Analysis of interference display meters, how to suppress the interference of meters

Eliminating noise sources is a proactive measure. For example, the plug connector is in poor contact, virtual soldering, etc., and this kind of interference source can be eliminated. In principle, noise sources should be eliminated.

However, in reality, many noise sources are difficult or impossible to eliminate. For example, sometimes the electromagnetic interference of the motor in the instrument in the pump room cannot be eliminated when the pump is running. In this case, protective measures must be taken to suppress interference.

1) Suppression of serial mode interference.

Serial-mode interference is in the same position as the signal being measured, so once it occurs, it is not easy to eliminate. Therefore, it should be prevented in the first place. The measures to prevent tandem interference are generally as follows:

1.Twisting of signal wires.

Because twisting the signal wires together can greatly reduce the area surrounded by the signal loop, and the distance between the two signal wires and the interference source can be roughly equal, and the distributed capacitance can also be about the same, so the series mode interference that enters the loop through inductive coupling between the magnetic field and the electric field can be greatly reduced.

2.Shield. In order to prevent interference from electric fields, the signal wires can be wrapped in metal. It is common practice to wrap the wire with a metal mesh (or ferromagnetic material) and an insulating layer. The purpose of shielding is to isolate the coupling of "fields" and suppress the interference of various "fields".

3.Filtering. For DC signals with very slow change speed, a filter circuit can be added to the input of the instrument to minimize the interference mixed with the signal. However, in actual engineering design, this method is rarely used, and usually, this is already considered in the circuit design process of the instrument.

2) Suppression of common-mode interference.

Since most of the signals in the instrument system are low, common-mode interference can also distort the instrument signal, resulting in various measurement errors. The usual measures taken to prevent common-mode interference are as follows:

1.Earthing. Normally, both the meter and the signal source housing are connected to ground and maintain zero potential for safety reasons.

The signal source circuit, as well as the instrumentation system, also needs to be stabilized to ground. However, if the grounding method is not appropriate, a ground loop will be formed. As is the case in Figure 3, two points to ground, due to the presence of a potential difference between the ground, resulting in common-mode interference.

As a result, instrumentation circuits are typically grounded at a single point at the system. However, in fact, it is impossible to insulate the signal source side from the ground, so in this sense, it is impossible to completely eliminate the interference caused by the ground potential difference. Therefore, in order to improve the anti-interference ability of the instrument, the secondary instrument is usually "floated" in the low-level measuring instrument, that is, the secondary instrument is insulated from the ground.

In order to cut off the leakage path of the common mode interference voltage, so that the interference cannot enter. In practical applications, we usually combine shielding and grounding, which can often solve most of the interference problems. If the shield is grounded on both the signal side and the instrument side, the ground potential difference will form a loop through the shield, because the ground resistance is usually much smaller than the resistance of the shield, so a potential gradient will be formed on the shield, and it will be coupled to the signal circuit through the distributed capacitance between the shield and the signal wire, so the shield must also be grounded at one point.

In addition, the grounding of the signal wire shield should be on the same side as the system grounding.