Anti-interference measures in PCB design

In the design of electronic systems, in order to avoid detours and save time, the requirements of anti-interference 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 create interference:

(1) Interference source refers to the component, equipment or signal that generates interference, which is described in mathematical language as follows: du/dt, where 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 the interference propagates from the interference source to the sensitive device. Typical interference propagation paths are conduction through wires and radiation from space.

(3) Sensitive devices refer to objects that are easily disturbed. Such as: A/D, D/A converter, microcontroller, digital IC, weak signal amplifier, etc. The basic principle of anti-jamming design is to suppress the interference source, cut off the interference propagation path, and improve the anti-jamming performance of sensitive devices.

1 Suppress interference sources

Suppressing the interference source is to reduce the du/dt and di/dt of the interference source as much as possible. This is a more prioritized and important principle in anti-jamming design, and often results in a multiplier effect. Reducing the du/dt of the interference source is mainly achieved by connecting capacitors in parallel at both ends of the interference source. Reducing the di/dt of the interference source is achieved by connecting the inductance or resistance in series with the interference source loop 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 EMF 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) Connect a spark suppression circuit (usually an RC series circuit, with a resistance of several K to several tens of K, and a capacitor of 0.01uF) in parallel at both ends of the relay contact to reduce the influence of electrical sparks.

(3) Add a filter circuit to the motor, pay attention to the shortest possible lead wires of capacitors and inductors.

(4) Each IC on the circuit board should be connected in parallel with a 0.01μF ~ 0.1μF high-frequency capacitor to reduce the influence 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 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 emission.

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

According to the propagation path of interference, it can be divided into two types: conducted interference and radiated 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. It can be solved by adding a filter on the wire to cut off the propagation of high-frequency interference noise, and sometimes adding an isolation optocoupler. Power supply noise is more harmful and should be handled with special attention. The so-called radiated 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 a ground wire and add a shield on the sensitive device.

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

(1) Fully consider the influence of the power supply on the microcontroller. If the power supply is done well, the anti-interference of the whole circuit is solved more than half. Many single-chip microcomputers are very sensitive to power supply noise, and a filter circuit or a voltage regulator should be added to the power supply of the single-chip 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 conditions are not high.

(2) If the I/O port of the microcontroller is used to control noise devices such as motors, isolation should be added between the I/O port and the noise source (a π-shaped filter circuit is added). To control noise devices such as motors, isolation should be added between the I/O port and the noise source (a π-shaped filter circuit is added).

(3) Pay attention to the crystal oscillator wiring. The crystal oscillator and the MCU pins should be as close as possible, the clock area should be isolated by the ground wire, and the crystal oscillator shell should be grounded and fixed. This action resolves many difficult problems.

(4) The circuit board is reasonably partitioned, 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 then connected to the power ground at one point. A/D, D/A chip wiring is also based on this principle, the manufacturer has considered this requirement when assigning A/D, D/A chip pinout.

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

(7) The use of anti-interference components such as magnetic beads, magnetic rings, power filters, and shielding covers in key places such as single-chip microcomputer I/O ports, power lines, and circuit board connecting lines 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 reducing the pickup of interference noise as much as possible from the sensitive device side, and the method of recovering from an abnormal state as soon as possible.

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

(1) Minimize the area of the loop loop during wiring to reduce induced noise.

(2) When wiring, the power and ground wires should be as thick as possible. In addition to reducing voltage drop, it is more important to reduce coupled noise.

(3) For the idle I/O port of the single-chip microcomputer, do not float it, but connect it to ground or power supply. The idle terminals of other ICs can be connected to ground or power without changing the system logic.

(4) The use of power supply monitoring and watchdog circuits for single-chip microcomputers, such as: IMP809, IMP706, IMP813, X25043, X25045, etc., 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) The IC device should be soldered directly on the circuit board as much as possible, and the IC seat should be used less.

In order to achieve good anti-interference, we often see the wiring method with ground division on the PCB. But not all mixes of digital and analog circuits must be ground plane split. Because this division is to reduce