Analysis of the causes and solutions of series mode and common mode interference

There are two forms when the change of voltage and current is transmitted through the wire, which we call “common mode” and “differential mode”. The power cord of the equipment, the communication line of the telephone, etc., and the communication line exchanged with other equipment or peripheral equipment, have at least two wires, and these two wires are used as a round-trip line to transmit power or signals. But there is usually a third conductor in addition to these two wires, which is the “ground”. The interference voltage and current are divided into two types: one is that two wires are used for round-trip transmission; the other is that two wires are used for outgoing, and the ground wire is used for return transmission. The former is called “differential mode” and the latter is called “common mode”.

There are two forms when the change of voltage and current is transmitted through the wire, which we call “common mode” and “differential mode”. The power cord of the equipment, the communication line of the telephone, etc., and the communication line exchanged with other equipment or peripheral equipment, have at least two wires, and these two wires are used as a round-trip line to transmit power or signals. But there is usually a third conductor in addition to these two wires, which is the “ground”. The interference voltage and current are divided into two types: one is that two wires are used for round-trip transmission; the other is that two wires are used for outgoing, and the ground wire is used for return transmission. The former is called “differential mode” and the latter is called “common mode”.

Due to the complex environmental conditions of the Display instrument used in the field, and most of the measured parameters are converted into weak low-level voltage signals, and transmitted to the display instrument over a long distance, in addition to useful signals, there will be some signals related to the measured signal. Unrelated interference signals are mixed in, which will affect the correctness of the measurement results, and in severe cases, the instrument will not work properly. The way to produce interference and the measures to eliminate it are now introduced.

Common Interference Pathways

Interference sources may exist both inside and 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 all become sources of interference. Interference sources often enter the instrument through the following ways.

1. The connecting wire 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 the closed loop will induce an electric potential in this changing magnetic field. This induced potential is connected in series with the useful signal. When the distance between the sensor and the display instrument is long, the series mode interference is particularly prominent.

2. The interference source forms interference in the loop through capacitive coupling, which is the result of the interaction of the two electric fields. Through electrostatic coupling, a common voltage to the ground can be induced at the two input ends, which appears in the form of common mode interference. Since the common mode interference does not overlap with the signal, it does not directly affect the instrument. But it can form a leakage current to the ground through the measurement system, and the leakage current can directly act on the instrument and cause interference through the coupling of the resistance. Most of the interference caused by electromagnetic induction and electrostatic induction is power frequency interference voltage, but frequency converters, motors with commutators, etc. will produce harmonic interference. Due to the action of lightning, interference voltages will also be induced on the power line.

3. In some temperature measurement occasions, when the thermocouple electrode is directly welded to the metal part heated by electricity, since the metal part has a potential difference at each point parallel to the current direction, the interference voltage introduced at this time is also very large. At high temperature, the insulation resistance of the refractory material drops sharply, and the insulation performance of the ceramic protection tube and ceramic beads of the thermocouple will also decrease, and the power supply voltage of the electric furnace leaks to the thermocouple wire through the refractory brick, thermowell, ceramic bead, etc. , an interference voltage is generated between the thermocouple electrode and ground.

4. There is often a potential difference between different points in the earth, especially in the vicinity of high-power electrical equipment. When the insulation performance of these equipment decreases, the potential difference is larger. In the use of field instruments, sometimes there will be more than two grounding points in the loop, and the potential difference of different grounding points will be introduced into the display instrument to 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 on the signal line to the ground. This common voltage is not the signal voltage to be measured, but a manifestation of common mode interference.

Series Mode Interference Common Mode Interference

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

1. Series Mode Interference

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

Analysis of the causes and solutions of series mode and common mode interference

To measure the series-mode interference voltage, a tube voltmeter was recommended in the past, and a digital multimeter with an AC millivolt block can be used for measurement on site. As shown in the figure above, the voltmeter is connected across the positive and negative terminals of the meter input to measure, usually the series-mode interference voltage is mostly in the range of several millivolts to tens of millivolts.

2. Common mode interference

Common mode interference refers to the AC signal with the interference voltage appearing between any input terminal (positive or negative terminal) of the instrument and the ground. This interference is also called “ground interference” and “longitudinal interference”.

Analysis of the causes and solutions of series mode and common mode interference

To measure the common mode interference voltage, you can measure it with a high-resistance voltmeter, or you can measure it with the AC voltage block of a digital multimeter. As shown in the figure above, first connect the voltmeter between the positive terminal of the meter input and the ground to measure, and then connect the voltmeter between the negative terminal of the meter input and the ground to measure, usually the common mode interference voltage is mostly in several volts to tens of volts.

Anti-interference source anti-interference measures

To overcome and eliminate series-mode interference and common-mode interference on site, it is first necessary to find out the source of the interference, and then it is possible to take measures to overcome the interference.

The source of series-mode interference: high-power transformers, AC motors, frequency converters, etc. all have strong alternating magnetic fields. If the connecting wires measured and controlled by the instrument pass through the alternating magnetic fields, they will be affected by these alternating magnetic fields. The AC voltage is induced in the input loop of the device, which becomes an interference signal.

In order to overcome the influence of series mode interference on the instrument and control system, the following measures can be taken:

1. For example, the signal lines of thermocouples and analytical instruments should be transported away from strong electromagnetic fields, not too close to the power lines;

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

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

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

The source of common mode interference: the interference of high-voltage electric field; the interference introduced when measuring the temperature of the electric furnace. For example, at high temperature, the power supply of the electric furnace leaks to the thermocouple through the refractory brick and the porcelain protective sleeve of the thermocouple, so that the thermocouple and the thermocouple are connected to the thermocouple. Interference voltage is generated between grounds; interference introduced by different ground potentials; and when the electric heater of the ammonia synthesis tower heats up, it will also cause interference to the thermocouple. Most of the interference sources are AC voltages and may also be DC voltages.

In order to overcome the influence of common mode interference on the instrument and control system on site, the following measures can be taken:

1. Float the measuring thermocouple;

2. The instrumentation amplifier is also floated;

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

4. The thermocouple protection casing should be grounded reliably;

5. Equipotential shielding method is adopted when using shielded wire;

6. Install a bypass capacitor on the signal line.

The Links:   SKKT71/16D NL6448BC33-95D