Anti-Interference Measures in PCB Design

In electronic system design, to avoid detours and save time, it is essential to fully consider and meet interference immunity requirements, thereby avoiding the need for remedial measures after the design is complete. There are three fundamental elements that contribute to interference:

(1) Interference source: This refers to the components, equipment, or signals that generate interference. In mathematical terms, it is described as follows: areas where du/dt and di/dt are high constitute interference sources. For example: lightning, relays, thyristors, motors, high-frequency clocks, and other such sources can all become sources of interference.

(2) Propagation path: This refers to the route or medium through which interference travels from the source to the sensitive components. Typical propagation paths include conducted interference through wires and radiated interference through space.

(3) Sensitive components refer to devices that are susceptible to interference. Examples include A/D and D/A converters, microcontrollers, digital ICs, and weak-signal amplifiers. The basic principles of anti-interference design are: suppressing the interference source, blocking the propagation path, and improving the interference immunity of sensitive components.

1. Suppressing the Interference Source

Suppressing the interference source involves minimizing the du/dt and di/dt of the source as much as possible. This is a high-priority and crucial principle in interference-resistant design, often yielding twice the result with half the effort. Reducing the du/dt of the interference source is primarily achieved by connecting a capacitor in parallel across the source. Reducing the di/dt of the interference source is achieved by connecting an inductor or resistor in series with the source’s circuit and adding a freewheeling diode.

Common measures for suppressing interference sources are as follows:

(1) Add a freewheeling diode to the relay coil to eliminate back-EMF interference generated when the coil is de-energized. Adding only a freewheeling diode will cause a delay in the relay’s de-energization time; however, adding a zener diode allows the relay to operate more frequently per unit of time.

(2) Connect a spark suppression circuit in parallel across the relay contacts (typically an RC series circuit, with a resistor value ranging from a few kΩ to several tens of kΩ and a capacitor of 0.01 μF) to minimize the impact of electrical sparks.

(3) Add a filter circuit to the motor; ensure that the leads of the capacitors and inductors are as short as possible.

(4) Connect a 0.01 μF to 0.1 μF high-frequency capacitor in parallel with each IC on the circuit board to minimize the IC’s impact on the power supply. Pay attention to the routing of the high-frequency capacitors; connections should be made close to the power supply terminals and should be as thick and short as possible. Otherwise, this will effectively increase the capacitor’s equivalent series resistance (ESR), which will impair the filtering effect.

(5) Avoid 90-degree bends when routing traces to reduce high-frequency noise emissions.

(6) Connect an RC suppression circuit in parallel across the thyristor terminals to reduce noise generated by the thyristor (severe noise can potentially cause the thyristor to break down).

Based on the propagation path, interference can be classified into two categories: conducted interference and radiated interference.

Conducted interference refers to interference that propagates through conductors to sensitive components. Since high-frequency interference noise operates in a different frequency band than the useful signal, its propagation can be blocked by adding filters to the conductors; in some cases, isolating optocouplers can also resolve the issue. Power supply noise poses a significant risk and requires special attention. Radiated interference refers to interference that propagates to sensitive components through space. Common solutions include increasing the distance between the interference source and the sensitive components, isolating them with a ground wire, and shielding the sensitive components.

2. Common measures to block interference propagation paths are as follows:

(1) Give full consideration to the impact of the power supply on the microcontroller. A well-designed power supply resolves more than half of the entire circuit’s immunity to interference. Many microcontrollers are highly sensitive to power supply noise; therefore, a filter circuit or voltage regulator should be added to the microcontroller’s power supply to minimize the interference caused by power supply noise. For example, a π-type filter circuit can be constructed using ferrite beads and capacitors; of course, when requirements are not too stringent, a 100Ω resistor can be used in place of the ferrite bead.

(2) If the microcontroller’s I/O ports are used to control noisy devices such as motors, isolation should be added between the I/O ports and the noise sources (by adding a π-filter circuit). When controlling noisy devices such as motors, isolation should be added between the I/O ports and the noise sources (by adding a π-filter circuit).

(3) Pay attention to crystal oscillator routing. Position the crystal oscillator as close as possible to the microcontroller pins, isolate the clock area with a ground plane, and ground and secure the crystal oscillator housing. These measures can resolve many difficult-to-troubleshoot issues.

(4) Divide the circuit board into appropriate zones, such as high- and low-level signals, and digital and analog signals. Keep noise sources (such as motors and relays) as far away as possible from sensitive components (such as the microcontroller).

(5) Use ground traces to isolate the digital and analog sections; separate the digital ground from the analog ground, and connect both to the power supply ground at a single point. Follow this principle when routing A/D and D/A chip traces as well; manufacturers have already taken this requirement into account when assigning pin layouts for A/D and D/A chips.

(6) Microcontrollers and large