PCB design anti-interference measures

In electronic system design, in order to avoid detours and save time, anti-interference requirements should be fully considered and met, and remedial measures for anti-interference should be avoided after the design is completed. There are three basic elements that can cause interference:

(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. can all become sources of interference.

(2) Propagation path refers to the path or medium through which interference propagates from the interference source to sensitive devices. 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 means minimizing 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 freewheeling diodes.

The common measures to suppress interference sources are as follows:

(1) Add a freewheeling diode to the relay coil to eliminate the back electromotive force interference generated when the coil is disconnected. Adding only a freewheeling 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 K 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 make sure that the capacitor and inductor leads are as short as possible.

(4) Each IC on the circuit board should 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) Connect the two ends of the thyristor in parallel with an RC suppression circuit to reduce the noise generated by the thyristor (which may cause breakdown of the thyristor in severe cases).

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 supply noise is significant, and special attention should be paid to handling it. The so-called radiation interference refers to the interference that propagates through space radiation to sensitive devices. General solutions