How to identify common mode interference? Methods to eliminate common mode interference

The definition of common-mode interference, a simple method to measure common-mode interference, measuring with a high-impedance voltmeter, or measuring with the AC voltage range of a digital multimeter, methods to identify common-mode interference and eliminate common-mode interference, etc.

Common Mode Interference Definition and Measurement Method

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

Method for measuring common mode interference voltage:

To measure the common mode interference voltage, you can use a high resistance voltmeter or the AC voltage range of a digital multimeter.

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 in the range of a few volts to tens of volts.

2. Common mode interference

Common-mode interference refers to the interference voltage having the same amplitude on the signal line and its return line (generally called the signal ground line). The voltage here uses any nearby object (the earth, metal chassis, reference ground plate, etc.) as the reference potential, and the interference current loop flows in the loop formed by the conductor and the reference object.

1. How to identify common mode interference?

1) Judging from the interference source: the interference generated on the cable by lightning, nearby electric arcs, nearby radio stations or other high-power radiating devices is common-mode interference.

2) Judging from the frequency: Common mode interference is mainly concentrated above 1MHz. This is because common mode interference is induced to the cable through space, and this induction is easy to occur only at higher frequencies. However, there is an exception. When the cable passes by a strong magnetic field radiation source (for example, a switching power supply), it will also induce lower frequency common mode interference.

3) Measure with instruments: As long as you have a spectrum analyzer and a current caliper, you can measure and judge. The steps of judgment are as follows:

Place the caliper on the signal line or ground line (live or neutral) and record the interference intensity of a frequency of interest (f1);

Clamp the signal line and the ground line with the calipers at the same time. If interference at (f1) can be observed, the interference at (f1) contains common-mode interference components. To determine whether it contains only common-mode components, proceed to step 3.

Clamp the signal line and the ground line with the calipers respectively. If the amplitudes of the (f1) interference measured on the two lines are the same, then the (f1) interference only contains common-mode components; if they are different, then the (f1) interference also contains differential-mode components.

2. Methods to eliminate common mode interference

1) Use shielded twisted pair cables and ground them effectively.

2) In areas with strong electric fields, galvanized pipe shielding should also be considered.

3) When wiring, stay away from high-voltage lines, and do not bundle high-voltage power lines and signal lines together.

4) Do not share the same power supply with devices that may cause interference, such as electric locks.

5) Use a linear regulated power supply or a high-quality switching power supply (ripple interference is less than 50mV).

6) Use a differential circuit.