(1) Interference source refers to the component, device, or signal that generates interference, described in mathematical language as follows: du/dt, where di/dt is large, it is the interference source. For example, lightning, relays, thyristors, motors, high-frequency clocks, etc. may all become interference sources.

(2) Propagation path refers to the path or medium through which interference propagates from an interference source to a sensitive device. The typical interference propagation path is through the conduction of wires and spatial radiation.

(3) Sensitive devices refer to objects that are easily disturbed. For example: A/D, D/A converters, microcontrollers, digital ICs, weak signal amplifiers, 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.

1. Suppress interference sources

Suppressing interference sources is to minimize the du/dt, di/dt of interference sources as much as possible. This is a priority and important principle in anti-interference design, often achieving twice the result with half the effort. Reducing the du/dt of interference sources is mainly achieved by connecting capacitors in parallel at both ends of the interference source. Reducing the di/dt of interference sources is achieved by serializing inductors or resistors in the interference source circuit and adding a freewheeling diode.

The common measures to suppress interference sources are as follows:

(1) The relay coil adds a freewheeling diode to eliminate the back electromotive force interference generated when the coil is disconnected. Adding only a continuous current diode will cause the disconnection time of the relay to lag, and adding a voltage regulator diode will allow the relay to operate more times per unit time.

(2) Connect a spark suppression circuit (usually an RC series circuit, with a resistance of several to several tens of K and a capacitance of 0.01uF) in parallel at both ends of the relay contact to reduce the impact of electric sparks.

(3) Add a filtering circuit to the motor, and ensure that the capacitor and inductor leads are as short as possible.

(4) Each IC on the circuit board needs to be connected in parallel with a 0.01 μ F~0.1 μ F high-frequency capacitor to reduce the impact of IC on the power supply. Pay attention to the wiring of high-frequency capacitors, and the wiring should be close to the power supply end and as thick and short as possible. Otherwise, it will increase the equivalent series resistance of the capacitor and affect the filtering effect.

(5) Avoid 90 degree creases during wiring to reduce high-frequency noise emissions.

(6) The two ends of the thyristor are connected in parallel with an RC suppression circuit to reduce the noise generated by the thyristor (which may cause the thyristor to break down when the noise is severe).

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 that propagates through wires to sensitive devices. The frequency bands of high-frequency interference noise and useful signals are different. The propagation of high-frequency interference noise can be cut off by adding filters on wires, and sometimes isolation optocouplers can also be added to solve the problem. The harm of power noise is significant, and special attention should be paid to handling it. The so-called 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 a ground wire, and place a mask on the sensitive device.

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

(1) Fully consider the impact of power supply on the microcontroller. If the power supply is well done, the anti-interference of the entire circuit is largely solved. Many microcontrollers are sensitive to power noise, and a filtering circuit or voltage regulator should be added to the microcontroller power supply to reduce the interference of power noise on the single chip. For example, a π shaped filtering circuit can be composed of magnetic beads and capacitors, and of course, a 100 Ω resistor can also be used to replace the 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 (adding a π shaped filtering circuit). To control noise devices such as motors, isolation should be added between the I/O port and the noise source (adding a π shaped filtering circuit).

(3) Pay attention to crystal oscillator wiring. The crystal oscillator and microcontroller pins should be as close as possible, and the clock area should be isolated with a ground wire. The crystal oscillator shell should be grounded and fixed. This measure can solve many difficult problems.

(4) Reasonable partitioning of circuit boards, 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 microcontrollers) as much as possible.

(5) Use a ground wire to isolate the digital area from the analog area. The digital ground should be separated from the analog ground, and then 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 taken this requirement into account when allocating the pin arrangement of A/D and D/A chips.

(6) The ground wires of the microcontroller and high-power devices should be grounded separately to reduce mutual interference. High power devices should be placed on the edge of the circuit 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 areas such as microcontroller I/O ports, power lines, and circuit board connection lines can significantly improve the anti-interference performance of the circuit.