How to resist interference in the hottest digital

2022-08-09
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How to resist the interference of digital circuits

the design of electronic system includes the demand analysis and modeling technology of the system. In order to avoid detours and save time, we should fully consider and meet the requirements of wiping and cleaning the instruments and equipment for anti-interference, and avoid taking remedial measures against the interference after the design is completed. There are three basic elements to form interference:

(1) interference source refers to the components, equipment or signals that generate interference for their development laboratory. It is described in mathematical language as follows: du/dt, where di/dt is large, it is the interference source. For example, thunder

electricity, relays, thyristors, motors, high-frequency clocks, etc. may become interference sources

(2) propagation path refers to the path or medium through which interference propagates from interference source to sensitive devices. Typical interference propagation paths are conduction through wires and spatial

radiation

(3) sensitive devices refer to objects that are easy to be disturbed. Such as: a/d, d/a converter, MCU, digital IC, weak signal amplifier, etc

the basic principle of anti-interference design is to suppress interference sources, cut off interference propagation paths, and improve the anti-interference performance of sensitive devices. (similar to the pre

prevention of infectious diseases)

1 suppress the interference source

suppress the interference source is to reduce the du/dt and di/dt of the interference source as much as possible. This is the most priority and the most important principle in anti-interference design, which often has the effect of doing twice the work. Reducing du/dt of interference source is mainly achieved by paralleling capacitors at both ends of interference source. Reducing di/dt of interference source is realized by connecting inductance or resistance in series and adding freewheeling diode in interference source circuit

common measures to suppress interference sources are as follows:

(1) add freewheeling diode to the relay coil to eliminate the back EMF interference generated when disconnecting the coil. Only adding freewheeling diode will delay the breaking time of the relay, and the relay can act more times in unit time after adding zener diode

(2) connect the spark suppression circuit at both ends of the relay contact in parallel (generally RC series circuit, the resistance is generally selected from a few K to dozens of K, and the capacitance is selected as 0.01uF), so as to reduce the influence of

spark

(3) add a filter circuit to the motor, and pay attention to the capacitor and inductance leads to be as short as possible

(4) each IC on the circuit board should be connected with a 0.01 f ~ 0.1 f high-frequency capacitor in parallel to reduce the impact of the IC on the power supply. Pay attention to the wiring of high-frequency capacitors. The wiring should be

close to the power end and as short as possible. Otherwise, it will increase the equivalent series resistance of the capacitor and affect the filtering effect

(5) avoid 90 degree broken lines during wiring to reduce high-frequency noise emission

(6) both ends of the thyristor are connected with RC suppression circuit in parallel to reduce the noise generated by the thyristor (when the noise is serious, the thyristor may be broken down)

according to the propagation path of interference, it can be divided into conduction interference and radiation interference

the so-called conducted interference refers to the interference transmitted to sensitive devices through wires. The frequency band of high-frequency interference noise is different from that of useful signals. The propagation of high-frequency interference noise can be cut off by adding a

filter on the wire, and sometimes it can be solved by adding an isolation optocoupler. Power supply noise is the most harmful, so special attention should be paid to treatment. The

radiation interference refers to the interference transmitted to sensitive devices through space radiation. The general solution is to increase the distance between the interference source and the sensitive device, isolate them with ground wire and put a shield on the sensitive device

the common measures to cut off the interference propagation path are as follows:

(1) fully consider the influence of power supply on MCU. If the power supply is well done, the anti-interference of the whole circuit is solved by more than half. Many single-chip computers are very sensitive to power supply noise. BASF hopes to participate in the formulation of relevant standards in Jilin Province. It is necessary to add filter circuits or voltage regulators to the single-chip computer power supply to reduce the interference of power supply noise to the single-chip computer. For example, magnetic beads and capacitors can be used to form a

filter circuit. Of course, 100 resistors can also be used to replace magnetic beads when conditions are not high

(2) if the i/o port of the single chip microcomputer is used to control noise devices such as motors, isolation should be added between the i/o port and the noise source (adding a shaped filter circuit). To control noise devices such as

motors, isolation should be added between the i/o port and the noise source (adding shape filter circuit)

(3) pay attention to crystal oscillator wiring. The crystal oscillator shall be close to the pin of the single chip microcomputer as far as possible, and the clock area shall be isolated with ground wire, and the shell of the crystal oscillator shall be grounded and fixed. This measure can solve many difficult problems

(4) the circuit board shall be divided reasonably, such as strong and weak signals, digital and analog signals. Try to keep interference sources (such as motors, relays) away from sensitive components (such as single-chip

machines)

(5) isolate the digital area from the analog area with a ground wire, separate the digital ground from the analog ground, and finally connect to the power ground at one point. The wiring of a/d and d/a chips is also based on this principle, and the manufacturer has considered this requirement when allocating the pin arrangement of a/d and d/a chips

(6) the ground wires of single chip microcomputer and high-power devices should be grounded separately to reduce mutual interference. High power devices shall be placed on the edge of the circuit board as far as possible

(7) the anti-interference performance of the circuit can be significantly improved by using anti-interference components such as magnetic beads, magnetic rings, power filters and shielding covers in key places such as the i/o port of the single chip microcomputer, the power line and the connecting line of the circuit board

3 improving the anti-interference performance of sensitive devices

improving the anti-interference performance of sensitive devices refers to the method of reducing the pickup of interference noise as far as possible from the sensitive devices and recovering from abnormal conditions as soon as possible

common measures to improve the anti-interference performance of sensitive devices are as follows:

(1) try to reduce the area of the loop ring during wiring to reduce the induced noise

(2) when wiring, the power line and ground wire should be as thick as possible. In addition to reducing the voltage drop, it is more important to reduce the coupling noise

(3) for the idle i/o port of the single chip microcomputer, do not hang in the air, but connect it to the ground or power supply. The idle terminals of other ICs are grounded or powered on without changing the system logic

(4) the anti-interference performance of the whole circuit can be greatly improved by using power monitoring and watchdog circuits for single chip microcomputer, such as imp809, imp706, imp813, X25043, X25045, etc

(5) on the premise that the speed can meet the requirements, try to reduce the crystal vibration of single chip microcomputer and select low-speed digital circuit

(6) IC devices shall be directly welded on the circuit board as far as possible, and IC seats shall be used less

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