In high-speed PCB design, interference issues in digital-analog hybrid circuits remain a significant challenge. Specifically, the analog circuit typically serves as the signal source, making the accurate reception and conversion of signals a crucial consideration in PCB design. This article explores the general methods for addressing the interference in digital-analog hybrid circuits by analyzing the underlying mechanisms and integrating design practices, validated through design examples.
I. Introduction
A printed circuit board (PCB) acts as a foundation for circuit components and devices within electronic products, facilitating electrical connections between them. Today, many PCBs are not solely single-function; they consist of a blend of digital and analog circuits. Typically, data is collected and processed in the analog domain, while parameters like bandwidth and gain must be digitized for software control. Consequently, digital and analog circuits often coexist on the same board, sometimes sharing components. To mitigate mutual interference and its effects on circuit performance, careful principles of layout and routing must be adhered to. The specific requirements for power transmission lines in mixed-signal PCB design and the necessity of isolating noise between analog and digital circuits add complexity to the layout and routing process. This analysis focuses on high-density mixed-signal PCB layout and wiring design to achieve the desired PCB design objectives.
2. The Generation Mechanism of Digital-Analog Hybrid Circuit Interference
1. Compared to digital signals, analog signals are significantly more susceptible to noise because analog circuit operation relies on continuously varying current and voltage. Any minor interference can disrupt its normal function, while digital circuits operate based on the receiving end detecting high or low levels according to a defined voltage threshold, providing a degree of noise immunity. However, in a mixed-signal environment, digital signals can introduce noise to analog signals. When a digital circuit operates, the effective voltage remains at either a high or low level. When the digital logic output transitions from high to low, the ground pin of the device discharges, generating a switching current, which is the circuit’s switching action. The faster the digital circuit operates, the shorter the switching time typically required. When many switching circuits simultaneously transition from a logic high to a logic low, inadequate current-carrying capacity of the ground wire can lead to substantial switching currents. This results in fluctuations in the logic ground voltage, a phenomenon known as ground bounce. The ground bounce noise and power disturbances caused by the digital circuit, if coupled to the analog circuit, can impair its performance. As many interference sources originate from power and ground buses, ground wires contribute the most to noise interference, making the design of ground and power particularly critical in PCB design.
2. General Guidelines for PCB Design in Digital-Analog Hybrid Circuits
I previously discussed the mechanisms of interference in hybrid circuits. How can we minimize the mutual interference between digital and analog signals? Before commencing design, it’s essential to understand two fundamental principles of electromagnetic compatibility (EMC): The first principle is to minimize the area of the current loop. If the signal cannot return through the smallest possible loop, a large loop can form, acting like an antenna. The second principle is to utilize only one reference plane in the system. Conversely, having two reference planes may create a dipole antenna. It’s important to avoid these situations as much as possible during design.
3. Layout and Routing Principles. One of the primary considerations for component placement is to keep the analog circuit separate from the digital circuit. The analog signals should be routed within the designated analog areas of all layers of the circuit board, while digital signals should be confined to the digital areas. This ensures that the return current from the digital signal does not flow into the analog signal ground. For high-frequency lines with specific requirements, manual routing is advisable, employing differential or shielded lines when necessary. Sometimes, due to the placement of input/output connectors, the routing of digital and analog circuits may overlap, potentially causing mutual interference. It’s crucial to avoid routing digital clock lines and high-frequency analog signals close to the analog power layer, as noise from the power signal can couple into sensitive analog signals. To achieve a low-impedance power and ground network, it’s important to minimize the inductance of digital circuit traces and reduce capacitive coupling in the analog circuit. Given that digital circuits operate at high frequencies while analog circuits are highly sensitive, high-frequency digital signal lines should be kept as distant as possible from sensitive analog devices.
4. Power and Ground Management. In the design of complex hybrid circuit boards, the layout and management of ground connections are vital for enhancing circuit performance. It is recommended to separate the digital ground and analog ground on mixed-signal circuit boards to achieve isolation between them. However, this approach can lead to wiring crossing the separation gap, which may significantly increase electromagnetic radiation and signal crosstalk. Understanding the path and method of current return to ground is crucial for optimizing mixed-signal circuit board design. If the ground layer must be divided, and wiring needs to traverse the gap between divisions, a single-point connection can be established between the divided grounds to create a bridge connecting the two grounds, allowing for wiring through this connection.
I. Introduction
A printed circuit board (PCB) acts as a foundation for circuit components and devices within electronic products, facilitating electrical connections between them. Today, many PCBs are not solely single-function; they consist of a blend of digital and analog circuits. Typically, data is collected and processed in the analog domain, while parameters like bandwidth and gain must be digitized for software control. Consequently, digital and analog circuits often coexist on the same board, sometimes sharing components. To mitigate mutual interference and its effects on circuit performance, careful principles of layout and routing must be adhered to. The specific requirements for power transmission lines in mixed-signal PCB design and the necessity of isolating noise between analog and digital circuits add complexity to the layout and routing process. This analysis focuses on high-density mixed-signal PCB layout and wiring design to achieve the desired PCB design objectives.
2. The Generation Mechanism of Digital-Analog Hybrid Circuit Interference
1. Compared to digital signals, analog signals are significantly more susceptible to noise because analog circuit operation relies on continuously varying current and voltage. Any minor interference can disrupt its normal function, while digital circuits operate based on the receiving end detecting high or low levels according to a defined voltage threshold, providing a degree of noise immunity. However, in a mixed-signal environment, digital signals can introduce noise to analog signals. When a digital circuit operates, the effective voltage remains at either a high or low level. When the digital logic output transitions from high to low, the ground pin of the device discharges, generating a switching current, which is the circuit’s switching action. The faster the digital circuit operates, the shorter the switching time typically required. When many switching circuits simultaneously transition from a logic high to a logic low, inadequate current-carrying capacity of the ground wire can lead to substantial switching currents. This results in fluctuations in the logic ground voltage, a phenomenon known as ground bounce. The ground bounce noise and power disturbances caused by the digital circuit, if coupled to the analog circuit, can impair its performance. As many interference sources originate from power and ground buses, ground wires contribute the most to noise interference, making the design of ground and power particularly critical in PCB design.
2. General Guidelines for PCB Design in Digital-Analog Hybrid Circuits
I previously discussed the mechanisms of interference in hybrid circuits. How can we minimize the mutual interference between digital and analog signals? Before commencing design, it’s essential to understand two fundamental principles of electromagnetic compatibility (EMC): The first principle is to minimize the area of the current loop. If the signal cannot return through the smallest possible loop, a large loop can form, acting like an antenna. The second principle is to utilize only one reference plane in the system. Conversely, having two reference planes may create a dipole antenna. It’s important to avoid these situations as much as possible during design.
3. Layout and Routing Principles. One of the primary considerations for component placement is to keep the analog circuit separate from the digital circuit. The analog signals should be routed within the designated analog areas of all layers of the circuit board, while digital signals should be confined to the digital areas. This ensures that the return current from the digital signal does not flow into the analog signal ground. For high-frequency lines with specific requirements, manual routing is advisable, employing differential or shielded lines when necessary. Sometimes, due to the placement of input/output connectors, the routing of digital and analog circuits may overlap, potentially causing mutual interference. It’s crucial to avoid routing digital clock lines and high-frequency analog signals close to the analog power layer, as noise from the power signal can couple into sensitive analog signals. To achieve a low-impedance power and ground network, it’s important to minimize the inductance of digital circuit traces and reduce capacitive coupling in the analog circuit. Given that digital circuits operate at high frequencies while analog circuits are highly sensitive, high-frequency digital signal lines should be kept as distant as possible from sensitive analog devices.
4. Power and Ground Management. In the design of complex hybrid circuit boards, the layout and management of ground connections are vital for enhancing circuit performance. It is recommended to separate the digital ground and analog ground on mixed-signal circuit boards to achieve isolation between them. However, this approach can lead to wiring crossing the separation gap, which may significantly increase electromagnetic radiation and signal crosstalk. Understanding the path and method of current return to ground is crucial for optimizing mixed-signal circuit board design. If the ground layer must be divided, and wiring needs to traverse the gap between divisions, a single-point connection can be established between the divided grounds to create a bridge connecting the two grounds, allowing for wiring through this connection.
