Key Principles and Practical Tips for Mixed-Signal PCB Partition Design

Mixed Signal Circuit Board Design

Future trends indicate that mixed-signal systems will become an indispensable part of the system, and their proportion is expected to increase from 33% in 2005 to 66%. However, another view is that future systems will rely on ultra- With deep sub-micron CMOS technology, large digital chips will play a major role, while analog and mixed-signal circuits will gradually be external.

Currently, mixed-signal design and mixed-technology design are often conflated. However, in both terms, we rely less on analog technology and instead rely on digital circuits to perform analog functions; in mixed-technology design, In particular, power supply technology plays a more important role.

Mixed-signal PCB design requires the designer to have a basic understanding of analog and digital circuits to minimize the possibility of signal interference. Because modern systems have components in both the digital and analog domains, they need to be carefully designed to ensure Signal integrity of the entire system.

PCB layout is a crucial part of the mixed-signal development process, but component placement is only the beginning of the process. Other factors need to be considered, including how to manage the various layers of the board and how to effectively reduce errors caused by parasitic capacitance. Potential interference, as this interference problem can occur between different layers of the PCB.

Another important step is the design of the ground wire. Although the ground wire issue has always been controversial, it is not easy to develop a universal standard method. High-quality ground wire design may have a significant impact on the performance of high-performance mixed-signal PCBs. The overall layout has a significant impact and cannot be ignored.

Methods for mixed-signal PCB partition design

It has been suggested to separate the digital and analog grounds on mixed-signal circuit boards to achieve isolation between them. Although this method is effective in some cases, in complex large systems, it may bring Many potential problems. One of the most critical issues is to avoid wiring across the dividing line, because once the dividing line is crossed, electromagnetic radiation and signal crosstalk problems will increase dramatically, which is one of the common problems in PCB design, especially When signal lines cross separate grounds or power supplies, EMI problems may occur.

Consider the following situation. If we adopt the above-mentioned segmentation method and the signal line spans the gap between the two grounds, then the question arises: Where is the return path of the signal current? Normally, the two divided grounds It will be connected with a single point connection somewhere. In this case, the ground current will form a large loop. When high-frequency current flows through this large loop, it will produce radiation and high ground inductance. If it flows through the large loop is a low-level analog current, so interference from external signals can easily affect it. What is even more worrying is that when separate ground wires are connected together at the power supply, a huge current loop will be generated .In addition, if the analog ground and digital ground are connected to each other through long wires, a dipole antenna effect will also be formed.

To delve deeper into the interference of digital signals on analog signals, we first need to understand the characteristics of high-frequency currents. High-frequency currents tend to travel along the path of lowest impedance (least inductance), usually the path directly underneath the signal. This means that the return current will flow through nearby circuit layers, whether these layers are power layers or ground layers.

In practical work, it is usually preferred to use a unified ground wire and divide the PCB into an analog part and a digital part. Analog signals are routed in the analog area of the circuit board, and digital signals are routed in the digital circuit area. In this case, the digital The return current of the signal will not flow through the ground wire of the analog signal.

Only when digital signals are routed on the analog part of the circuit board, or analog signals are routed on the digital part, will the problem of digital signal interference with the analog signal occur. This problem does not occur because there is no separate ground wire, the real reason The problem lies in improper wiring of digital signals.

Key Points of Mixed Signal PCB Design

1\Divide the PCB into independent analog and digital parts.

2\Arrange parts reasonably.

3\Special attention is required when placing the A/D converter.

4\Avoid ground wire separation and lay unified ground wires on the entire circuit board.

5\Digital signals can only be routed in the digital part, analog signals can only be routed in the analog part, and they are independent.

6\Ensure analog and digital power supplies are separated.

7\Do not run wiring across the gap that separates power and ground.

8\If wiring must be conducted across the gap that separates the power ground, the signal line should be located on the wiring layer adjacent to the large ground area to reduce the inductance.

9\Analyze the specific path of the ground current.

10\Use appropriate

Wiring rules.

11\In terms of component placement, creating a system floor plan is necessary to ensure the integrity of the system design and reduce potential problems with high noise signal interference.

12\The power module needs to be isolated from other circuits, but still needs to be close to the power supply components.

Devices in complex systems may have multiple power supply pins. In this case, you can consider using dedicated power modules to power the analog and digital parts separately to reduce the possibility of digital interference.

On the other hand, power routing should be short and straight, using wide traces to reduce inductance and avoid current limiting.