Do you know the possible factors that cause the analog quantity to be interfered with?

Everyone knows the analog quantity, but do you know the factors that may interfere with it? There are two forms of voltage and current changes when transmitted through wires. We call this "common mode" and "differential mode". The power cord of the equipment, the communication line of the telephone, etc., and the communication line that exchanges with other equipment or peripheral equipment, have at least two wires, and these two wires serve as round-trip lines to transmit power or signals. But there is usually a third conductor in addition to these two wires, which is the "ground wire". There are two types of interference voltage and current: one is that two wires are used as round-trip transmission lines; the other is that two wires are used as outgoing paths and the ground wire is used as return path transmission. The former is called "differential mode" and the latter is called "common mode".

Due to the complex environmental conditions of the display instruments used on site, and the fact that most of the measured parameters are converted into weak low-level voltage signals and transmitted to the display instruments over long distances, in addition to the useful signals, there are also some signals related to the measured signals. Irrelevant interference signals are mixed in, which will affect the accuracy of the measurement results, and in severe cases, the instrument will not work properly. Now we will introduce the ways to generate interference and the measures to eliminate it.

interference pathway

Interference sources may exist inside or outside the instrument. For example, outside the instrument, high-power electrical equipment, high-power transformers, and power grids may become sources of interference. Inside the instrument, power transformers, coils, relays, switches and power lines may become sources of interference. Interference sources often enter the instrument through the following channels.

1. The connecting wires between the signal source and the instrument and the wiring inside the instrument cause interference in the circuit through magnetic coupling. There are strong AC magnetic fields in high-power transformers, AC motors, power lines and surrounding spaces, and a closed loop will induce an electric potential in this changing magnetic field. This induced potential is connected in series with the useful signal. This series mode interference is particularly prominent when the sensor is far away from the display instrument.

2. The interference source forms interference in the loop through capacitive coupling, which is the result of the interaction of two electric fields. Through electrostatic coupling, a common voltage to ground can be induced at the two input terminals, which appears in the form of common-mode interference. Since common-mode interference does not superimpose with the signal, it does not directly affect the instrument. However, it can form a leakage current to the ground through the measurement system. This leakage current can directly act on the instrument through the coupling of the resistor and cause interference. The interference caused by electromagnetic induction and electrostatic induction is mostly power frequency interference voltage, but frequency converters, motors with commutators, etc. can produce harmonic interference. Interference voltages will also be induced on power lines due to lightning.

3. In some temperature measurement situations, when the thermocouple electrode is directly welded to a metal part that is energized and heated, since there is a potential difference in the metal part at various points parallel to the direction of the current, the interference voltage introduced at this time is also very large. At high temperatures, the insulation resistance of the refractory material drops sharply, and the insulation performance of the thermocouple's porcelain protection tube and porcelain beads also decreases. Then the electric furnace power supply voltage leaks to the thermocouple wire through the refractory bricks, thermowell sleeves, porcelain beads, etc. On the thermocouple electrode, an interference voltage is generated between the thermocouple electrode and ground.

4. There are often potential differences between different points in the earth, especially near high-power electrical equipment. When the insulation performance of these equipment decreases, the potential difference is even greater. When field instruments are in use, sometimes there are more than two grounding points in the loop without paying attention, which will introduce the potential difference of different grounding points into the display instrument and form common mode interference.

5. When the bridge power supply of the instrument is grounded, in addition to the unbalanced signal voltage output by the bridge, there is a common voltage between the signal line and the ground. This common voltage is not the signal voltage to be measured, but a manifestation of common mode interference.

Series mode interference and common mode interference

1. Series mode interference

Series mode interference (differential mode interference) and common mode interference (ground interference). Take the two PCB traces on the motherboard (the wires connecting the components of the motherboard) as an example. The so-called cross-mode interference refers to the interference between the two traces; while the common-mode interference is the interference between the two traces and the PCB ground. Interference caused by potential differences between lines.

To measure the series-mode interference voltage, it was previously recommended to use a tube voltmeter. On-site measurements can be made using a digital multimeter with an AC millivolt block. As shown in the figure above, connect the voltmeter across the positive and negative terminals of the meter input to measure. Usually the series mode interference voltage is mostly in the range of a few millivolts to tens of millivolts.

2. Common mode interference

Common mode interference refers to the AC signal where the interference voltage appears between any input terminal (positive terminal or negative terminal) of the instrument and the ground. This interference is also called "interference to ground" and "vertical interference".

To measure the common-mode interference voltage, you can use a high-impedance voltmeter to measure it, or you can use the AC voltage block of a digital multimeter to measure it. As shown in the figure above, first connect the voltmeter between the positive terminal of the instrument input and the ground for measurement, and then connect the voltmeter between the negative terminal of the instrument input and the ground for measurement. Usually the common mode interference voltage is mostly within a few degrees. Voltage to within the range of several dozen volts.

Anti-interference sources and anti-interference measures

To overcome and eliminate series-mode interference and common-mode interference at the scene, you must first understand the source of the interference, and only then can you take measures to overcome the interference. Sources of series mode interference: High-power transformers, AC motors, frequency converters, etc. all have strong alternating magnetic fields. If the connecting wires for instrument measurement and control pass through the alternating magnetic fields, they will be affected by these alternating magnetic fields. The instrument will AC voltage is induced in the input circuit and becomes an interference signal.

In order to overcome the impact of series mode interference on instruments and control systems on site, the following measures can be taken:

1. Signal lines such as thermocouples and analytical instruments should be transported away from strong electromagnetic fields and not too close to power lines;

2. Do not place instrument signal lines, control signal lines and power lines in parallel in the same bridge tray, or wear them in the same conduit. If necessary, signal lines should use shielded wires or shielded cables, and the shielding layer of the line should be grounded at one end. The way;

3. Add a filter circuit to the input end of the instrument;

4. For smart instruments, the digital filter constants should be set according to the on-site conditions and the number of stages of the filter circuit should be increased if necessary.

Sources of common mode interference: interference from high-voltage electric fields; interference introduced when measuring the temperature of an electric furnace. For example, at high temperatures, the power supply of an electric furnace leaks to the thermocouple through the refractory bricks and the porcelain protection sleeve of the thermocouple, causing the thermocouple to interact with the thermocouple. Interference voltage is generated between the ground; interference is introduced due to different ground potentials; and when the electric heater of the ammonia synthesis tower is heated, it will also cause interference to the thermocouple. Most of the interference sources are AC voltage and possibly DC voltage.

In order to overcome the impact of common mode interference on instruments and control systems on site, the following measures can be taken:

1. Float the measuring thermocouple;

2. The instrumentation amplifier is also floating;

3. If the measurement object permits, do not use an exposed-end thermocouple to avoid grounding the hot electrode;

4. The thermocouple protection sleeve must be reliably grounded;

5. Use equipotential shielding when using shielded wires;

6. Install a bypass capacitor on the signal line. The above is an analysis of some factors that may interfere with analog quantities. I hope it can help you.