Because digital circuits use pulse signals with short rising/falling edges, they emit unwanted electromagnetic waves (noise) including high-frequency components to the outside, and they respond sensitively to electromagnetic waves (noise) from outside, causing malfunctions. In addition, there are also problems in the circuit, such as intermodulation distortion between lines, and power supply voltage fluctuations caused by sudden changes in current when digital devices are turned on/off. In this way, it is necessary to consider the distributed constant circuit composed of wiring inductance and parasitic capacitance in the digital circuit to prevent overshoot and undershoot from causing waveform chaos and signal reflection, delay, attenuation, and intermodulation distortion of electromagnetic interference between lines. The filters and shields that solve this problem are all analog technologies.
Due to the application of digital circuit technology in the control of automobiles, trains, and radios, it has achieved high reliability with high reliability that could not be achieved with analog technology before. However, noise can cause system and circuit malfunctions, and it is a fatal problem especially for machines. Even if the analog circuit has noise, it only temporarily reduces the accuracy of the data. Once the noise disappears, it has the characteristics of self-recovery function. Therefore, combining high-functional digital circuits and analog circuits with self-recovery/self-confirmation capabilities will be a safe solution to prevent malfunctions caused by noise in mobile control systems and digital circuits. Special attention should be paid to the circuit design. After the circuit design, in order to verify the work, it is necessary to assemble the circuit for experimentation. But as a result, it often appears that it does not work as designed. For example, the designed amplifier has become an oscillator. In the analog circuit, the noise from the digital circuit is mixed, which causes the waveform of the analog signal to be distorted, the operation is unstable, and the data cannot be obtained smoothly.
For low-frequency circuits, no matter who assembles them, as long as the wiring is not connected wrongly, there is almost no difference in various installation, wiring, and circuit characteristics, and the same data can be obtained. But the high frequency is different. Due to the different installation methods, data with different characteristics will generally be obtained. In high-frequency circuits and high-speed digital circuits, if there is one line, an inductance component (parasitic) will be formed, and if there are two lines, a parasitic capacitance component and a mutual inductance component (parasitic) will be formed between the lines, the so-called three parasitics. The formed three parasitic values are very small, so it is almost no problem at low frequencies, but the influence of the C and L components cannot be ignored in the high frequency range.
In order to improve the performance of the machine, various circuits such as low-frequency to high-frequency analog circuits, high-speed digital circuits, micro-analog circuits, and high-current circuits are often mixed together, which will cause circuit instability and deterioration of frequency characteristics. The main reason is that the above-mentioned three parasites are not fully considered in the design, and reliability and safety cannot be maintained. In addition, the circuit diagram only uses two-dimensional representation of the semiconductor device and the lumped parameters of R, C, and L, but this does not represent the performance and function of the actual circuit. The actual action is three-dimensional space, including frequency is four-dimensional space. Therefore, the micro-current circuit formed by the combination of intermodulation distortion, reflection, static electricity, and electromagnetic will affect the characteristics and functions of the high-frequency circuit. According to the requirements of the times, many of the recent ICs are high-speed devices that are sensitive to high-frequency noise. Therefore, when using the device, select the corresponding components according to the circuit function, and try to avoid using higher-speed ICs than required.
In the circuit diagram, the impedance of the power supply, ground wire, and signal wire are usually considered as zero ohms. But in fact, there is no zero ohm, and the higher the frequency, the greater the influence of inductance and parasitic capacitance. As a result, the combination of circuits and the influence of external electromagnetic fields are too large to be ignored, resulting in circuit instability and deterioration of frequency characteristics. The problem of error, noise and time delay should be solved in analog circuits; while in digital circuits, anti-noise is solved, and it is not affected by time delay through synchronization, which is very important to improve circuit characteristics. We must pay attention to the influence of dynamic noise "static electricity". There are many noise sources that can cause electrical equipment to malfunction, such as fluorescent lamps, dust collectors, radio transceivers, transformers, and converters around us. These are all sources of electromagnetic field noise. In addition, the source of noise that causes malfunctions is electrostatic discharge.
Due to the electrostatic discharge current and the instantaneous high voltage, the IC will be destroyed, which will cause the system or equipment to malfunction and malfunction. In order to prevent electrostatic discharge, necessary measures must be taken from the purchase of components to the design, production and packaging of equipment. The following measures can be taken in terms of design:
(1) Avoid using high-speed ICs that exceed the requirements, especially pay attention to the input circuit. When possible, the input circuit adopts a differential mode. The filter circuit should be connected close to the IC.
