Anti-interference measures for measurement and control systems based on single chip microcomputers

The circuit of the single-chip microcomputer measurement and control system is relatively complex, and there are many reasons for interference. The following are some commonly used anti-interference measures.

1. Cut off the transmission path of interference

1) Increase the distance between interference sources (such as motors, relays) and sensitive devices (such as microcontrollers), isolate them with ground wires or add shielding covers to sensitive devices.

2) Reasonably divide the circuit board into different areas, and arrange the strong signal, weak signal, digital signal, and analog signal circuits in different areas.

3) The ground wires of the microcontroller and high-power devices should be grounded separately to reduce mutual interference. High-power devices should be placed at the edge of the circuit board as much as possible.

4) Using anti-interference components in key places such as microcontroller I/O ports and circuit board connecting lines can significantly improve the anti-interference performance of the circuit.

5) Place the crystal oscillator and microcontroller pins as close as possible, isolate the clock area with a ground wire, and ground and fix the crystal oscillator casing.

2. Try to use a single-chip microcomputer with strong anti-interference performance

1) Reduce the power supply noise inside the microcontroller

In traditional digital integrated circuit design, the power supply and ground terminals are usually arranged on two symmetrical sides. For example, the lower left corner is the ground and the upper left corner is the power supply. This allows the power supply noise to pass through the entire silicon chip. The improved method arranges the power supply and ground of the microcontroller on two adjacent pins, which not only reduces the current passing through the entire silicon chip, but also facilitates the design of power supply decoupling capacitors on the printed circuit board to reduce system noise.

2) Reduce the clock frequency

The clock circuit of the microcontroller measurement and control system is a frequency modulation noise source, which can not only interfere with the system, but also interfere with the outside world, making the electromagnetic compatibility test of other systems fail to meet the standards. Under the premise of ensuring the reliability of the system, the use of a microcontroller with a low clock frequency can reduce the noise of the system. Taking the 8051 microcontroller as an example, when the shortest instruction cycle is 1US, the clock is 12MHZ. The manufacturer of the MOTOROLA compatible microcontroller with the same speed reduces the clock frequency to 1/3 of the original without sacrificing the computing speed. In particular, the newly launched 68HC08 series microcontroller of MOTOROLA uses phase-locked frequency multiplication technology internally, which divides the external clock to 32KHZ, while the internal bus speed is increased to 8MHZ, or even higher.

3) EFT technology

With the development of VLSI, the anti-interference technology inside the microcontroller is also constantly improving. The 68HC08 series microcontroller newly launched by MOTOROLA uses EFT technology to further improve the anti-interference ability of the microcontroller. When the sine wave signal of the oscillation circuit is interfered by the outside world, some glitches will be superimposed on its waveform. If it is shaped by the Schmitt circuit, this burr will become a trigger signal to interfere with the normal clock signal. However, alternating the Schmitt circuit and RC filtering can make this kind of burr ineffective. This is the EFT technology.

3. Frequency jitter technology in single-chip microcomputer measurement and control system

Although the frequency pull signal is superimposed on the random noise of the input signal, the total noise of the conversion will increase, but the added noise is used to compensate for the output digital quantization noise, so that the quantization error is no longer a function of the input signal but a function of the instantaneous value of the dither noise. Therefore, the correlation between the quantization noise and the input signal can be removed by using the frequency dither signal. The size of the frequency pull signal is usually about 1/3LSB effective value. For example, without frequency pull, the quantization noise output by the ADC is a function of the instantaneous input signal amplitude. After quantization pull, because the amplitude of the dither signal does not depend on the input signal, the quantization noise is independent of the input signal, thereby eliminating the harmonic components of the ADC output, but this is at the expense of increasing the total noise. It should be pointed out that it is not necessary to actually apply dither noise at the ADC input. The thermal noise of the converter can also be used as a frequency dither signal, but the ADC must have enough output bits to ensure that the correlation between the input signal and the quantization noise can be removed.

4. Technology to prevent leakage current

The intelligent single-chip microcomputer measurement and control system should be used in a clean, dry, ventilated and suitable environment. When the system is damp, the insulation resistance decreases and leakage current will be generated. Light measurement will increase the measurement error and poor control; severe measurement will cause moderate failure and damage components. For example, when the printed circuit board is damp, the input impedance of the A/D converter decreases, the reading is inaccurate, and there is a jump phenomenon. This is also the main reason why some digital voltmeters cannot work properly during thunderstorms. For damp printed circuit boards, wipe the surface with anhydrous alcohol and then dry it with a hair dryer to eliminate leakage.

Some precision integrated circuits are specially equipped with guard rings to prevent inter-electrode leakage. For example, the ICL7650 chopper-stabilized precision operational amplifier and the HI7195A 5 1/2-bit A/D converter with a microprocessor produced by Harris Corporation of the United States are both equipped with two guard ring leads. When designing the circuit, grounding the two guard rings can eliminate the influence of slight leakage on the printed circuit board on the measurement.

5. Filtering technology

Filtering refers to the method of obtaining useful signals from signals mixed with interference or noise. The component that can achieve the above function is called a filter. There are three main types of filters commonly used in digital instruments: passive filters, active filters, and digital filters.

