Grounding technique
Grounding technology is one of the effective techniques to suppress interference and an important guarantee of shielding technology. Correct grounding can effectively suppress external interference, improve the reliability of the test system, and reduce the interference factors generated by the system itself. The purpose of grounding is twofold: safety and suppression of interference. Therefore, grounding is divided into protection grounding, shielding grounding and signal grounding. For safety purposes, the chassis and chassis of the sensor measurement device should be grounded. Want to find ground resistance at 10? The following. Shielded grounding is the interference voltage to the ground to form a low resistance path to prevent interference with the measuring device. The ground resistance should be less than 0.02. .
Signal ground is the zero signal potential common line between the input and output of an electronic device, which may itself be insulated from the ground. Signal ground wire is divided into analog signal ground wire and digital signal ground wire, analog signal is generally weak, so the ground wire requirements are higher: digital signal is generally strong, so the ground wire requirements can be lower.
Different sensor detection conditions also have different requirements for grounding methods, you must choose the appropriate grounding methods, common grounding methods include one-point grounding and multi-point grounding. These two different grounding treatment measures are given below.
One point grounding
In the low frequency circuit, it is generally recommended to use a point ground, which has a radiating ground line and a bus-type ground line. Radioactive grounding is that the functional circuit in the circuit is directly connected to the zero potential reference point with a wire: busbar grounding is to use a high-quality conductor with a certain cross-sectional area as a ground bus, directly connected to the zero potential point, and the ground of each functional block in the circuit can be connected to the nearest bus. At this time, if the use of multi-point grounding, in the circuit will form a plurality of ground circuit, when the low-frequency signal or pulsed magnetic field through these loops, it will cause electromagnetic induction noise, due to the different characteristics of each grounding loop, the potential difference in different loop closure points, the formation of interference. To avoid this situation, it is best to use a point of grounding method.
The sensor and the measuring device constitute a complete detection system, but the two may be far apart. Because the telluric current in the industrial site is very complex, the potential between the major points of the two parts of the housing is generally not the same, if the zero potential of the sensor and the measuring device are grounded at two places respectively, that is, the two points are grounded, there will be a large current flowing through the signal transmission line with very low internal resistance to produce a voltage drop, resulting in series mode interference. Therefore, the one-point grounding method should also be used in this case.
Multipoint grounding
Multipoint grounding is generally recommended for high frequency circuits. At high frequency, even a small section of ground will have a large impedance voltage drop, coupled with the role of distributed capacitors, it is impossible to achieve a point of grounding, so the plane grounding method can be used, that is, the multi-point grounding method, the use of a good conductive plane (such as the use of a layer of the multi-layer circuit board) to connect to the zero potential reference point, the ground of the high-frequency circuit to the nearest conductive plane. Because the high frequency reactance of the conductive plane is very small, the same potential is basically guaranteed at each place, and the bypass capacitor is added to reduce the voltage drop. Therefore, the multi-point grounding method is used in this case.
Filtering technique
Filter is one of the effective means to suppress AC series mode interference. The common filter circuits in the sensor detection circuit are Rc filter, AC power filter and true current power filter.