(2) Input protection for semiconductors. In the input part of the connector, a limiter circuit is added in order to control the noise below the semiconductor withstand voltage value. Because the CMOS gate has weak anti-static noise performance, it is not easy to use in the input part of the connector. (3) Avoid using edge-delayed ICs, and use strobing methods or circuits with latches.
(4) In order to suppress the occurrence rate of misoperation, low effective logic should be made at the control end and output end.
(5) Filter the high-sensitivity signal input. Filter out the high frequency outside the frequency band, which is very important for the operational amplifier not to input too large signals. Also pay attention to the lead inductance of the capacitor used.
(6) Some measures have also been taken in terms of software. Since electrostatic discharge is a one-time transient pulse, wrong data can be detected through multiple verifications. A watchdog circuit (monitoring circuit) is installed in the microcomputer to prevent accidental stop.
(7) The electronic circuit and wiring should be kept away from the metal case that discharges static electricity.
(8) The metal and metal connecting parts of the chassis should be tightly connected with the paint removed, and screwed as much as possible.
In order to reduce the influence of the electromagnetic field generated by the discharge current, the following measures should be taken on the printed circuit board:
(1) Reduce the ring area. Due to the cross-linking of the magnetic flux in the formed ring, current will be induced in the ring. The larger the area of the ring, the more the cross-linking of the magnetic flux, the greater the induced current. Therefore, in order to minimize the loop area formed by the power and ground wires, the power and ground wires should be as close as possible to the wiring. Install a high-frequency bypass capacitor between the power supply and the ground wire to reduce the loop area. In order to reduce the area of the loop formed between the signal line and the ground line, route the signal close to the ground line.
(2) Make the wiring the shortest. It is necessary to consider the distribution of signal line lengths. When designing, lengthen the low-effective signal line and make the high-effective signal line the shortest. The wiring between the devices is made the shortest, and the devices connected to the input and output lines are installed near the terminals.
(3) Use multi-layer circuit boards, which is seen in analog circuits and high-speed digital circuits. In high-speed digital circuits, the frequency spectrum of the pulse signal has a very wide range of high-order harmonic components. The higher the operating frequency used, the greater the influence of parasitic capacitance and inductance. Assuming that a high-frequency current I flows on a pattern with an inductance L, the voltage drop generated by the inductance L is: V=L·di/dt. The pattern is like an antenna, sending out the radiated noise. Making the ground wire a surface can reduce the impedance of the ground wire and reduce the voltage drop caused by the discharge current.
(4) Anti-static measures should be taken for the interface cable: the two ends of the shielded wire of the cable are connected to the casing. Add bypass capacitors for high-frequency short-circuits where ground loops may occur. It should not be connected to logical ground when there is no shell ground. For flat cables, a ground wire can be added between the signal wire and the signal wire. Problems that should be paid attention to when switching power supply is used as an analog signal power supply: The so-called switching power supply is a form of power supply circuit that stabilizes the output voltage through pulse modulation. Since this method only consumes power in the switching part, the faster the switching speed, the higher the efficiency of the power supply. Therefore, high-speed switching devices are generally used. Due to its high efficiency, this power supply is widely used from high-power machines to small and lightweight machines. However, with high-speed switching, there is a shortcoming of switching noise leakage. This kind of power supply for analog circuits will cause many problems.
When the switching power supply is used as the power supply of the analog circuit, high-frequency noise will enter the frequency band of the analog signal, and the signal/noise ratio of the analog signal will deteriorate. Although the switching noise is generally only 50-100mVpp, which is quite small, due to the large dynamic range of the analog signal, such noise often causes problems. Especially when used in equipment such as A/D converters, when noise is superimposed on the signal at the time of determining the level of the conversion, conversion errors will occur, and the expected accuracy will not be obtained. In order to solve the problems of using switching power supplies in analog circuits, you can pay attention to the following two aspects when selecting switching power supplies: (1) The noise level of the switching power supply is as small as possible; (2) Switching noise components do not enter the signal frequency band. Due to the high level of the analog signal, the switching noise has no effect on the signal-to-noise ratio. In order to prevent switching noise from entering the signal frequency band, the simplest method is to select a power supply with a higher switching frequency than the highest frequency band of the analog signal.
When the above method cannot be selected, it is necessary to find a way to reduce the switching noise generated by the power supply. These methods include: (1) Add capacitors externally. (2) Switching noise generated by external power supply. (3) Combined use of series regulators. The transformer of the power supply uses three windings, and noise can be eliminated between the windings. This type of power supply is a high-efficiency power supply that can be used in communication devices that supply power through a transmission line. The receiving part of the communication machine is an analog circuit that uses very low inductance signals. When this low-noise switching power supply is used, it can solve both efficiency and noise problems at the same time.
Our other product:
