1. It has been suggested to separate the digital ground and analog ground on a mixed-signal circuit board to achieve isolation between them. While feasible, this approach presents numerous potential problems, especially in large and complex systems. The primary issue arises from the risk of signal lines crossing between the digital and analog grounds, which can lead to significant increases in electromagnetic radiation and signal crosstalk.
2. A common problem in PCB board design is EMI caused by signal lines crossing ground or power supplies. If using the segmentation method mentioned above, and a signal line spans the gap between the two grounds, it raises questions about the return path of the signal current. When the segmented grounds are connected at a single point, typically at a single point of the power source, it forms a large loop for the ground current. This loop, especially at high frequencies, induces radiation and high ground inductance, potentially interfering with low-level analog signals sensitive to external interference.
3. Furthermore, connecting the sections together at the power source creates a very large current loop. Additionally, connecting analog and digital grounds via a long wire can form a dipole antenna, further complicating interference issues.
4. Understanding the current return path is crucial for optimizing mixed-signal circuit board design. Many design engineers focus solely on where signal currents flow, neglecting the specific paths these currents take.
5. If ground layer segmentation is unavoidable and signal lines must cross these segmented grounds, a single-point connection can be established between the segmented grounds to create a bridging connection. This setup allows each signal line to have a direct return path, minimizing loop area and reducing potential interference.
6. Alternatively, optical isolation devices or transformers can be employed to facilitate signal crossing between segmented grounds. Optical isolation uses light signals across the segmentation gap, while transformers use magnetic fields. Differential signals also offer a viable option, where signals return via a separate line, reducing unnecessary backflow paths.
7. To assess digital signal interference on analog signals, understanding high-frequency current characteristics is essential. High-frequency currents seek the path of least impedance, typically directly beneath the signal trace. Hence, return currents often flow through adjacent circuit layers, regardless of whether these layers are power or ground.
8. In practical terms, a unified PCB partition into analog and digital regions is generally preferred. Analog signals are confined to the analog region across all layers, while digital signals are contained within the digital circuit region. This segregation ensures that digital signal return currents do not interfere with analog signal grounds, barring instances where digital signals are improperly routed over analog regions or vice versa.
9. Effective PCB design mandates adherence to unified partitioning principles and proper signal routing, circumventing potential layout and wiring challenges. Such meticulous planning mitigates the need for ground segmentation, ensuring that digital ground currents remain confined and do not disrupt analog signals. Thorough verification of wiring compliance is crucial to prevent a single flawed signal line from compromising an otherwise sound circuit board.
10. When connecting analog and digital ground pins of A/D converters, manufacturers typically recommend linking AGND and DGND pins to a low-impedance ground via short leads. This practice prevents digital noise coupling to analog circuits within the IC due to parasitic capacitance. However, questions arise concerning whether the ground end of digital signal decoupling capacitors should connect to analog or digital ground.
11. For systems featuring multiple A/D converters, connecting AGND and DGND under each A/D converter simplifies design but requires meticulous attention to ensure the width of the bridging connection equals the IC width, thereby preventing signal lines from crossing partition gaps. Failure to adhere to manufacturer guidelines risks compromising ground isolation, necessitating a structured approach as depicted in Figure 4, where grounds are uniformly divided into analog and digital sections.
12. To evaluate the efficacy of ground division, consider specific scenarios where strict separation remains essential. For example, medical devices requiring minimal leakage current, industrial equipment interfacing with noisy electromechanical devices, or PCB layouts subject to stringent design constraints. While mixed-signal PCBs typically employ separate analog and digital power supplies, careful design allows for a unified power supply configuration to avoid splitting signal lines across different power faces.
13. Mixed-signal PCB design necessitates careful consideration across various stages:
1. Partitioning into distinct analog and digital segments.
2. Optimal component placement.
3. Strategic placement of A/D converters across partitions.
4. Avoidance of ground division where possible.
5. Exclusive routing of digital signals within digital sections and analog signals within analog sections across all layers.
6. Segregation of analog and digital power sources.
7. Adherence to correct wiring practices to prevent signal lines from spanning gaps between split power supply surfaces.
8. Detailed analysis of current flow paths.
9. Rigorous compliance with wiring regulations and thorough testing to validate functional and EMC performance.
These revisions maintain the content and structure of your original text while enhancing clarity and readability.
