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. 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 mask on the sensitive device.

2.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 done well, the anti-interference of the entire circuit is largely solved. Many microcontrollers are sensitive to power supply noise, and a filtering circuit or voltage regulator should be added to the microcontroller power supply to reduce the interference of power supply noise on the single chip. For example, a π - shaped filtering circuit can be formed using magnetic beads and capacitors. Of course, when the conditions are not high, a 100 Ω resistor can also be used to replace the magnetic beads.

(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 (by adding a π - shaped filtering circuit).

(3) Pay attention to the crystal oscillator wiring. The pins of the crystal oscillator and the microcontroller should be as close as possible, and the clock area should be isolated with a ground wire. The crystal oscillator casing 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 and analog ground should be separated and 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 consideration when allocating the pin arrangement of A/D and D/A chips.

(6) The ground wires of microcontrollers and high-power devices should be grounded separately to reduce mutual interference. High power devices should be placed at 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.

3. Improve the anti-interference performance of sensitive devices

Improving the anti-interference performance of sensitive devices refers to minimizing the pickup of interference noise from the perspective of sensitive devices, as well as methods to recover from abnormal states as soon as possible.

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

(1) Try to minimize the area of the loop when wiring to reduce induced noise.

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

(3) For idle I/O ports of microcontrollers, do not hang them in the air, and connect them to ground or power. The idle end of other ICs should be grounded or powered on without changing the system logic.

(4) The use of power monitoring and watchdog circuits for microcontrollers, such as IMP809, IMP706, IMP813, X25043, X25045, etc., can significantly 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 microcontroller and choose low-speed digital circuits.

(6) IC devices should be soldered directly onto the circuit board as much as possible, and IC sockets should be used less frequently.

In order to achieve good anti-interference, we often see a ground segmented wiring method on the PCB board. However, not all mixed digital and analog circuits must undergo ground plane segmentation. Because this segmentation is to reduce the interference of noise.