Trying to eliminate or suppress the interference of electronic circuits is always a problem that needs to be solved in circuit design and application. The sensor circuit is usually used to measure weak signals and has high sensitivity. If the influence of various types of interference cannot be solved, it will bring a large error to the circuit and its measurement, and even cause the circuit to flood the normal measurement signal due to the interference signal. Can not work normally. Here, the internal noise and external interference in the design of the sensor circuit are studied, and it is concluded that reasonable and effective anti-interference measures can be taken to ensure the normal operation of the circuit and improve the reliability, stability and accuracy of the circuit. The sensor circuit is usually used to measure weak signals and has high sensitivity, but it is also easy to receive external or internal irregular noise or interference signals. If the magnitude of these noises and interferences can be compared with the useful signals, then the sensor The useful signal at the output of the circuit may be overwhelmed, or because the useful signal component and the noise interference component are difficult to resolve, it will hinder the measurement of the useful signal. Therefore, in the design of sensor circuits, anti-interference design is the key to the success of sensor circuit design.
1 Internal noise of the sensor circuit
1.1 High frequency thermal noise
High frequency thermal noise is caused by the random movement of electrons inside the conductor.
The higher the temperature, the more intense the electronic motion. The random motion of the electrons inside the conductor creates many tiny current fluctuations inside it. Because it is disordered, its average total current is zero, but when it is used as a component (or as part of the circuit) it is amplified. After the circuit, its internal current is amplified into a noise source, especially for high-frequency thermal noise of circuits operating in the high-frequency band.
Usually in the power frequency, the thermal noise of the circuit is proportional to the passband, and the wider the passband, the greater the influence of the thermal noise of the circuit. In the passband Δf, the effective value of the circuit thermal noise voltage:
Taking a 1 kΩ resistor as an example, if the passband of the circuit is 1 MHz, the effective value of the open circuit voltage noise across the resistor is 4 μV (the temperature is room temperature T = 290 K). It seems that the electromotive force of the noise is not large, but if it is connected to a gain circuit with a gain of 106 times, the output noise can reach 4 V, and the interference to the circuit is great. 1.2 Low-frequency noise Low-frequency noise is mainly caused by the discontinuity of internal conductive particles. In particular, the carbon film resistor has many tiny particles inside the carbonaceous material, and the particles are discontinuous. When the current flows, the electrical conductivity of the resistor changes to cause a change in current, resulting in a flash arc similar to poor contact. . In addition, transistors may also produce similar burst noise and flicker noise, the mechanism of which is similar to the discontinuity of the particles in the resistor, and also to the degree of doping of the transistor. 1.3 Shot noise generated by semiconductor devices The amount of charge accumulated in this region changes due to the change in the voltage of the barrier region across the semiconductor PN junction, thereby exhibiting a capacitive effect. When the applied forward voltage rises, the electrons in the N region and the holes in the P region move toward the depletion region, which is equivalent to charging the capacitor. When the forward voltage is reduced, it again causes electrons and holes to move away from the depletion region, which is equivalent to a capacitor discharge. When a reverse voltage is applied, the change in the depletion region is reversed. When a current flows through the barrier region, this change causes a small fluctuation in the current flowing through the barrier region, thereby generating current noise. The magnitude of the noise generated is proportional to the temperature and the bandwidth Δf. 1.4 Interference of electromagnetic components on the circuit board Many circuit boards have electromagnetic components such as relays and coils. When the current passes, the inductance of the coil and the distributed capacitance of the outer casing radiate energy to the surroundings, and the energy is generated by the surrounding circuits. interference. Components such as relays work repeatedly. When the power is turned off and off, an instantaneous reverse high voltage is generated to form an instantaneous surge current. This momentary high voltage will have a great impact on the circuit, thereby seriously disturbing the normal operation of the circuit. 1.5 The noise resistance of a resistor comes from the inductance of the resistor, the capacitive effect, and the thermal noise of the resistor itself. For example, a solid core resistor with a resistance of R can be equivalent to a series-parallel connection of the resistor R, the parasitic capacitor C, and the parasitic inductance L. In general, the parasitic capacitance is 0.1 to 0.5 pF and the parasitic inductance is 5 to 8 nH. These parasitic inductances and capacitances cannot be ignored at frequencies above 1 MHz. All types of resistors generate thermal noise. When a resistance of R (or the body resistance of BJT or the channel resistance of the FET) is not connected to the circuit, the thermal noise voltage generated in the bandwidth B is:
