In the design of high-speed PCB boards, managing interference in digital-analog hybrid circuits has perennially posed challenges. Analog circuits typically serve as signal sources, crucial for ensuring correct signal reception and conversion in PCB design. This factor is pivotal for circuit performance. This paper analyzes hybrid circuit interference mechanisms, integrating design practices to explore general processing methods, substantiated through a design example.

A Printed Circuit Board (PCB) supports circuit elements and facilitates electrical connections within electronic products. Today, many PCBs transcend single-function circuits, incorporating both digital and analog circuitry. Analog circuits typically handle data collection and reception, requiring digitization of bandwidth and gain for software control. Consequently, digital and analog circuits often coexist on the same board, sometimes sharing components. Given their potential mutual interference and impact on circuit performance, circuit layout and wiring must adhere to specific principles.

In mixed-signal PCB design, particularly, special attention is needed for power transmission line requirements and the imperative to isolate noise coupling between analog and digital circuits. These demands amplify the complexity of layout and routing during design. The achievement of PCB design goals necessitates thorough analysis of layout and routing design for high-density mixed-signal PCBs.

1. The generation mechanism of digital-analog hybrid circuit interference

Analog signals are far more susceptible to noise compared to digital signals due to their reliance on continuously varying currents and voltages. Even slight interference can disrupt their operation. In contrast, digital circuits determine signal levels based on predefined thresholds, offering inherent noise immunity. However, in mixed-signal environments, digital signals can act as noise sources for analog circuits. When digital circuits switch between high and low voltage states, they generate switching currents through the ground pins, known as ground bounce (Figure 1). Such disturbances, coupled with power supply noise, can degrade analog circuit performance. Therefore, meticulous design of power and ground distribution is crucial in PCB design.

2. General processing principles for PCB design of digital-analog hybrid circuits

Understanding electromagnetic compatibility (EMC) principles is essential to mitigating interference between digital and analog signals. Firstly, minimize current loop areas to prevent unintentional antenna formation. Secondly, maintain a single reference plane to avoid forming dipole antennas. When laying out components, separate analog and digital sections on different board layers to prevent digital signal return currents from interfering with analog signal grounds. Use differential or shielded lines for high-frequency and critical connections where necessary. Avoid routing digital clock and high-frequency analog signal lines adjacent to analog power planes to prevent power noise coupling. Ensure a low impedance network for power and ground to minimize inductive reactance in digital circuits and capacitive coupling in analog circuits. Keep high-frequency digital signal lines away from sensitive analog devices.

Handling power and ground traces is critical in improving circuit performance on complex hybrid circuit boards. Although separating digital and analog grounds is a common approach, it can increase electromagnetic radiation and signal crosstalk. Optimize signal paths by using single-point connections between divided ground sections or employing isolation techniques like transformers. Despite challenges, integrated ground solutions often outperform divided approaches in both functionality and EMC performance. Additionally, use separate power planes for digital and analog circuits, ensuring physical isolation to minimize RF current coupling.

Hybrid devices such as crystal oscillators and high-speed AD converters contain both digital and analog circuitry. Connect AGND and DGND pins to a low-impedance analog ground plane with minimal lead length to reduce digital noise coupling. Similarly, connect analog and digital power pins separately, using bypass capacitors close to each pin to suppress noise interference. Use inductors for isolation if needed, ensuring digital currents do not disrupt analog operations.

Decoupling capacitors are effective in eliminating high-frequency interference by providing a low impedance path to ground. Place ceramic chip capacitors (0.01mF~0.1mF) near each integrated circuit to manage high-frequency noise. Additionally, use electrolytic capacitors (10mF~100mF) at power inputs to stabilize supply voltages, ensuring leads are kept short to minimize inductive effects.

Design a large copper clad foil connected to the analog ground to shield and isolate analog signals, enhancing EMC and heat dissipation. Ensure power and ground lines are short and thick, particularly across magnetic beads bridging digital and analog power supplies, to reduce noise coupling.

3. PCB design example of hybrid circuit

In PCB layout, separate analog and digital circuits to prevent signal interference. Place analog circuits at the board edges and digital circuits near power connections to mitigate digital switching effects. Use dedicated layers for analog signals, ensuring short, drilled connections to maintain signal integrity. Surround analog signals with unified ground planes to prevent EMI issues. Route high-speed signals on dedicated layers next to ground planes, while critical lines are routed as striplines for enhanced signal integrity. Ensure separate power planes for digital and analog circuits, keeping them adjacent to respective ground layers. Connect high-speed AD devices to analog ground with minimal lead length and add decoupling capacitors for noise suppression.

4. Conclusion

Designing PCBs for hybrid circuits involves complex considerations such as component layout, power management, and signal integrity. Adhering to design rules ensures PCBs meet performance and EMC requirements, achieving reliable operation in mixed-signal environments.

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