The old expression is, “you can’t fit a square peg into a round hole.” During circuit board manufacturing, we regularly put square pins into round holes for soldering, but our topic here is mixing digital and analog signals in PCB design. Analog and digital signals tend to negatively affect each other and create problems when together on the same circuit board. Trying to design these two signal types together can sometimes feel like forcing a square peg into a round hole.
In a perfect layout, we wouldn’t ever mix digital and analog signals. However, the majority of circuit boards require both sections of circuitry. The analog section prepares signals for digitizing, and the digital section converts those signals into digital forms to be acted on by the rest of the board’s circuitry. To accommodate this mixture means designing layouts to avoid the signal integrity problems that mixed-signal boards can create. Here are some considerations for partitioning and layout of a mixed-signal PCB that can help get the best performance out of both the analog and digital sections of the board.
Key Considerations for Partitioning and Layout of a Mixed-Signal PCB
Both analog and digital signals can be disrupted by the noise they create. Analog circuitry, in particular, is highly susceptible to noisy digital signals, requiring extra care to protect it from unwanted interference. With all the complexities of designing high-speed digital circuitry, combining the design with analog circuitry can make the job even more difficult. It is essential to follow some basic design guidelines for both the analog and digital circuitry to achieve the best performance of a mixed-signal board:
- Begin the layout by partitioning the design into analog and digital sections.
- Complete the partitions by defining specific blocks of circuitry within each section.
- Create the partitions so that the analog and digital circuitry are isolated from each other.
- Create a consistent ground system for the entire circuit board.
- Keep return paths close to their originating signals using only one reference plane.
With these guidelines, you can create the partition for a layout to serve as a roadmap for placing components on the board. Still, follow some standard placement practices, such as locating memory devices close to processors, but do so according to the confines of the partitions. This practice will prevent the mixing of analog and digital signals by not routing circuitry from one section through the other. It will also allow you to use a single ground plane across the board instead of splitting the plane between ground and analog sections. Split planes can create EMI and crosstalk for those signals that must cross between the two sections, making partitioning the analog and digital sections the better option.
With the partitions of the circuit board configured, it’s time to start the layout.
Mixed-Signal Component Placement and Routing
Before placing and routing a mixed-signal PCB design, define the board layer stackup and the design rules for layout. The configuration of the power and ground planes in the stackup is crucial for the performance of the high-speed circuitry to determine the board layer and trace widths for impedance-controlled routing. Additionally, the ground plane will serve as the reference layer for the different return paths of a mixed-signal design. The design rules are critical for maintaining the necessary spacing between components and traces in the design.
Component placement
When placing components, make sure to follow the already set partitions and don’t allow the placement of one area to infringe upon the other. The exception to this rule is the analog to digital converters of the design. Place these where they will straddle the dividing line between the analog and digital sections of the board. All components must follow standard PCB layout practices, ensuring that they obey good design for manufacturability (DFM) rules for the highest manufacturing yields.
Trace routing
Analog and digital signals should only be routed in their respective sections. Never route the signals of one section through the other section. If there isn’t enough room for the routing as the board is partitioned, adjust the partitioning and associated circuitry to create additional room. Those signals that cross between sections must be routed entirely over the circuit board’s ground plane.
Power and ground planes
When designing the partitions of the board, make sure that you have allowed enough room for signal routing and the return paths of those signals on the adjacent reference ground plane. The reference plane mustn’t be chewed up with dense areas of vias or cutouts that can block the clear return path of the signals. It is also necessary to ensure that there is enough separation between circuitry areas to prevent blending of the return paths on the reference plane.
The successful partitioning and layout of a mixed-signal PCB start first with how the design is set up. Here is where partnering with an experienced PCB contract manufacturer can help.
Involve Your Manufacturing Partner Early On in the Design Phase
Using the services of an experienced PCB contract manufacturer is a valuable resource. The CMs’ design engineers will review your circuitry to recommend changes and alterations to improve your board’s performance. At the same time, their procurement team will be looking for ways to reduce expenses in the components required for your board’s assembly. And when it comes to your PCB layout, their engineers will help with DFM reviews, test strategies, and recommendations on how best to partition your mixed-signal design. These steps decrease cost and production times while increasing yields and ensuring designs are optimized for peak performance.
At VSE, we have been building mixed-signal designs for our customers for over 30 years. Based in Silicon Valley in California’s Bay Area, we have been serving local clients in San Francisco and San Jose with the same level of quality and expertise that we provide to clients all over the world. We have the experience in analog and digital PCB designs that you are looking for and look forward to partnering on your next mixed-signal design.
