Anti-jamming design scheme of sensor circuit

Trying to eliminate or suppress the interference of electronic circuits is a problem that always 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 interferences cannot be resolved, it will bring large errors to the circuit and its measurement, and even cause the circuit to be overwhelmed by the interference signal. Can not work normally.

Here, the internal noise and external interference in the sensor circuit design 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.

Sensor circuits are usually used to measure weak signals, with high sensitivity, but it is also easy to receive some irregular noise or interference signals from outside or inside. If the magnitude of these noises and interferences can be compared with the useful signal

Then the useful signal at the output end of the sensor circuit may be overwhelmed, or because the useful signal component and the noise interference component are difficult to distinguish, it will inevitably hinder the measurement of the useful signal.

Therefore, in the design of sensor circuits, anti-interference design is often the key to the success of sensor circuit design.

1.1 High frequency thermal noise

High-frequency thermal noise is caused by the irregular movement of electrons inside the conductor.

The higher the temperature, the more intense the electron movement. The irregular movement of electrons inside the conductor will form a lot of small current fluctuations inside it. Because it is a disorderly movement, its average total current is zero, but when it is used as a component (or as a part of a circuit) it is connected to amplify After the circuit is completed, the internal current will be amplified and become a noise source, especially the high-frequency thermal noise of the circuit working in the high-frequency band.

Usually in the power frequency, the thermal noise of the circuit is proportional to the passband. The wider the passband, the greater the influence of the thermal noise of the circuit. In the pass band △f, 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 present at both ends of the resistor is 4μV (set temperature as room temperature T=290 K).

It seems that the electromotive force of the noise is not large, but if it is connected to an amplifying circuit with a gain of 106 times, its output noise can reach 4 V, and the interference to the circuit is very large at this time.

1.2 Low frequency noise

Low-frequency noise is mainly caused by the discontinuity of internal conductive particles.

Especially for carbon film resistors, there are many tiny particles inside the carbonaceous material, and the particles are discontinuous. When current flows, the conductivity of the resistor will change and the current will change, resulting in a flash arc similar to poor contact. .

In addition, transistors may also produce similar popping noise and flicker noise, the mechanism of which is similar to the discontinuity of particles in the resistor, and is also related to the degree of doping of the transistor.

1.3 Shot noise generated by semiconductor devices

The change in the voltage of the barrier region at both ends of the semiconductor PN junction causes the amount of charge accumulated in this region to change, thereby exhibiting a capacitance effect.

When the applied forward voltage increases, the electrons in the N zone and the holes in the P zone move to the depletion zone, which is equivalent to charging the capacitor. When the forward voltage decreases, it keeps electrons and holes away from the depletion zone, which is equivalent to capacitor discharge.

When a reverse voltage is applied, the depletion zone changes in the opposite direction. When the current flows through the barrier area, this change will cause small fluctuations in the current flowing through the barrier area, thereby generating current noise. The size of the noise produced is proportional to the temperature and the bandwidth △f.

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 through, the inductance of the coil and the distributed capacitance of the shell radiate energy to the surroundings, and the energy will interfere with the surrounding circuits.

Components such as relays work repeatedly, and when the power is turned on and off, they will generate instantaneous reverse high voltage, forming an instantaneous surge current. This instantaneous high voltage will have a great impact on the circuit, which will seriously interfere with the normal operation of the circuit.

1.5 The noise of the resistor

The interference of resistance comes from the inductance and capacitance effect in the resistance and the thermal noise of the resistance itself.

For example, a solid core resistance with a resistance value of R can be equivalent to a series-parallel connection of resistance R, parasitic capacitance C, and parasitic inductance L.

Generally speaking, the parasitic capacitance is 0.1 to 0.5 pF, and the parasitic inductance is 5 to 8 nH. When the frequency is higher than 1 MHz, these parasitic inductances and capacitances cannot be ignored.

All kinds of resistors will produce thermal noise. When a resistor with a resistance value of R (or BJT body resistance or FET channel resistance) is not connected to the circuit, the thermal noise voltage generated in the bandwidth B is:

In the formula: k is Boltzmann's constant; T is the absolute temperature (unit: K). The thermal noise voltage itself is a non-periodic time function, so its frequency range is very wide. Therefore, the wide-band amplifier circuit is more affected by noise than the narrow-band.

