Anti interference measures for PCB design

In electronic system design, in order to avoid detours and save time, 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 components, equipment, or signals that generate interference, described in mathematical language as follows: du/dt, where di/dt is large is the interference source. For example, lightning, relays, thyristors, motors, high-frequency clocks, etc. can all become sources of interference.

(2) The propagation path refers to the pathway or medium through which interference propagates from the interference source to the sensitive device. The typical interference propagation path is through the conduction of wires and radiation in space.

(3) Sensitive devices refer to objects that are easily disturbed. Such as 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 their du/dt and di/dt 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 the interference source is mainly achieved by paralleling capacitors at both ends of the interference source. Reducing the di/dt of the interference source is achieved by connecting an inductor or resistor in series with the interference source circuit and adding a freewheeling diode.

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 a delay in the disconnection time of the relay, while 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 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, paying attention to keeping the capacitor and inductor leads as short as possible.

(4) Each IC on the circuit board should be connected in parallel with a high-frequency capacitor of 0.01 μ F to 0.1 μ F to reduce the impact of the IC on the power supply. Pay attention to the wiring of high-frequency capacitors. The connection 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, which will affect the filtering effect.

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

(6) Connect RC suppression circuit at both ends of the thyristor 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 categories: 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, which can be cut off by adding filters on the wires to cut off the propagation of high-frequency interference noise. 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 that propagates 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.

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 solved by half. Many microcontrollers are sensitive to power noise, and it is necessary to add filtering circuits or voltage regulators to the microcontroller power supply to reduce the interference of power noise on the microcontroller. For example, a π - shaped filtering circuit can be composed of magnetic beads and capacitors. Of course, when conditions are not high, a 100 Ω resistor can also be used instead of magnetic beads.

(2) If the I/O port of the microcontroller is used to control noisy devices such as motors, isolation should be added between the I/O port and the noise source (by adding a π - shaped filtering circuit). Control noise components such as motors, and isolate them 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 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 housing 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 and relays) as far away as possible from sensitive components (such as microcontrollers).

(5) Isolate the digital area from the analog area with a ground wire, separate the digital ground from the analog ground, and connect them 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 allocating the pin arrangement of A/D and D/A chips.

(6) The ground wires of microcontrollers and high-power devices should be separately grounded 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 picking up of interference noise from the perspective of sensitive devices, as well as methods for recovering from abnormal states as soon as possible.

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

(1) When wiring, try to minimize the area of the loop 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. They should be grounded or powered on. The idle terminals of other ICs can 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 as much as possible.

(6) IC devices should be soldered directly onto the circuit board as much as possible, with less use of IC sockets.

In order to achieve good anti-interference, we often see the wiring method with ground division on the PCB board. However, not all mixed digital and analog circuits necessarily require ground plane segmentation. Because this segmentation is done to reduce