Designing Mixed-Signal PCBs: A Practical Block-by-Block Approach

 Introduction

In the world of electronics, most real-world systems aren't just analog or just digital. They are a combination of both. Such systems are called mixed-signal systems, and designing their PCBs (Printed Circuit Boards) requires a good understanding of both analog and digital rules. Today, we will learn how to design a mixed-signal PCB using a simple, step-by-step, block-by-block approach.

We will use a fictional but realistic example: a 100mm x 100mm, 2-layer PCB divided into different functional areas (blocks). This method teaches not only routing and layout but also why certain decisions are made.

Overview of Our Example PCB

Let’s assume our 100mm x 100mm PCB has 5 major blocks:

1. Analog Signal Block: Processes sensitive analog signals.
2. Digital Block: Contains a microcontroller (MCU) and logic ICs.
3. Switching Power Supply (Buck/Boost): Converts power, but generates noise.
4. DSP Audio Block: Handles audio input/output.
5. Impedance-Controlled Filter Block: Filters AC signals with high signal integrity.

Later, we can add more blocks like RF, connectors, power entry, or battery management.

Why Partition Into Blocks?

• Noise control: Separate noisy and sensitive parts.
• Troubleshooting: Easier to isolate issues.
• Design clarity: Focus on one function at a time.

Think of your PCB like a city. Each block is a neighborhood with its own purpose, and each must be carefully placed and connected.

Grounding Strategy

In 2-layer PCBs, ground is precious. Here are some rules:
• Use the bottom layer as a near-continuous ground plane.
• Avoid cutting the ground plane with traces.
• Make sure each block has a clear return path to ground.
• Don’t route signals across split ground zones.

If you must split grounds (analog and digital), reconnect them at a single star point near the power supply.

Step-by-Step Routing Plan

 

Step 1: Identify Noise Sources and Victims

• Source: Buck converter (fast switching, high EMI).
• Victim: Analog signal lines, audio section.

Lesson: Keep them far apart. Place analog block on one side, buck converter on the opposite side.

Step 2: Power Distribution

• Use separate filtered power for analog and digital sections.
• Place bypass capacitors close to every IC power pin.
• Use ferrite beads to isolate analog from digital power.

Step 3: Route Ground Carefully

• Keep a solid ground plane under high-speed or analog signals.
• Ensure return paths are direct. If a signal trace goes from analog to digital, its return path must also be unbroken.

Step 4: Place and Route Digital Logic (MCU)

• Keep clock and data lines short.
• Don’t route digital lines near analog input traces.

Step 5: Route Analog and Audio Lines

• Route analog signals away from digital and switching areas.
• Shield them using ground guard traces.
• Keep analog traces short and direct.

Step 6: Impedance-Controlled Traces

• If using high-frequency or differential lines (e.g. I2S, USB), use controlled impedance routing.
• Teach students to use online calculators to choose trace width based on board thickness.

Teaching Techniques Using This Model

 

Mini-Demos

1. Noise Injection: Place buck converter near analog line. Show noise on oscilloscope.
2. Return Path Break: Cut ground under signal. Show degraded waveform.
3. ESD Entry Point: Touch unprotected I/O. Show MCU resets.

Block-Wise Design Exercise

1. Students start with analog and buck blocks.
2. Route each one with isolation and clean power.
3. Add digital and DSP blocks next.
4. Finally, connect impedance-controlled and I/O blocks.

Goal: Teach integration while minimizing noise.

Summary

Designing mixed-signal PCBs is not just about connecting components. It’s about managing interference, grounding, routing, and planning. By breaking a complex PCB into understandable blocks, students can learn step-by-step how to:

• Identify and separate noisy and sensitive sections
• Route clean power and ground
• Handle signal integrity and layout planning

Optional Extensions

• Add a block with a Bluetooth or Wi-Fi module.
• Add USB or external analog I/O.
• Add protection circuits (TVS diodes, fuses, etc.).

Final Thought

Good PCB design is not art, it's architecture. You don't just draw lines; you plan paths.