Where: k is the Boltzmann constant; T is the absolute temperature (unit: K). The thermal noise voltage itself is a function of the time of a non-periodic change, so its frequency range is very broad. Therefore, the wideband amplifying circuit is affected by noise more than the narrow band. In addition, the resistor also generates contact noise, and its contact noise voltage is:
Where: I is the mean square value of the current flowing through the resistor; f is the center frequency; k is a constant related to the geometry of the material. Since Vc plays an important role in the low frequency band, it is the main noise source of the low frequency sensor circuit. 1.6 Transistor Noise The noise of transistors mainly includes thermal noise, shot noise, and flicker noise. The thermal noise is caused by the irregular thermal motion of the carriers passing through the body resistance of the three regions in the BJT and the corresponding lead resistance. The noise generated by rbb' is the main one. The current in the so-called BJT is just an average. In fact, the number of carriers injected into the base region through the emitter junction is different at each instant, and thus the emitter current or the collector current has irregular fluctuations, which generate shot noise. The noise caused by the poor cleaning of the transistor surface due to the semiconductor material and the manufacturing process level is called flicker noise. It is related to the recombination of minority carriers on the semiconductor surface, which is characterized by the fluctuation of the emitter current, and its current noise spectral density is inversely proportional to the frequency, also known as 1/f noise. It plays a major role mainly in the low frequency (below kHz) range.
1.7 Noise of integrated circuits There are two types of noise interference in integrated circuits: one is radiated and the other is conductive. These noise spikes can have a large impact on other electronic devices connected to the same AC grid. The noise spectrum extends beyond 100 MHz. In the laboratory, you can use a high-frequency oscilloscope (above 100 MHz) to observe the waveform between an integrated circuit power supply and a ground pin on a general-purpose microcontroller system board. You can see that the noise spikes peak-to-peak up to hundreds of millivolts. Even the volt level. 2 External interference of the sensor circuit 2.1 Power supply interference The DC power supply of most electronic circuits is provided by the grid AC power supply after filtering and voltage regulation. If the power system is not cleaned, it will interfere with the test system. At the same time, the start and stop of large AC power equipment near the sensor test system will generate a very high frequency surge voltage superimposed on the grid voltage. In addition, lightning induction can also generate high-frequency surge voltages of high amplitude on the grid. If these interference signals enter the sensor interface circuit along the AC power line, it will interfere with its normal operation and affect the system's test accuracy.
2. 2 Interference sensor interface of the ground wire Each circuit often shares a DC power supply, or although one power supply is not shared, there is often one ground between different power supplies. Therefore, when the current of each part of the circuit flows through the common ground resistance (ground) When the line conductor is blocked, a voltage drop is generated, and the voltage drop becomes a noise interference signal that affects each other. At the same time, in the long-distance measurement, the sensor and the detecting instrument are grounded at two places respectively, so there is a large ground potential difference between the two "grounds", and a common mode interference voltage is easily formed at the input end of the meter. Common sources of common mode interference are equipment leakage to ground, ground potential difference, and the line itself has ground disturbance. Due to the unbalanced state of the line, common mode interference is converted into normal mode interference, which is difficult to remove. 2.3 Signal channel interference Usually the sensor is located at the production site, and the measuring device such as display and recording is installed in the control room at a certain distance from the site. This requires a long signal transmission line, and the signal is easily interfered during transmission. Causes the transmitted signal to be distorted or distorted. The interference encountered by long-distance signal transmission is: (1) electromagnetic induction interference of long-line electromagnetic fields in the surrounding space. (2) Crosstalk between signal lines. When a strong signal line (or a line whose signal changes rapidly) is close to the weak signal line, inter-line interference is generated by the distributed capacitance and mutual inductance between the lines. (3) Ground interference of long-line signals. The longer the signal line is, the longer the signal ground line is, that is, the ground line resistance is large, forming a large potential difference.
