While smaller and more powerful electronics delight users, engineering these marvels can challenge the printed circuit board designer. Electronics used to contain multiple boards for digital processing, power supply, and analog functions, but that functionality typically packs into one dense PCB design in today’s devices. Analog circuitry takes sensory inputs and converts them into digital signals for processing through the board’s CPU and memory circuitry. At the same time, clean power and ground from the board’s power delivery network to support the operation of the digital and analog circuitry.
A mixed-signal PCB design simultaneously incorporates the functions of both digital and analog circuits. To operate successfully, designers need to exercise caution in the layout of these boards to avoid signal and power integrity problems. Here are some PCB design rules for mixed-signal circuit boards that can help.
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Component Placement for Mixed-Signal PCB Design
As with any PCB layout, a mixed-signal design must start with accurate library parts and a well-built layer stackup. When considering the grounding needs of a mixed-signal board, the layer stackup requires even greater attention to detail for the best signal and power integrity. Once the circuit board design incorporates library parts and a layer stackup, the next step is partitioning the design into different circuitry areas.
A design partition enforces isolation between different circuitry groups. Digital and analog signals can disrupt each other, and analog circuitry is sensitive to interference from noisy digital signals. Many PCB CAD systems have partitioning features, but guidelines for component placement will be system-agnostic:
General Mixed-Signal Design Strategies
- Map out the digital, analog, and power partitions on the board.
- Structure the partitions to isolate the different circuitry areas from each other naturally.
- Allow room for the blocks of circuitry within the partitions as defined in the schematic.
- Also, allow room for bus routing, and remember that high-speed signals need space for return paths on an adjacent ground plane that are tightly coupled and unobstructed.
With the board partitions mapped out, board placement can begin. While standard PCB part placement design rules apply, they must follow the constraints of the partitions:
Best Practices for Partitioning
- Start with fixed components such as connectors and other interface parts, and place back from there.
- Position large processors and memory devices more towards the center of the board to leverage heat dissipation through the board.
- Follow the schematic signal paths for placing high-speed circuitry and keep these parts as closely as possible.
- Decoupling capacitors should sit close to their assigned supply pins on large devices.
- Power supply circuits should be placed close to their supply circuits without being clustered with other power supplies.
- Power supply parts must also be placed tightly together and on the same side of the board for the best power integrity.
With the parts placed on the board, routing can begin. Follow some best practices for mixed-signal grounding layout.
PCB Design Rules for Mixed-Signal Circuit Grounding
Mixed-signal designs with signals between their digital and analog circuitry should use a single ground plane for the entire design. Although a split or isolated ground system would be cleaner, it is not for a fundamental reason: the signals that cross between the two circuitry sections also need a clear and unobstructed return path on a ground plane. Those signals that don’t have a clear return path will find an alternate return path that will be undesirable, such as going through the system’s chassis. This configuration usually results in a large loop that radiates EMI and ruins the signal integrity of the design.
A well-designed partition will help keep the digital and analog sections of circuitry isolated from each other and prevent their noise from affecting each other. Additionally, a single ground plane will aid signal and power integrity with the following advantages:
The Importance of a Direct Return Path
- Clear and unobstructed signal return paths for all signals.
- A lower impedance ground system to help eliminate ground bounce noise.
- Built-in shielding to protect the board from incoming EMI.
- A natural barrier to prevent any generated EMI from escaping.
- Heat dissipation of hot components such as a processor or memory parts.
Careful consideration must go into the design of the power delivery network (PDN) and the ground system of the board (along with the component placement) to minimize changes during routing. After part placement, it’s time to route the connections and the PDN system layout.
Trace Routing on a Mixed-Signal PCB Design
As with any circuit board layout, working with complete PCB design rules and constraints is essential for success. Design rules and constraints will govern the clearances between parts and regulate the trace widths and spacings. However, there are some other PCB design rules for mixed-signal circuit boards to keep in mind while trace routing:
How To Design With Signal Integrity in Mind
- Digital and analog circuitry routing need sufficient isolation from each other.
- For a signal return path, keep high-speed traces on one layer with an adjacent ground plane.
- Don’t route over breaks, splits, or voids in the reference ground plane, which will ruin the return path and create a lot of noise.
- Keep signal paths between high-speed components as short and as direct as possible.
- Don’t route sensitive digital or analog signals through power supply areas.
- Route connections between power supply components with short and wide traces on one layer only to lower the inductance.
Although many other design rules apply to mixed-signal PCB designs, the rules we’ve looked at in this article cover the basics to ensure good signal and power integrity in your layout. Your contract manufacturer will also have other tips and recommendations to help you navigate the challenges of designing a mixed-signal printed circuit board. At VSE, we regularly work with our customers on mixed-signal and other PCB design technologies to ensure their success. Let us show you how we can help you get the highest performance, quality, and production yields on your next design.
