In the design of electronic systems, in order to minimize detours and save time, the requirements for immunity should be fully considered and met to avoid remedial measures for immunity after the design is completed. There are three basic elements in the formation of interference:
(1) Interference source, refers to the components, equipment or signals that generate interference. Such as: lightning, relays, thyristors, motors, high-frequency clocks, etc. can be a source of interference.
(2) Propagation path, refers to the path or medium through which the interference propagates from the interference source to the sensitive device. Typical interference propagation paths are conduction through wires and radiation through space.(3) Sensitive devices, referring to objects that are easily disturbed. Such as: A/D, D / A converter, microcontroller, digital IC, weak signal amplifier. The basic principle of anti-interference design is: inhibit the source of interference, cut off the interference propagation path, improve the anti-interference performance of sensitive devices.
1 Suppression of interference sources
Reduce the du/dt of the interference source is mainly through the interference source parallel capacitance to realize. Reduce the di/dt of the interference source is in the interference source circuit series inductance or resistance as well as increase the continuity diode to realize.
Common measures to suppress sources of interference are as follows:
(1) Adding a current-continuing diode to the relay coil eliminates the interference of the reverse electromotive force generated when disconnecting the coil. Adding only the continuity diode causes the relay's disconnect time to lag, and adding the voltage regulator diode allows the relay to actuate more times per unit of time.
(2) In the relay contacts at both ends of the parallel spark suppression circuit (generally RC series circuit, resistance is generally selected a few K to dozens of K, capacitance selection 0.01uF), reduce the impact of sparks.
(3) add filter circuit to the motor, pay attention to capacitance, inductance leads should be as short as possible.
(4) Each IC on the board should be connected to a 0.01μF ~ 0.1μF high-frequency capacitors to reduce the impact of IC on the power supply. Pay attention to the wiring of high-frequency capacitors, the wires should be close to the power supply and try to be as short as possible, otherwise, it is equivalent to increasing the equivalent series resistance of the capacitor, which will affect the filtering effect.
(5) Avoid 90-degree folding line when wiring to reduce high-frequency noise emission.
(6) SCR parallel RC suppression circuit at both ends to reduce the noise generated by the SCR (this noise may be serious when the SCR breakdown).
According to the propagation path of the interference can be divided into two categories: conduction interference and radiation interference.
The so-called conduction interference refers to the interference propagated through the wire to the sensitive device. High-frequency interference noise and useful signals of different frequency bands, you can increase the filter on the wire method to cut off the propagation of high-frequency interference noise, and sometimes can also be added to isolate the optocoupler to solve the problem. Power supply noise is more harmful, to pay special attention to deal with. The so-called radiation interference refers to the interference propagated through the space radiation to sensitive devices. The general solution is to increase the distance between the source of interference and sensitive devices, land line to isolate them and sensitive devices on the shield.
2 Common measures to cut the propagation path of interference are as follows:
(1) Fully consider the impact of the power supply on the microcontroller. Power supply to do a good job, the whole circuit of anti-jamming on a large part of the solution. Many microcontrollers are very sensitive to power supply noise, to the microcontroller power supply filter circuit or voltage regulator to reduce the power supply noise on the microcontroller interference. For example, you can use magnetic beads and capacitors to form a π-shaped filter circuit, of course, the conditions do not require high requirements can be used in place of 100Ω resistor beads.
(2) If the I/O port of the microcontroller is used to control noisy devices such as motors, isolation should be added between the I/O port and the noise source (increase the π-shaped filter circuit). If the I/O port of the microcontroller is used to control a noise device such as a motor, isolation should be added between the I/O port and the noise source (add a π-shaped filter circuit).
(3) Pay attention to the crystal wiring. Crystal and microcontroller pins as close as possible, use the ground wire to isolate the clock area, crystal shell grounded and fixed. This measure can solve many difficult problems.
(4) Reasonable partition of the circuit board, such as strong and weak signals, digital and analog signals. As far as possible, the source of interference (such as motors, relays) and sensitive components (such as microcontroller) away.
(5) land line to the digital area and analog area isolated, digital and analog ground to be separated, after a point connected to the power supply ground.A/D, D/A chip wiring is also the principle of the manufacturer to allocate A/D, D/A chip pin arrangement has been taken into account when this requirement.
(6) microcontroller and high-power devices should be grounded separately to reduce mutual interference. High-power devices are placed at the edge of the board as far as possible.
(7) in the microcontroller I / O ports, power lines, circuit board connecting lines and other key places to use anti-jamming components such as magnetic beads, magnetic rings, power filters, shields, can significantly improve the circuit's anti-interference performance.
3 Improvement of anti-interference performance of sensitive devices
Improve the anti-interference performance of sensitive devices is to consider from the sensitive device side to minimize the pickup of interference noise, as well as from the abnormal state as soon as possible to recover the method.
Common measures to improve the anti-interference performance of sensitive devices are as follows:
(1) Minimize the area of the loop ring when wiring to reduce induced noise.
(2) When wiring, the power and ground wires 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 microcontroller idle I / O port, do not hang, to ground or connect to the power supply. Other IC's idle end in the case of not changing the system logic grounded or connected to the power supply.
(4) the microcontroller using power monitoring and watchdog circuits, such as: IMP809, IMP706, IMP813, X25043, X25045, etc., can significantly improve the anti-interference performance of the entire circuit.
(5) In the speed to meet the requirements of the premise, try to reduce the crystal of the microcontroller and the choice of low-speed digital circuit.
(6) IC devices as far as possible directly welded to the circuit board, less IC seat.