2.4 Space electromagnetic wave interference space Electromagnetic wave interference mainly includes:
(1) Lightning, changes in the electric field of the atmosphere, changes in the ionosphere, and electromagnetic radiation from sunspots;
(2) Communication equipment, television, radar, etc. in the regional space emit strong electromagnetic waves through the antenna;
(3) Interference caused by electromagnetic waves in local space to circuits and equipment, such as glow discharge interference generated by gas discharge facilities such as xenon lamps and fluorescent lamps, and interference caused by electric waves generated by arc discharge.
3 Measures to suppress the noise of the sensor circuit 3.1 Reasonably select the semiconductor components with low noise according to different operating frequencies. In the low frequency band, the transistor has large noise due to problems such as barrier capacitance and diffusion capacitance. Since the junction field effect transistor is a majority carrier, the current non-uniformity of the barrier region does not exist. Moreover, the reverse current between the gate and the conductive trench is small, and the generated shot noise is small. Therefore, the field effect transistor should be used in the middle and low frequency pre-stage circuits, which can not only reduce the noise but also have a higher input impedance. In addition, if it is necessary to replace a semiconductor component such as a transistor, it must be compared and selected, even if the parameters of the semiconductor device of the same type are different. Similarly, the noise resistance of the carbon film resistor and the metal film resistor in the circuit is also different. The noise of the metal film resistor is smaller than that of the carbon film, especially when the small signal input of the front stage is used, a metal film with low noise can be considered. resistance. 3.2 Choosing the appropriate amplifier circuit according to different working frequency bands and parameters Selecting the appropriate amplifier circuit not only has a direct impact on the circuit of this stage, but also has an important impact on the working parameters and working state of the whole circuit.
When the cascode configuration is connected, the circuit has a higher amplification gain, and its noise has less influence on the latter stage. In the case of a common configuration, there is a higher input impedance and a better frequency response. Therefore, according to different circuits, the parameters should have different requirements. Choosing a good circuit can not only simplify the circuit structure, but also reduce the interference of noise on the whole circuit. Under the conditions allowed by the circuit performance parameters, digital circuits with better anti-interference ability should be used as much as possible. 3.3 Adding the filter link in the sensor circuit In the amplifier circuit, the wider the frequency band, the larger the noise, and the frequency of the useful signal is often within a certain range. Therefore, a filter link can be added to the circuit to filter out or attenuate the interference as much as possible. Signals to achieve the purpose of improving signal-to-noise ratio to suppress interference. The filtering technique is particularly effective for suppressing interference caused by the coupling of the wires to the circuit, and the filter of the corresponding frequency band is connected to the signal transmission channel, and various filters are one of effective measures for suppressing differential mode interference.
Commonly used filters in automatic detection systems are: (1) RC filters. When the signal source is a sensor with slow signal change such as thermocouple or strain gauge, the use of a small-volume, low-cost passive RC filter will have a better suppression effect on the series mode interference. (2) AC power filter. The power network absorbs various high and low frequency noises. The common LC filter is used to suppress the noise mixed into the power supply. For example, a high frequency filter composed of a 100 μH inductor and a 0.1 μF capacitor can absorb high frequency noise in the short and medium wavelength bands. interference. (3) DC power supply filter. DC power is often shared by several circuits. In order to avoid mutual interference between several circuits through the internal resistance of the power supply, an RC or LC decoupling filter should be added to the DC power supply of each circuit to filter out low frequency noise. 3.4 Negative feedback circuit to suppress noise Negative feedback circuit can stabilize the circuit by sampling and control of feedback signal, improve the signal-to-noise ratio of the amplifier, and improve the dynamic performance of the amplifier circuit. The negative feedback signal can stabilize the static operating point of the circuit, thereby stabilizing the temperature, current, voltage and other parameters of the circuit.
