PCB design anti-interference measures

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) the source of interference, refers to the components, equipment or signals that generate interference, described in mathematical language as follows: du/dt, di/dt large place is the source of interference. Such as: lightning, relays, thyristors, motors, high-frequency clocks, etc. can be a source of interference.

　　(2) Propagation path, refers to the interference from the source of interference propagation to the sensitive device path or medium. Typical interference propagation paths are conduction through wires and radiation in space.

　　(3) Sensitive device, refers to the object that is susceptible to interference. For example: 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, and improve the anti-interference performance of sensitive devices.

　　1 Suppress the source of interference

　Suppressing the source of interference is to reduce the source of interference as much as possible du/dt, di/dt. This is the anti-interference design of the more priority consideration and important principles, will often play a multiplier effect. 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 the interference source are as follows:

　　(1) Adding a current-continuing diode to the relay coil eliminates the reverse electromotive force generated when the coil is disconnected. Adding only the continuity diode causes the relay's disconnection time to lag, and the addition of the voltage regulator diode allows the relay to operate 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 the 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 of conduction interference and radiation interference.

　　The so-called conduction interference refers to the interference propagated through the wire to the sensitive devices. 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 off the interference propagation path are as follows:

　　(1) Fully consider the impact of the power supply on the microcontroller. Power to do a good job, the whole circuit of anti-interference will solve a large part. Many microcontrollers are very sensitive to power supply noise, to the microcontroller power supply filtering circuit or voltage regulator, in order to reduce the power supply noise on the microcontroller interference. For example, you can use the beads and capacitors to form a π-shaped filter circuit, of course, the conditions do not require high requirements can be used instead 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) circuit board reasonable partition, 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) with the land line to the digital area and analog area of isolation, digital ground and analog ground to be separated, at a point connected to the power supply ground.A/D, D/A chip wiring is also based on this principle, the manufacturer allocates A/D, D/A chip pin arrangement has taken into account 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 connection lines and other key places to use anti-interference components such as magnetic beads, magnetic rings, power filters, shielding, can significantly improve the circuit's anti-interference performance.

　　3 Improve the 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 normal state as soon as possible to restore the method.

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

　　(1) Wiring to minimize the area of the loop ring to reduce induced noise.

　　(2) When wiring, the power and ground lines 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.

　　In order to achieve good anti-jamming, so we often see PCB boards have a ground division of the wiring method. But not all digital circuits and analog circuits are mixed must be divided into ground planes. Because this split is to reduce the interference of noise.