In addition, the resistance will also produce contact noise, and its contact noise voltage is:

In the formula: I is the mean square value of the current flowing through the resistance; 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 The noise of the transistor

The noise of the transistor mainly includes thermal noise, shot noise, and flicker noise.

Thermal noise is generated when the irregular thermal movement of carriers passes through the bulk resistance of the three regions in the BJT and the corresponding lead resistance. Among them, the noise generated by rbb' is the main one.

Generally speaking, the current in BJT is just an average value. In fact, the number of carriers injected into the base region through the emitter junction is different at each instant, so the emitter current or collector current has irregular fluctuations, which will produce shot noise.

The noise caused by the poor cleaning of the surface of the transistor due to the semiconductor material and manufacturing process level is called flicker noise.

It is related to the recombination of minority carriers on the semiconductor surface, which is manifested as the fluctuation of the emitter current, and its current noise spectral density is approximately inversely proportional to the frequency, also known as 1/f noise. It mainly plays a major role in the low frequency (below kHz) range.

1.7 Noise of integrated circuits

There are generally two types of noise interference from integrated circuits: one is radiation type and the other is conduction type. These noise spikes will have a greater impact on other electronic devices connected to the same AC power grid. The noise spectrum extends above 100 MHz.

In the laboratory, you can use a high-frequency oscilloscope (above 100 MHz) to observe the waveform between the power and ground pins of an integrated circuit on the general microcontroller system board, and you will see noise spikes up to hundreds of millivolts. Even volts.

2.1 Interference from the power supply

The DC power supply of most electronic circuits is provided by the AC power supply of the grid after being filtered and stabilized. If the power system is not purified, it will interfere with the test system.

At the same time, the start and stop of large-scale AC power equipment near the sensor test system will generate high-frequency surge voltage superimposed on the grid voltage.

In addition, lightning induction will also generate high-frequency surge voltages with high amplitude on the power 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 test accuracy of the system.

2. 2 Ground wire interference

The circuits of the sensor interface often share a DC power supply, or although they do not share a power supply, different power supplies often share the same ground. Therefore, when the current of each part of the circuit flows through the common ground resistance (ground conductor resistance), it will 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 detection instrument are grounded at two places, so there is a large ground potential difference between the two "grounds", and common mode interference voltage is easily formed at the input of the instrument.

The source of common mode interference is generally the leakage of the equipment to the ground, the ground potential difference, and the line itself has interference to the ground. Due to the unbalanced state of the line, common mode interference will be converted into normal mode interference, which is more difficult to remove.

2.3 Interference of signal channel

Usually the sensor is set in the production site, and the display, recording and other measuring devices are 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 the transmission process, resulting in the transmission of the signal. Distortion or distortion.

The interference encountered in long-term signal transmission includes:

(1) The electromagnetic induction interference of the electromagnetic field in the surrounding space on the long line.

(2) Crosstalk between signal lines. When a strong signal line (or a line with a fast signal change speed) is close to a weak signal line, inter-line interference occurs through the distributed capacitance and mutual inductance between the lines.

(3) Ground interference of long-term signals. The longer the signal wire, the longer the signal ground wire, that is, the greater the resistance of the ground wire, which results in a larger potential difference.

2.4 Interference of electromagnetic waves in space The electromagnetic interference in space mainly includes:

(1) Thunder and lightning, changes in the electric field of the atmosphere, changes in the ionosphere, and electromagnetic radiation from sunspots, etc.;

(2) In the regional space, communication equipment, televisions, radars, etc. emit strong electromagnetic waves through antennas;

(3) The interference of local electromagnetic waves on circuits and equipment, such as the glow discharge interference produced by gas discharge facilities such as neon lamps and fluorescent lamps, and the interference caused by electric waves generated by arc discharge.

3.1 Reasonably choose low-noise semiconductor components according to different operating frequencies

In low frequency bands, transistors are noisy due to problems such as barrier capacitance and diffusion capacitance. However, because the junction field effect transistor is conductive by majority carriers, there is no problem of uneven current in the barrier region.