In a multi-stage circuit, the first-stage circuit is often a small-signal configuration, so a common-emitter circuit configuration with a large gain is often used. Unless otherwise required, the common-fire configuration circuit is often without negative feedback. Therefore, the noise generated by the first stage circuit can only be suppressed by the negative feedback circuit of the latter stage. For a multi-stage circuit, the static operating point of the current stage is stabilized by a negative feedback signal, which can suppress the generation and propagation of noise of the circuit of the current stage. Therefore, in multi-stage circuits, the negative feedback circuit is an important means of suppressing noise. 3.5 Suppressing and Reducing the Noise Input of the Input Bias Circuit The bias circuit noise is typically generated by the input bias shunt resistor. When the DC current flowing through the bias resistor is too large, excess energy is generated to generate current noise. If a suitable bias circuit is selected, the noise can be short-circuited to the ground through the bypass capacitor, which can suppress the noise output and reduce the influence on the next-stage circuit. In addition, high-quality signal source is also an important guarantee for circuit anti-interference.
4 Measures to reduce sensor circuit interference 4.1 Reasonable layout Reasonable circuit layout can reduce the mutual interference between circuits in different working frequency bands, and also make the filtering of interference signals relatively simple. 4.1.1 Anti-interference measures for grounding layout In order to overcome the interference caused by the unreasonable grounding of the grounding line, when designing the printed circuit, the circuits of different circuits should be avoided to flow through a certain common grounding line at the same time. Especially in high-frequency circuits and high-current circuits, it is more important to pay attention to ground connection. Separating the "communication ground" from the "DC ground" is an effective way to reduce noise crosstalk through the ground. 4.1.2 Anti-interference measures for power supply wiring When wiring, first separate the AC power supply part from the DC power supply part, do not share the grounding wire, or separate the “AC ground†and “DC ground†to reduce noise through the ground wire. Crosstalk. In addition, in the DC power supply loop, changes in load can cause power supply noise. Configure decoupling capacitors to suppress noise due to load changes. The specific configuration method is to connect a 10~100μF electrolytic capacitor to the power input terminal. If the position of the printed circuit board allows, the anti-interference effect of the electrolytic capacitor above 100μF will be better. When wiring the power line, increase the width of the power line as much as possible according to the current of the printed circuit board to reduce the loop resistance. At the same time, the direction of the power line and ground line and the data signal transmission are consistent, which helps to enhance the anti-interference ability.
4.1.3 Anti-interference measures for component layout (1) Suppress electromagnetic interference. Components that may affect or interfere with each other should be separated or shielded as much as possible. Try to shorten the wiring between the components of the high-frequency part, reduce their distribution parameters and mutual electromagnetic interference. (If you need to use the metal shield for the high-frequency part, you should also leave the area occupied by the shield on the board. ). Components that are susceptible to interference should not be too close. The components of the high voltage section (220 V) and the weak current section (DC power supply), input stage and output stage should be separated as much as possible. When the DC power lead is long, filter components should be added to prevent 50 Hz interference. Components such as speakers, electromagnets, and permanent magnet meters generate a constant magnetic field, and high-frequency transformers, relays, etc. generate alternating magnetic fields. These magnetic fields not only interfere with surrounding components, but also affect the surrounding printed conductors.
Such interference should be treated differently according to the situation. Generally, attention should be paid to reducing the cutting of the magnetic wires to the printed conductors. When determining the position of the two inductive components, try to make their magnetic field directions perpendicular to each other and reduce the coupling between them. The interference source is magnetically shielded, and the shield should be well grounded. When the signal is directly transmitted using the high-frequency cable, the shield of the cable should be grounded at one end. (2) Suppress thermal interference. Interference caused by elevated temperatures should also be noted in printed board design. When designing a printed board, measures should be taken to thermally isolate the components. For example, temperature-sensitive components, such as transistors, integrated circuits and other heat-sensitive components, large-capacity electrolytic capacitors, etc., should not be placed near the heat source or in the upper part of the device. The long-term operation of the circuit causes the temperature to rise, which will affect the working state and performance of these components.
