1、 Preface

The application of sensor transmitters is very extensive, whether in industry, agriculture, national defense construction, or in daily life, education, scientific research 

and other fields, analog sensors can be seen everywhere. But in the design and use of analog sensors, there is a problem of how to achieve the highest measurement 

accuracy.

However, numerous interferences have been affecting the measurement accuracy of sensors, such as: there are many energy consuming devices on site, especially 

the start stop of high-power inductive loads, which often cause sharp pulse interference of hundreds or even thousands of volts in the power grid;

Under voltage or overvoltage in industrial power grids (the power supply voltage of Shexian Iron and Steel Plant fluctuates between 160V and 310V), often reaching 

about 35% of the rated voltage. This harsh power supply can sometimes last for several minutes, hours, or even days; When various signal lines are tied together or 

run on the same multi-core cable, the signal will be interfered with, especially when the signal line and AC power line run in the same long pipeline;

Poor performance of multi-channel switches or retainers can also cause channel signal interference;

Various electromagnetic, meteorological conditions, lightning, and even changes in the geomagnetic field in space can interfere with the normal operation of sensors;

In addition, changes in on-site temperature and humidity may cause changes in circuit parameters. The effects of corrosive gases, acid and alkali salts, outdoor 

sandstorms, rain, and even rodent bites and insect infestations can all affect the reliability of sensors.

The output of analog sensors is generally small signals, which have issues with small signal amplification, processing, shaping, and anti-interference. This means 

that the weak signals of the sensor are accurately amplified to the required unified standard signal (such as 1VDC~5VDC or 4mADC~20mADC) and meet the required 

technical specifications.

This requires designers to pay attention to certain issues that are not shown on the analog sensor circuit diagram, namely anti-interference problems. Only by 

understanding the sources and modes of interference of analog sensors, designing circuits to eliminate interference or measures to prevent interference, can the 

optimal state of applying analog sensors be achieved.

2、 Interference sources, types, and phenomena

Sensors and instruments are subject to various interferences during on-site operation. The principle of anti-interference is to analyze the specific situation and take 

different measures for different interferences. This flexible and adaptable strategy is undoubtedly contradictory to universality, and the solution is to adopt a modular 

approach. In addition to the basic components, instruments can be equipped with different options for different operating scenarios to effectively resist interference 

and improve reliability. Before further discussing the selection of circuit components, circuit and system applications, it is necessary to analyze the sources and types 

of interference that affect the accuracy of analog sensors.

1. Main sources of interference

(1) Electrostatic induction

Electrostatic induction is due to the presence of parasitic capacitance between two branch circuits or components, causing charges on one branch to be transferred 

to the other branch through the parasitic capacitance, hence also known as capacitive coupling.

(2) Electromagnetic induction

When there is mutual inductance between two circuits, the change in current in one circuit will be coupled to the other circuit through a magnetic field, and this 

phenomenon is called electromagnetic induction. For example, leakage magnetic flux of transformers and coils, parallel wires when energized, etc.

(3) Electric leakage current should

Due to poor insulation of internal components such as brackets, terminals, printed circuit boards, capacitors, or casings in electronic circuits, especially in the 

application environment of sensors with high humidity, the insulation resistance of insulators decreases, leading to an increase in leakage current and interference. 

Especially when leakage current flows into the input stage of the measurement circuit, its impact is particularly severe.

(4) RF interference

The main interference is the starting and stopping of large power equipment, as well as high-order harmonic interference. Such as interference from thyristor 

rectification systems.

(5) Other interferences

In addition to the aforementioned interferences, the on-site safety production monitoring system is also susceptible to mechanical interference, thermal 

interference, and chemical interference due to its poor working environment.

2. Types of interference

(1) Normal mode interference

Constant mode interference refers to interference signals whose intrusion is consistent across two round-trip lines. The source of normal mode interference is 

generally the strong alternating magnetic field around, which affects the instrument and generates AC electromotive force to form interference. This interference 

is difficult to eliminate.

(2) Common mode interference

Common mode interference refers to the interference signal flowing through a portion of each of the two lines, with ground as the common circuit, while the signal 

current only flows through the two round-trip lines. The sources of common mode interference are generally equipment leakage to ground, ground potential difference,

 and line interference to ground. Due to the unbalanced state of the circuit, common mode interference can be converted into normal mode interference, making it 

difficult to eliminate.

