PCB design anti-interference measures
In the design of electronic systems, in order to avoid detours and save time, the anti-interference requirements should be fully considered and met, and anti-interference remedial measures should be avoided after the design is completed. There are three basic elements that form interference:
(1) Interference source refers to the component, device or signal that generates interference. It is described in mathematical language as follows: du/dt, where the di/dt is large is the interference source. Such as: lightning, relays, thyristors, motors, high-frequency clocks, etc. may become sources of interference.
(2) Propagation path refers to the path or medium through which interference propagates from the interference source to the sensitive device. Typical interference propagation paths are conduction through wires and radiation in space.
(3) Sensitive devices refer to objects that are easily disturbed. Such as: A/D, D/A converter, single chip microcomputer, digital IC, weak signal amplifier, etc. The basic principles of anti-jamming design are: suppress the source of interference, cut off the propagation path of interference, and improve the anti-jamming performance of sensitive devices.
1、Suppress interference sources
To 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 most important principle in antijamming design, and it will often get twice the result with half the effort. Reducing the du/dt of the interference source is mainly realized by connecting capacitors in parallel at both ends of the interference source. Reducing the di/dt of the interference source is achieved by connecting an inductance or resistance in series with the interference source circuit and adding a freewheeling diode.
Common measures to suppress interference sources are as follows:
(1) A freewheeling diode is added to the relay coil to eliminate the back electromotive force interference generated when the coil is disconnected. Only adding a freewheeling diode will delay the disconnection time of the relay. After adding a Zener diode, the relay can operate more times per unit time.
(2) A spark suppression circuit (generally an RC series circuit, generally a few K to dozens of K for resistance, and 0.01uF for capacitance) is connected in parallel at both ends of the relay contact to reduce the impact of electric sparks.
(3) Add a filter circuit to the motor, and pay attention to the shortest lead wires of capacitors and inductors.
(4) Each IC on the circuit board should be connected with a 0.01μF～0.1μF 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 supply terminal and as thick and short as possible. Otherwise, the equivalent series resistance of the capacitor will be increased, which will affect the filtering effect.
(5) Avoid 90-degree broken lines when wiring to reduce high-frequency noise emissions.
(6) Both ends of the thyristor are connected in parallel with an RC suppression circuit to reduce the noise generated by the thyristor (when the noise is severe, the thyristor may be broken down).
According to the propagation path of interference, it can be divided into two types: conducted interference and radiated interference.
The socalled conducted interference refers to the interference transmitted to sensitive devices through wires. The frequency bands of highfrequency interference noise and useful signals are different, and the propagation of highfrequency interference noise can be cut off by adding a filter on the wire, and sometimes an isolation optocoupler can be added to solve the problem. Power supply noise is the most harmful, so special attention should be paid to it. The socalled 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 sensitive devices, isolate them with ground wires and add shields to sensitive devices.
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. If the power supply is well done, more than half of the anti-interference of the entire circuit will be solved. Many singlechip microcomputers are very sensitive to power supply noise, and a filter circuit or voltage regulator should be added to the power supply of the singlechip microcomputer to reduce the interference of power supply noise to the single chip. For example, magnetic beads and capacitors can be used to form a π-shaped filter circuit. Of course, 100Ω resistors can be used instead of magnetic beads when the requirements are not high.
(2) If the I/O port of the singlechip microcomputer is used to control noise devices such as motors, isolation should be added between the I/O port and the noise source (add a πshaped filter circuit). To control noise devices such as motors, isolation should be added between the I/O port and the noise source (add a πshaped filter circuit).
(3) Pay attention to the wiring of the crystal oscillator. The pins of the crystal oscillator and the singlechip microcomputer should be as close as possible, and the clock area should be isolated with the ground wire, and the shell of the crystal oscillator should be grounded and fixed. This action can solve many difficult problems.
(4) Reasonable division of the circuit board, such as strong and weak signals, digital and analog signals. Keep interference sources (such as motors, relays) away from sensitive components (such as microcontrollers) as much as possible.
(5) Use the ground wire to isolate the digital area from the analog area. The digital ground and the analog ground should be separated, and finally connected 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 assigning the pins of A/D and D/A chips.
(6) The ground wires of the singlechip microcomputer and highpower devices should be grounded separately to reduce mutual interference. Place highpower components on the edge of the board as much as possible.
(7) Use antiinterference components such as magnetic beads, magnetic rings, power filters, and shields in key places such as the I/O port of the microcontroller, power lines, and circuit board connections, which can significantly improve the anti-interference performance of the circuit.
3、Improve the anti-interference performance of sensitive devices
Improving the anti-interference performance of sensitive devices refers to the method of minimizing the pick-up of interference noise and recovering from abnormal states as soon as possible from the perspective of sensitive devices.
Common measures to improve the anti-interference performance of sensitive devices are as follows:
(1)When wiring, try to reduce the area of the loop loop to reduce the 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 idle I/O port of the microcontroller, do not leave it in the air, but connect it to ground or power supply. The idle ends of other ICs can be grounded or connected to power without changing the system logic.
(4)Use power monitoring and watchdog circuits for single-chip microcomputers, such as: IMP809, IMP706, IMP813, X25043, X25045, etc., which can greatly improve the anti-interference performance of the entire circuit.
(5)On the premise that the speed can meet the requirements, try to reduce the crystal oscillator of the single-chip microcomputer and select low-speed digital circuits.
(6)IC devices should be directly soldered on the circuit board as much as possible, and IC sockets should be used less.
In order to achieve good anti-interference, we often see the wiring method with ground division on the PCB board. However, not all digital circuits and analog circuits must be divided into ground planes. Because this division is to reduce noise interference.