4.2 Shielding technology The shielding technology can effectively prevent the interference of electric or magnetic fields. Shielding can be divided into electrostatic shielding, electromagnetic shielding and low frequency magnetic shielding. 4.2.1 Electrostatic shielding is made of a metal with good electrical conductivity such as copper or aluminum. A sealed metal container is fabricated and connected to the ground wire, and the circuit to be protected is placed therein so that the external interference electric field does not affect its internal circuit. In turn, the electric field generated by the internal circuit does not affect the external circuit. For example, in a sensor measuring circuit, a conductor with a gap is inserted between the primary and secondary of the power transformer, and grounded to prevent electrostatic coupling between the two windings. 4.2.2 Electromagnetic shielding For high-frequency interference magnetic fields, the eddy current principle is used to make high-frequency interference electromagnetic fields generate eddy currents in the shielding metal, consuming energy that interferes with the magnetic field, and the eddy current magnetic field cancels the high-frequency interference magnetic field, thereby making the protected circuit Protected from high frequency electromagnetic fields. If the electromagnetic shielding layer is grounded, it also has the function of electrostatic shielding. The output cable of the sensor is generally shielded by a copper mesh, which has both electrostatic shielding and electromagnetic shielding. The shielding material must be selected from low-resistance materials with good electrical conductivity, such as copper, aluminum or silver-plated copper.
4.2.3 Low-frequency magnetic shielding interference is a low-frequency magnetic field. At this time, the eddy current phenomenon is not obvious. The anti-interference effect is not very good only by the above method. Therefore, it is necessary to use a high-magnetic magnetic material as a shielding layer in order to The low-frequency interference magnetic line is limited to the inside of the magnetic shielding layer with small magnetic resistance, so that the protected circuit is protected from the coupling interference of the low-frequency magnetic field. The metal housing of the sensor inspection instrument acts as a low frequency magnetic shield. If it is further grounded, it will also function as electrostatic shielding and electromagnetic shielding. Based on the above three common shielding technologies, in the place where the interference is relatively serious, a composite shielded cable can be used, that is, the outer layer is a low-frequency magnetic shielding layer, and the inner layer is an electromagnetic shielding layer, which achieves double shielding. For example, the capacitive sensor's parasitic capacitance is a key problem that must be solved in actual measurement. Otherwise, its transmission efficiency and sensitivity should be low. The sensor must be electrostatically shielded, and the electrode lead-out line adopts double-layer shielding technology. It is the drive cable technology. In this way, the parasitic capacitance of the sensor during use can be effectively overcome.
4.3 Grounding Technology Grounding technology is one of the effective techniques to suppress interference and is an important guarantee for shielding technology. Proper grounding can effectively suppress external interference, and at the same time improve the reliability of the test system and reduce the interference factors generated by the system itself. There are two purposes for grounding: safety and interference suppression. Therefore, the grounding is divided into protective grounding, shielding grounding, and signal grounding. The protective grounding is for safety purposes, and the casing, chassis, etc. of the sensor measuring device are grounded. The grounding resistance is required to be less than 10 Ω; the shielding grounding is to form a low-resistance path to the ground by the interference voltage to prevent interference with the measuring device. The grounding resistance should be less than 0.02 Ω; the signal ground is the common line of the zero signal potential of the input and output of the electronic device, which itself may be insulated from the earth. The signal ground line is further divided into an analog signal ground line and a digital signal ground line. The analog signal is generally weak, so the ground line requirement is high; the digital signal is generally strong, so the ground line requirement can be lower. Different sensor detection conditions also have different requirements for the grounding method. A suitable grounding method must be selected. The common grounding method has a little grounding and multi-point grounding.
