We discussed what printed circuit board (PCB) designers are most likely to mean when they talk about doing mixed-signal designs. As part of this, we considered the different types of circuits that might be involved, and we touched on the high-level differences and challenges involved with each.
Now, you might be wondering why this really matters, and why it’s harder than implementing a purely analog PCB design.
Mixed-signal PCB design requires a basic understanding of both analog and digital circuits to minimize, if not prevent, signal interference. The components that make up a modern system operate in both the digital and analog domains and must be carefully designed to ensure signal integrity throughout the system.
As an important part of the mixed-signal development process, PCB layout can be daunting, and component placement is just the beginning. There are other factors that must be considered, including the layers of the board and how to properly manage these layers to minimize interference caused by parasitic capacitance (such capacitance that can be inadvertently developed between layers between planes of the PCB).
Grounding is also an important step in PCB layout design for mixed-signal systems. While grounding is a topic of constant debate in the industry, developing a standardized approach isn’t necessarily the easiest task for engineers. For example, a single problem with high-quality grounding can affect the entire layout of a high-performance mixed-signal PCB design. Therefore, this aspect should not be ignored.
Don’t get us wrong…
Let’s get one thing clear right away – all PCB design is challenging, mixed signal or not. Developing a design from start to finish means a series of steps and decisions, all mixed with trade-offs and compromises, ultimately resulting in a finished product that (hopefully) meets the requirements you inherited. Let’s quickly walk through some of these design steps from a PCB design perspective.
· Define your requirements
o In our opinion, this is undoubtedly the most undervalued and underused step in design work today. Without clear requirements, achieving success will be much more difficult. (Perhaps we’ll explore this topic in more depth in a future column.)
· Planning and Design
o Early selection of critical and core components. If you’re using an analog PCB with 16 identical analog input channels, do your homework and research to find the perfect op amp to use. If you are developing a Wi-Fi module, read about available PA/LNA devices and RF transceivers. Maybe it makes more sense to use a full module. The key is to do this work before launching the schematic capture tool. Plan, plan, plan—then you can worry about execution.
o Draw a block diagram. Power trees, signal flow, floor planning, and system interconnect are just a few examples that can benefit from early diagrams.
o Consider PCB stackup, materials and constraints.
· Capture Schematic
o Once planning is complete and key components selected, you should feel confident starting to capture circuit design intent.
o Don’t forget to document all your design decisions. Historical references are important; capturing key decisions will be invaluable to you two years later when you need to revisit your PCB for various reasons.
· Layout printed circuit board
o Place copper on FR-4 (or whatever PCB material you may decide to use) and prepare to order your PCB.
The above provides a good overview that any PCB design should keep in mind, but each type of circuit design has its own nuances and challenges.
Imagine performing the steps we just outlined for a typical PCB that E3 might consider applicable to most industries:
· ARM processor and/or FPGA
· High-speed serial communication
· Analog signal conditioning and processing
· Power conversion
This is a fairly common circuit combination for embedded controllers in a variety of industries. There are many things here that interact in expected ways, and you also need to prevent them from interacting in unexpected ways. Consider a few examples:
· You don’t want your high-speed communications coupling into analog signals.
· You don’t want the high current loop from the power conversion coupling into anything.
Now start adding more types of circuits to the mix — such as RF designs — and the problem starts to compound and further constrain your design. Soon, you will face a daunting challenge.
Mixed Signal PCB Design: What Makes It Difficult?
Summary (for now)
Don’t worry! Of course, doing a job like this is challenging and can sometimes seem like an absolutely impossible task, even for the most seasoned designers, but we guarantee it can be done. We are living proof.
We didn’t know much about it when we graduated, but 10 years into our careers we were considered pretty good design engineers (forgive our lack of humility).
The same things that make mixed-signal design difficult also make it fun. It pushes you to learn new things and think critically about fundamental engineering principles. We don’t even cover constraints beyond your own design requirements – such as harmonized standards for safety, EMC/EMI and environmental regulations. Then there are product-driven constraints, but we digress…
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