Moreover, the reverse current between the grid and the conductive channel is very small, and the shot noise generated is very small. Therefore, FETs should be used in the middle and low frequency front-end circuits, which can not only reduce noise but also have higher input impedance.

In addition, if you need to replace semiconductor components such as transistors, you must go through comparison and selection, even if the parameters of semiconductor devices of the same model are different.

Similarly, the noise figure of carbon film resistors and metal film resistors in the circuit are also different. The noise of metal film resistors is smaller than that of carbon film. Especially in the front-level small signal input, you can consider using metal film with low noise. resistance.

3.2 Choose the appropriate amplifier circuit according to different working frequency bands and parameters

Choosing an appropriate amplifier circuit not only has a direct impact on the circuit at this level, but also has an important impact on the working parameters and working status of the entire circuit.

For example, when the common emission configuration is connected, the circuit has a higher amplification gain, and its noise has a smaller effect on the subsequent stage. In the collective configuration, there is a higher input impedance and also a better frequency response.

Therefore, according to different circuits, there should be different requirements on the parameters. Choosing a good circuit can not only simplify the circuit structure, but also reduce the interference of noise to the entire circuit.

Under the condition that the circuit performance parameters allow, use digital circuits with better anti-interference ability as much as possible.

3.3 Add a filter link to the sensor circuit

In the amplifying circuit, the wider the frequency band, the greater the noise, and the frequency of the useful signal is often within a certain range, so a filter link can be added to the circuit to filter or attenuate the interference signal as much as possible to improve the signal-to-noise ratio suppression The purpose of the interference.

Filtering technology is particularly effective in suppressing the interference coupled to the circuit via the wire. The filter of the corresponding frequency band is connected to the signal transmission channel. Various filters are one of the effective measures to suppress the differential mode interference.

The filters commonly used in automatic detection systems are:

(1) RC filter. When the signal source is a sensor whose signal changes slowly, such as a thermocouple or a strain gauge, the use of a small, low-cost passive RC filter will have a better suppression effect on the series mode interference.

(2) AC power filter. The power supply network absorbs various high and low frequency noises. For this, LC filters are commonly used to suppress the noise mixed into the power supply. For example, a high frequency filter composed of an inductance of 100 μH and a capacitor of 0.1 μF can absorb high frequency noise in the short and medium bands. interference.

(3) DC power filter. The DC power supply is often shared by several circuits. In order to avoid mutual interference between several circuits caused by 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 Suppress noise through negative feedback circuit

The negative feedback circuit can stabilize the circuit through the sampling and control of the feedback signal, improve the signal-to-noise ratio of the amplifier, and improve the dynamic performance of the amplifier circuit in many aspects.

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 the multi-stage circuit, the first-stage circuit is the original small signal, so it is often used in the common-emitter circuit configuration with larger gain.

Unless it is a special need, the common injection 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 multi-stage circuits, the static operating point of this stage can be stabilized by negative feedback signals, which can suppress the generation and propagation of noise in the circuit of this stage. Therefore, in the multi-stage circuit, the negative feedback circuit is an important means of suppressing noise.

3.5 Suppress and reduce the noise of the input bias circuit

The noise of the input bias circuit is generally generated by the input bias shunt resistor. When the DC current flowing through the bias resistor is too large, the energy will be excessive and current noise will be generated.

If a proper 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 impact on the next-level circuit. In addition, a high-quality signal source is also an important guarantee for the circuit's anti-interference.

4.1 Reasonable layout

A reasonable circuit layout can reduce the mutual interference between circuits of different operating frequency bands, and at the same time make the filtering of interference signals relatively simple.

4.1.1 Anti-interference measures for ground wire layout

In order to overcome the interference caused by the unreasonable ground wire layout, when designing the printed circuit, it is necessary to avoid the circuits of different loops flowing through a certain section of the common ground wire at the same time.

Especially in high-frequency circuits and high-current circuits, more attention must be paid to the connection of the ground wire. Separating the "AC ground" and "DC ground" is an effective way to reduce the crosstalk of noise through the ground wire.

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, that is, separate the "AC ground" from the "DC ground" to reduce the crosstalk of noise through the ground wire.

In addition, in the DC power supply loop, the change of the load will cause power supply noise. The configuration of decoupling capacitors can suppress noise caused by load changes.