4.3.1 One-point grounding It is generally recommended to use one-point grounding in low-frequency circuits. It has a radial grounding wire and a busbar grounding wire. Radial grounding is the direct connection of each functional circuit in the circuit with the zero potential reference point; the bus-type grounding is to use a high-quality conductor with a certain cross-sectional area as the grounding bus, directly connected to the zero potential point, the ground of each functional block in the circuit. Can be connected to the bus. At this time, if multi-point grounding is used, multiple ground loops will be formed in the circuit. When low-frequency signals or pulsed magnetic fields pass through these loops, electromagnetic induction noise will be generated. Due to the different characteristics of each ground loop, the loops are closed in different loops. A potential difference is generated at the point to form an interference. To avoid this, it is best to use a grounding method. The sensor and measuring device form a complete inspection system, but the distance between the two may be far apart. Because the earth current in the industrial site is very complicated, the potential between the two parts of the outer casing is generally different; if the zero potential of the sensor and the measuring device are grounded at two places, that is, two points are grounded, there will be A large current flows through a signal transmission line with a very low internal resistance to cause a voltage drop, causing series mode interference. Therefore, a grounding method should also be used in this case.
4.3.2 Multi-point grounding It is generally recommended to use multi-point grounding for high-frequency circuits. At high frequencies, even a small piece of ground wire will have a large impedance drop, and with the effect of distributed capacitance, it is impossible to achieve a little grounding. Therefore, a planar grounding method, that is, a multi-point grounding method can be used, and a good The conductive planar body (such as one of the multilayer circuit boards) is connected to the zero potential reference point, and the ground of each high frequency circuit is closely connected to the conductive planar body. Since the high-frequency impedance of the conductive plane body is small, the potential of each potential is basically ensured, and a bypass capacitor is added to reduce the voltage drop. Therefore, this situation requires a multi-point grounding method. 4.4 Isolation Technology In the interface circuit, if more than two points are grounded, common-resistance coupling interference and ground-loop current interference may be introduced. The method of suppressing such interference is to use isolation techniques. There are usually two types of electromagnetic isolation and photoelectric isolation.
4.4.1 Electromagnetic coupling isolation The isolation transformer is used to cut off the circulating current. Since the ground loop is cut off, the two circuits have independent ground potential references, so no interference is caused, and the signals are transmitted through the coupling form. 4.4.2 Photocoupler Isolation Optocoupler is an electro-optical-electrical coupling device consisting of a light-emitting diode and a phototransistor package. The input and output are electrically insulated, so the device is Used for photoelectric control, it is now being used more and more to improve the system's resistance to common mode interference. This way, even if the input loop has interference, as long as it is within the threshold, it will not affect the output. 4.5 Other anti-interference technology (1) Voltage regulation technology.
At present, there are two types of regulated power supplies commonly used in the development of smart sensors and instrumentation: one is a series-regulated power supply provided by an integrated voltage regulator chip, and the other is a DC-DC regulated power supply, which interferes with the grid voltage fluctuations. Normal work is very effective. (2) Techniques for suppressing common mode interference. Using a differential amplifier to increase the input impedance of the differential amplifier or reduce the internal resistance of the signal source can greatly reduce the effects of common-mode interference. (3) Software compensation technology. External factors such as changes in temperature and humidity can also cause changes in certain parameters, causing deviations. Software can be used to correct according to changes in external factors and error curves to remove interference. 5 Conclusion Anti-jamming is a very complicated and practical problem. An interference phenomenon may be caused by several factors.
Therefore, in the design of the sensor circuit and the measurement and control system, not only anti-interference measures should be taken in advance, but also the phenomena encountered during the debugging process should be analyzed in time, the circuit principle of the sensor and its system, the specific wiring, shielding, and power supply. Anti-interference ability, digital ground or analog ground processing and protection forms are continuously improved to improve circuit reliability and stability.
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