents Ic1 and Ic2 through a leakage path to the ground, as shown in 3b).
Since the common mode interference does not overlap with the signal, it does not directly affect the meter. But it measures the leakage current generated by the system to the ground. This leakage current can directly act on the meter through the coupling of the resistor, causing interference. Thus an interference voltage will be generated at both inputs.
After understanding the different sources of interference, you can take appropriate measures to eliminate or avoid them for different situations. Since all sources of interference affect the meter through a certain coupling channel, interference can be suppressed by cutting off the coupled coupling channel.

2. The suppression of interference is commonly used in anti-interference measures. To suppress interference, it is necessary to comprehensively analyze and understand the interference. It is necessary to eliminate or suppress the interference source, destroy the interference path and weaken the sensitivity of the receiving circuit to noise interference. Take measures in all aspects.
Solving the poor contact and soldering of the connector is a proactive measure to eliminate the interference source; in addition, for the DC signal, a filter circuit can be added at the input of the meter to minimize the interference mixed with the signal; In the middle, it is also necessary to use isolation to avoid the formation of interference fields. Pay attention to keeping the signal wires away from the power line. Signal lines with different signal amplitudes should not be worn in the same conduit, and reasonable wiring can reduce the generation of stray magnetic fields. Electrical components such as transformers are magnetically shielded. However, in fact, many sources of interference are difficult to eliminate or cannot be eliminated. In this case, it is necessary to take protective measures to suppress interference according to the type of interference in the instrument application.
2.1 Series mode interference suppresses series mode interference and signal superposition. Once generated, it is not easy to eliminate, and should be prevented from being generated. The measures generally have the following items.
2.1.1 Twisting of signal wires Twisting the signal wires together can greatly reduce the area enclosed by the signal circuit. It can be seen that the induced potential En is also greatly reduced. In addition, the twisting of the signal wires causes the two signal wires to interfere. The distances of the sources are approximately equal, and the distributed capacitances can also be approximately equal, that is, C120. As can be seen from the equation, the induced potential Ec is greatly reduced. Therefore, the twisting of the signal wires can greatly reduce the series mode interference caused by the inductive coupling of the magnetic field and the electric field into the loop.
2.1.2 Shielding In order to prevent the interference of the electric field, the signal wire can be wrapped with a metal mesh as a shielding layer, and then an insulating layer is wrapped around it, and the metal shielding wire can be selected as the signal transmission wire. The purpose of shielding is to isolate the field coupling and suppress interference from various fields. However, after shielding, the shield must be properly grounded to reduce the distributed capacitance between the interferer and the signal conductor, minimizing interference.
If the shielding layer is a non-ferromagnetic material, the magnetic field of the power frequency of 50 Hz is not shielded, and the wire can be magnetically shielded by penetrating the signal wire into the iron pipe.
2.2 Common mode interference E
The suppression E of c is superimposed on the interference between any input terminal of the secondary meter and the ground, mainly caused by the difference of ground potential. To prevent common mode interference, the combination of shielding and grounding is usually used to suppress interference.
For safety reasons, both the secondary meter and the signal source housing are normally grounded to maintain zero potential. The signal source circuit and the instrumentation system also need to be grounded stably. As shown, two points are grounded, and common mode interference occurs due to the ground potential difference. Therefore, system grounding is typically done at a single point on the signal source side or secondary instrument loop as shown. In order to improve the anti-jamming capability of the instrument, the instrument manufacturer generally floats the amplifier to cut off the leakage path of the common mode interference, so that the interference cannot be entered. In addition, the signal source side cannot be insulated to the ground. The grounding of 4a) is adopted. The method cannot completely eliminate the interference introduced by the ground potential difference, so in order to improve the anti-interference ability of the secondary meter, the grounding method shown in 4b) is often used.
In practical applications, shielding and grounding are often combined and often solve most of the interference problems. If the shielding layer is grounded on both the signal side and the instrument side, the ground potential difference will form a loop through the shielding layer. Since the ground resistance is usually much smaller than the resistance of the shielding layer, a potential gradient is formed on the shielding layer and passed. The distributed capacitance between the shield and the signal conductor is coupled into the signal circuit, so the shield must also be grounded at all. Also, the signal conductor shield ground should be on the same side as the system ground, as shown in 4. That is, when the ungrounded signal source is connected to the grounded secondary instrumentation amplifier, the shielding layer should be connected to the common terminal of the amplifier as shown in 4a), and when the signal source is grounded and the amplifier is floating, the shielding layer should be as in 4b) Connected to the signal source common.
In fact, the outer casing of the secondary instrument is grounded for safety. There must be distributed capacitance and leakage resistance between the input end of the meter and the outer casing. It is impossible for the floating ground to completely cut off the leakage path. Therefore, if necessary, double-layer shielding floating protection is usually adopted.
That is, an inner shielding layer is disposed in the outer casing of the secondary meter, and the inner shielding layer is not electrically connected with the signal input end and the outer casing, and the inner shielding layer leads a wire to be connected with the shielding layer of the signal wire, at the signal source. The grounding is done at one point, so that the input protection shielding and signal shielding of the secondary instrument are stabilized to the signal source, and the device is in an equipotential state, which can greatly improve the anti-interference ability of the secondary instrument.
The above describes several types of anti-interference measures that are often used in actual engineering for the way of interference generation in instrument applications. In actual use, the interference situation at the industrial production site is complicated. It is often difficult to solve the problem by using an anti-interference method. The various methods of twisting, shielding, grounding, filtering, and isolating the signal line should be used for different situations. In order to get a satisfactory result.