The specific configuration method is to connect a 10-100μF electrolytic capacitor at the power input terminal. If the location of the printed circuit board allows, the anti-interference effect of using an electrolytic capacitor above 100μF will be better.

When wiring the power line, according to the current of the printed circuit board, try to increase the width of the power line as much as possible to reduce the loop resistance.

At the same time, keeping the power line and ground line in the same direction as the data signal transmission will help enhance the anti-interference ability.

4.1.3 Anti-jamming measures for component layout

(1) Suppress electromagnetic interference. Components that may affect or interfere with each other should be separated or shielded as far as possible. Try to shorten the connection between the high-frequency components, reduce their distribution parameters and mutual electromagnetic interference (if you need to use a 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 cannot be placed too close.

The components of the strong current part (220 V) and the weak current part (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.

Speakers, electromagnets, permanent magnet instruments and other components will generate a constant magnetic field, and high-frequency transformers, relays, etc. will generate an alternating magnetic field.

These magnetic fields not only interfere with the surrounding components, but also affect the surrounding printed wires.

This type of interference should be treated differently according to the situation. Generally, several points should be noted:

Reduce the cutting of the printed wires by the magnetic field lines. When determining the position of the two inductive components, try to make their magnetic field directions perpendicular to each other to reduce the coupling between them;

Magnetically shield the interference source, and the shield should be well grounded;

When using high-frequency cables to directly transmit signals, the shielding layer of the cable should be grounded at one end.

(2) Suppress thermal interference. The interference caused by the temperature rise should also be paid attention to in the printed board design. When typesetting and designing printed boards, measures should be taken to conduct thermal isolation between components.

For example, temperature-sensitive components, such as transistors, integrated circuits and other thermal components, large-capacity electrolytic capacitors, etc., should not be placed near the heat source or on the upper part of the equipment.

The temperature rise caused by the long-term operation of the circuit will affect the working status and performance of these components.

4.2 Shielding technology

The use of 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

Use metals with good conductivity such as copper or aluminum as materials to make a closed metal container and connect it to the ground wire. Place the circuit to be protected in it so that the external interference electric field does not affect its internal circuit. In turn, the internal circuit produces The electric field will not affect the external circuit.

For example, in the sensor measurement circuit, insert a conductor with a gap between the primary and secondary of the power transformer and ground it to prevent electrostatic coupling between the two windings.

4.2.2 Electromagnetic shielding

For the high-frequency interference magnetic field, the principle of eddy current is used to make the high-frequency interference electromagnetic field generate eddy currents in the shielding metal, which consumes the energy of the interference magnetic field. The eddy current magnetic field cancels the high-frequency interference magnetic field, thereby protecting the protected circuit from the influence of the high-frequency electromagnetic field. .

If the electromagnetic shielding layer is grounded, it also serves as an electrostatic shield. The output cable of the sensor is generally shielded by copper mesh, which has the function of both electrostatic shielding and electromagnetic shielding.

Shielding materials must be low-resistance materials with good conductivity, such as copper, aluminum, or silver-plated copper.

4.2.3 Low-frequency magnetic shielding

If the interference is a low-frequency magnetic field, the eddy current phenomenon is not obvious at this time, and the anti-interference effect of the above-mentioned method is not very good. Therefore, a high-permeability material must be used as a shielding layer to limit the low-frequency interference magnetic line of induction The inside of the magnetic shielding layer with very small resistance protects the protected circuit from the interference of low-frequency magnetic field coupling.

The iron shell of the sensor detection instrument plays the role of low-frequency magnetic shielding. If it is further grounded, it will act as electrostatic shielding and electromagnetic shielding at the same time.

Based on the above three commonly used shielding technologies, composite shielded cables can be used where interference is serious, that is, the outer layer is a low-frequency magnetic shielding layer, and the inner layer is an electromagnetic shielding layer to achieve a double shielding effect.

For example, the parasitic capacitance of a capacitive sensor is a key problem that must be solved during actual measurement. Otherwise, its transmission efficiency and sensitivity will be reduced, and the sensor must be electrostatically shielded, and the electrode lead wire adopts double-layer shielding technology, which is generally called It is drive cable technology. This method can effectively overcome the parasitic capacitance of the sensor during use.