(3) Long term interference

Long term interference refers to interference that exists for a long time. The characteristic of this type of interference is that the interference voltage exists for a long 

time and does not change much. It is easy to detect with detection instruments. For example, electromagnetic interference from power lines or adjacent power lines 

is continuous AC 50Hz power frequency interference.

(4) Unexpected instantaneous interference

Unexpected instantaneous interference mainly occurs during the operation of electrical equipment, such as closing or opening, and sometimes also occurs during 

lightning strikes or the operation of wireless equipment.

Interference can be roughly divided into three aspects:

(a) Localized generation (i.e. unnecessary thermocouples);

(b) The coupling within the subsystem (i.e. the path problem of the ground wire);

(c) External interference (Bp power frequency interference).

3. Interference phenomenon

In applications, the following main interference phenomena are often encountered:

(1) When giving instructions, the motor rotates irregularly;

(2) When the signal is equal to zero, the digital display meter jumps around;

(3) When the sensor is working, its output value does not match the signal value corresponding to the actual parameter, and the error value is random and irregular;

(4) When the measured parameter is stable, the difference between the sensor output value and the signal value corresponding to the measured parameter is a 

stable or periodically changing value;

(5) Devices that share the same power supply as the AC servo system, such as displays, are not functioning properly.

There are two main channels for interference entering the positioning control system: signal transmission channel interference, which enters through the signal input 

channel and output channel connected to the system; Power supply system interference.

The signal transmission channel is the way for the control system or driver to receive feedback signals and issue control signals, because pulse waves can experience 

delay, distortion, attenuation, and channel interference on the transmission line. Therefore, interference from long lines is the main factor in the transmission process. 

Any power source and transmission line has internal resistance, which causes noise interference from the power source. Without internal resistance, any type of noise 

will be absorbed by the power source short circuit, and no interference voltage will be established in the line; In addition, the AC servo system driver itself is also a 

strong interference source, which can interfere with other devices through the power supply.

3、 Anti interference measures

1. Anti interference design of power supply system

The most serious hazard to the normal operation of sensors and instruments is the peak pulse interference of the power grid. Electrical equipment that generates 

peak interference includes welding machines, large motors, controllable machines, relay contactors, inflatable lighting fixtures with ballasts, and even soldering irons. 

Peak interference can be suppressed by a combination of hardware and software.

(1) Suppressing the impact of spike interference using hardware circuits

There are three common methods:

① Connect an interference controller designed according to the principle of spectrum equalization in series with the input end of the instrument's AC power supply

 and distribute the concentrated energy of the peak voltage to different frequency bands, thereby reducing its destructive power;

② Add a super isolation transformer to the input end of the instrument's AC power supply and use the principle of ferromagnetic resonance to suppress spike pulses;

③ Connect a varistor in parallel at the input of the instrument's AC power supply, and use the decrease in resistance value when the peak pulse arrives to reduce the 

voltage that the instrument receives from the power supply, thereby weakening the impact of interference.

(2) Using software methods to suppress peak interference

For periodic interference, programming can be used for time filtering, which means controlling the thyristor to conduct without sampling at the moment of 

conduction, effectively eliminating interference.

(3) Using a combination of hardware and software watchdog technology to suppress the impact of spike pulses

Software: Before the timer expires, the CPU accesses the timer once to restart the timer. The program runs normally, and the timer will not generate overflow 

pulses, so the watchdog will not work. Once a "flying program" occurs due to peak interference, the CPU will not access the timer before the scheduled time, resulting 

in a timing signal that triggers a system reset interrupt to ensure that the smart instrument returns to its normal program.

(4) Implement power supply grouping, for example: separate the driving power supply of the motor from the control power supply to prevent interference between 

devices.

(5) The use of noise filters can also effectively suppress the interference of AC servo drives on other devices. This measure can effectively suppress the 

above-mentioned interference phenomena.

(6) Adopting isolation transformer

Considering that high-frequency noise passes through transformers mainly not through mutual inductance coupling between the primary and secondary coils, but 

through parasitic capacitance coupling between the primary and secondary coils, a shielding layer is used to isolate the primary and secondary of the isolation 

transformer, reducing its distributed capacitance and improving its ability to resist common mode interference.

(7) Adopting power supplies with high anti-interference performance, such as those designed using spectrum equalization method. This power supply is very effective 

in resisting random interference. It can convert high peak disturbance voltage pulses into low peak voltage (voltage peak less than TTL level), but the energy of the 

interference pulses remains unchanged, thus improving the anti-interference ability of sensors and instruments.