Interference in precision meter applications

The formation of precise meter interference is due to the existence of interference sources and influences on instruments and meters through certain coupling channels. In order to reduce these effects, we should consider the problem of interference suppression and try to improve the anti-interference ability when designing instruments. In practice, the coupling channel should be cut off and combined with twisting, shielding, grounding, balancing, filtering, isolation and other methods to suppress interference. At the same time, it is required that the display instrument has high temperature, low temperature, high pressure, corrosion, high viscosity and better dynamic characteristics, so as to reduce the measurement error of the measured parameters.

1. suppression of serial mode interference (cascade interference is the interference voltage at the input of instrument and meter to be superimposed on the detected signal)

Serial mode interference may occur at the source of the signal and is likely to be sensed or received from the wire. Because the interference between the mode and the signal is the same, the harmful effect is not easy to eliminate once the series mode interference occurs, so it should be avoided first.

(1) signal wire twisting for electromagnetic induction, try to lead away from the strong electricity equipment and power network, adjusting wire direction and reduce wire loop area is necessary, only adjust the precision table line direction and the two signal line with a short pitch stranding, interference voltage can be reduced to the original 1/10~1/100; for electrostatic induction, when the two signal line with double stranded form twist and the two signal lines to the interference source is roughly equal to the length of time (usually a wire cutter becomes 20 times the diameter of the pitch), can make the signal loop encircling area is reduced, and the electric potential in series mode interference loop through the inductive coupling in the two signal line difference is reduced.

(2) shielding: in order to further prevent the interference of electric field, the signal wire can be wrapped with metal mesh (or metal skin), and then a layer of insulation or signal wire is coated on the outside, and the shielding cable is used directly to shield the grounding. The non-magnetic shielding layer has no effect on the magnetic field of 50 Hz. If necessary, the signal wire can be inserted into the iron tube, so that the signal wire can be magnetically shielded. After electrostatic shielding, the induced potential is reduced to the original 1/100~1 /1000.

(3) filter: the DC signal with very slow change, adding filter circuit at the input of instrument and meter, so that the interference that is mixed with the effective signal is attenuated to the minimum. The R-C filter circuit is added to the input stage two to three before the input stage, and the double T filter with lower internal resistance is better.

(4) cancellation: double integrator and pulse width modulation digital instruments, the average value of the input signal, rather than the instantaneous value of the A/D conversion, can be some of the average interference.

(5) try to separate the signal line from the power line. Reasonable wiring, under the condition of the current direction of wire processing to reduce mutual interference in the opposite direction, the magnetic field generated; do not allow signal and power lines in parallel together, also should not be a threading hole into the instrument inside. The low level signal line should be connected to the adjacent position of the signal terminal as short as possible, so as to reduce the area of inductive interference, and absolutely forbid the power line and signal line with the same cable. Do not use the same wiring plug-in for high and low level lines. In the event of an emergency, the high and low level lines are opened next to the connectors, with the earth wire terminals and the spare terminals in the middle.

2. suppression of common mode interference (common mode interference is the interference between the input of any instrument and the earth)

(1) correct earthing. The meaning of grounding can be understood as obtaining a equipotential point or surface, which is the reference potential of a circuit or system, but not necessarily for the earth potential. For safety reasons, both the instrument and signal source housings are grounded and zero potential is maintained. But when the ground is not handled properly, a loop will be formed to introduce the interference into the instrument. In order to improve the anti-interference ability of instrument, usually in the low level measuring instrument in the instrumentation amplifier and the shell (the earth) (namely "floating insulation amplifier"), to cut off the way of common mode interference voltage leakage, the interference can not enter. In the low level test, the signal line should have only one point grounding line and the signal shielding layer should also have one point grounding, both signal lines and instruments are required to be shielding, grounding and shielding properly combined, can often solve the problem of interference most. When there is a not connected to a signal source and a grounding amplifier, common signal line shielding layer connected to the amplifier. When there is a grounded signal source and a grounded signal source connected to the amplifier, even if termination is not the land public end of the shielding layer of the signal wire or cables to the signal source, keep zero potential, can effectively cut off the leakage current potential, improve the signal anti-interference ability, the the method is used in the measurement system.

(2) the instrument adopts double shield floating ground protection technology: in order to improve the instrument's ability to resist common mode interference, while the input part of the amplifier is floating, the instrument adopts double shield floating protection. In addition to using the watch case as a shield, the amplifier input portion is shielded by an inner shield in the instrument and meter. No electrical connection is made between the two shielding layers, between the amplifier input section and the inner shield layer. Not with the inner shielding layer casing of instrument is connected and the shielding layer alone leads to a line as a protective shield end and the signal line is connected, so that the protective shield extends to the signal line length, while shielding the signal line in the signal source at the grounding point, so that the input signal shielding and shielding instrument the stability of the signal source, at the same potential state. So shielding can be used to reduce the common mode voltage that is coupled to the wire.

(3) applying a balanced circuit: the stability of a system depends on the balance of the signal source, signal lead, load, and other stray distributed parameters. In order to improve the capability of instrument against common mode interference, balancing measures are adopted to make the voltage converted on the two lines equal, so as to reduce the part common mode voltage which is coupled to the load.

(4) the interference of the interference introduced by the power supply: the main interference in the precision meter is the leakage current produced by the small power transformer. To prevent the leakage of current interference, the primary winding of the transformer on the shielding layer, and the shielding layer grounding, the phase voltage of a primary winding of the transformer through the distributed capacitance on the shielding layer, the leakage current flows directly, without interference into the amplifier, measuring circuit and signal source. In order to prevent the interference of power transformer is introduced, using three layers of shielding structure is the primary of the power transformer shielding layer grounding directly with the case, the secondary winding power supply device with all the shielding layer is connected to the secondary winding of the power amplifier, the shielding layer and the amplifier in equipotential state. The pulse shaped interference caused by power supply, has a great influence on the digital circuit, the high-frequency filter should be installed in the power line, iron shielding box filter should be installed in the input and output leads through the capacitor filtering in the.