4.3 Grounding technology

Grounding technology is one of the effective technologies to suppress interference and an important guarantee for shielding technology. Correct grounding can effectively suppress external interference, and at the same time can 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 interference suppression. Therefore, grounding is divided into protective grounding, shielding grounding and signal grounding. Protective grounding is for safety purposes, and the housing and chassis of the sensor measuring device must be grounded.

The grounding resistance is required to be below 10 Ω; the shielding grounding is a low-resistance path formed by the interference voltage to the ground to prevent interference with the measuring device. The grounding resistance should be less than 0.02Ω; the signal grounding is the common line of the zero signal potential of the input and output of the electronic device, and it may be insulated from the earth.

The signal ground wire is divided into analog signal ground wire and digital signal ground wire. The analog signal is generally weak, so the ground wire requirement is higher; the digital signal is generally strong, so the ground wire requirement can be lower.

Different sensor detection conditions also have different requirements for the grounding method, and a suitable grounding method must be selected. Common grounding methods include one-point 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 radial grounding wires and bus-type grounding circuits.

Radial grounding means that the functional circuits in the circuit are directly connected to the zero potential reference point with wires;

Bus-type grounding is to use a high-quality conductor with a certain cross-sectional area as the grounding bus, which is directly connected to the zero potential point, and the ground of each functional block in the circuit can be connected to the bus nearby.

If multi-point grounding is used at this time, 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 caused. Since each ground loop has different characteristics, it is closed in different loops At the same point, a potential difference is generated and interference is formed. To avoid this situation, it is best to use a point grounding method.

The sensor and the measuring device constitute a complete detection system, but there may be a long distance between the two.

Because the earth current of the industrial site is very complicated, the potentials between the large connection points of the two parts of the housing are 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 larger current flows through a signal transmission line with a very low internal resistance, resulting in a voltage drop, causing series mode interference. Therefore, one point grounding method should also be used in this case.

4.3.2 Multi-point grounding

It is generally recommended that the high-frequency circuit should be grounded at multiple points. At high frequencies, even a small section of the ground wire will have a large impedance voltage drop. With the effect of distributed capacitance, it is impossible to achieve one-point grounding. Therefore, a flat grounding method, that is, a multi-point grounding method, can be used. The conductive plane body (such as one layer of a multilayer circuit board) is connected to the zero potential reference point, and the ground of each high-frequency circuit is connected to the conductive plane body.

Because the high-frequency impedance of the conductive plane body is very small, it basically guarantees the uniformity of the potential at each place, and at the same time, the bypass capacitor is added to reduce the voltage drop. Therefore, in this case, a multi-point grounding method should be used.

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 to suppress this type of interference is to use isolation technology. Usually there are 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 there is no interference, and the signal is transmitted through the coupling form.

4.4.2 Photoelectric coupling isolation

The photocoupler is an electrical-optical-electric coupling device, which is composed of a light-emitting diode and a phototransistor package. Its input and output are electrically insulated. Therefore, in addition to being used for photoelectric control, this device is now It is increasingly used to improve the system's ability to resist common mode interference. In 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-jamming technologies

(1) Voltage stabilization technology. At present, there are two types of stabilized power supplies commonly used in the development of smart sensors and instruments: one is a series-regulated power supply provided by an integrated stabilized chip, and the other is a DC-DC stabilized power supply, which can prevent the power grid voltage fluctuations from interfering with the instrument Normal work is very effective.

(2) Suppress common mode interference technology. Using a differential amplifier, increasing the input impedance of the differential amplifier or reducing the internal resistance of the signal source can greatly reduce the impact 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 make corrections based on changes in external factors and error curves to remove interference.

Anti-interference is a very complex and practical problem. An interference phenomenon may be caused by several factors.

Therefore, in the design of sensor circuits and measurement and control systems, not only should anti-interference measures be taken in advance, but also the phenomena encountered in the debugging process should be analyzed in time, and the circuit principle, specific wiring, shielding, and power supply of the sensor and its system should be analyzed in time. The anti-interference ability, the processing of digital ground or analog ground, and the protection form continue to improve to improve the reliability and stability of the circuit.