1) Passive filter

Passive filters are composed of R, L, and C components. According to the characteristics of the interference signal, low-pass filters, high-pass filters, and band-pass filters can be selected. In addition, there are other types such as band-stop filters. For 50HZ electromagnetic field interference, a double-T filter can be added at the input end of the measurement and control system.

2) Active filter

Active filters are various filters that contain active devices (such as transistors and operational amplifiers). Compared with passive filters that simply use R, L, and C components, they can save bulky inductance components and facilitate miniaturization and integration. Active filters are suitable for filtering at lower frequencies. The typical circuit of a second-order active bandpass filter is shown in the figure below.

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6. Isolation Technology

The so-called isolation technology refers to the technology that isolates the noise source from the signal line. In the microcontroller series, optocouplers are often used to achieve isolation between sensors and input channels, I/O interfaces, and internal circuits. In addition, when transmitting signals over long distances, optocoupler technology is also required. A network communication interface circuit using optocoupler technology is shown in the figure below. When isolating the transmitted analog signal, it is advisable to use a linear optocoupler, whose current transfer ratio is close to a constant.

In addition, transformer isolation, relay isolation, mother-of-pearl isolation and other technologies are often used in test systems.

7. Resist interference on the transmission line

Transmission lines should be twisted pairs or shielded coaxial cables. Although the frequency band of twisted pairs is narrow, they have high wave impedance and strong anti-common mode interference capability. The electromagnetic induction interference of each small loop on the twisted pair can offset each other, and their distributed capacitance is large, which can play the role of integral capacitance, so it has a certain resistance to electromagnetic fields. When using long transmission lines, attention should be paid to impedance matching. Otherwise, reflected waves will be generated on the transmission line, causing signal distortion.

8. Choose the correct grounding point

Inside the single-chip microcomputer measurement and control system, there are roughly 6 types of ground lines: signal line, analog ground, digital ground, power ground, AC ground and shielding ground.

When designing the whole circuit, you should decide whether to float or ground, which ground wire to use, and whether to choose single-point grounding or multi-point grounding based on technical conditions and actual conditions.

9. Sensor shielding

When using integrated temperature sensors in industrial sites, it is easy to introduce interference. To improve the signal-to-noise ratio, the sensor can be shielded. Some temperature sensors are packaged in TO-52 metal shells, and a dedicated pin is connected to the shell. When in use, this pin is grounded, and the shell can act as a shield. For integrated temperature sensors in plastic packages, a thin copper tube can be used as an outer shield if necessary. After the sensor is installed, it is sealed with epoxy resin to keep the two insulated. This fully sealed sensor is particularly suitable for measuring the temperature of liquids and vapors.

10. Shielding of plastic machines

At present, plastic chassis made of ABS engineering reverse material are popular among people for their beautiful appearance and light weight. In order to make the chassis have a shielding effect, a layer of conductive film can be covered on the inner surface of the chassis by spraying, vacuum, deposition and other methods. You can also process the shielding layer yourself, stick a layer of aluminum foil on the inner surface of the chassis, and then connect it to the public ground.

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11. Selection of components

To reduce the noise of components, metal film resistors and low-noise active devices should be used as much as possible. To reduce temperature drift, all components need to undergo high and low temperature aging treatment. If necessary, the preamplifier and active filter should be added with temperature compensation circuit. The integral capacitor of the A/D converter should be a polypropylene or polystyrene capacitor with low leakage and low dielectric loss factor. Tantalum capacitors should be used for the ICL7650 precision op amp and IC7660 DC/DC converter pump capacitor.

If low-speed devices can be used, high-speed devices will not be used. High-speed devices are only used in critical places.

12. Noise cancellation circuit

The bandgap reference voltage source is a high-stability voltage source used as a voltage reference. It is currently widely used in digital instruments, intelligent instruments and test systems. A 0.1UF noise-eliminating capacitor is connected in parallel to the reference voltage output to filter out high-frequency noise.

The maximum value of the noise voltage allowed to be added to the input terminal when the output state of the circuit remains unchanged is called the voltage noise margin. The higher the noise margin, the better the device's anti-interference ability. CMOS circuits should be used to replace TTL circuits. The noise margin of CMOS circuits can reach 40% of the power supply voltage, while that of TTL circuits is only about 16% of the power supply voltage.

13. Power supply is coupling capacitor

Many circuits in the microcontroller measurement and control system share a DC power supply. This requires that the power line should not introduce interference between the circuits, and the DC power line should not fluctuate when the load changes. But in fact, it is difficult to do this because of the internal impedance of the DC power supply and the AC impedance of the power supply lead cannot be zero. Using decoupling capacitors can not only reduce the internal impedance of the DC power supply, but also avoid mutual interference between the circuits through the power line.

A large number of digital ICs are used in the single-chip measurement and control system, and each digital IC itself is a pulse interference source, and they will also interfere with each other through the power line. The solution is to connect a 10UF~100UF tantalum capacitor in parallel to the power input end of the printed circuit board for power decoupling, and at the same time connect a high-frequency, low-distribution inductance ceramic capacitor in parallel to the power input end of each chip. The capacity is generally 0.1UF. When the frequency exceeds 15MHZ, 0.01JF can be used. The circuit is shown in the figure. The pins of the decoupling capacitor should be as short as possible.