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Mixed-Signal Design
Learn2026-01-14

Mixed-Signal Design

#electronics#mixed-signal#analog-digital

Overview

Mixed‑signal boards combine analog and digital domains and require careful partitioning, grounding, and reference management. This chapter gives practical rules for co‑existing analog and digital subsystems on the same PCB.

Prerequisites

  • Experience with PCB layout and data converters

Learning objectives

  • Partition analog and digital sections and manage return paths
  • Implement star grounding, split planes, and quiet reference routing
  • Minimise digital noise coupling into sensitive analog paths

Tools & materials

  • PCB CAD, oscilloscope, function generator, optional isolation components

Hands-On Mini Task

  1. Given a mixed-signal schematic, create a board plan that segregates analog/digital areas, routes sensitive analog traces with short returns, and places references and decoupling appropriately.
  2. Measure noise on an analog input with the MCU running and with the MCU in low-activity mode to compare coupling effects.

Expected result: reduced analog noise when following partitioning and grounding rules; measurable difference in coupling before/after changes.

Partitioning strategies

  • Physically separate noisy digital circuits from sensitive analog areas; use quiet analog zones for ADCs and sensor inputs.
  • Group related components (voltage references, input filters) to keep critical nets short.

Reference and ground handling

  • Use a single, well routed reference (voltage reference) and avoid burying analog reference traces under noisy planes.
  • Consider isolated islands or star points for reference distribution in sensitive systems.

Isolation and buffering

  • Use op‑amp buffers for high‑impedance sensors, and consider isolation (digital isolators or transformers) for noisy domains.

Worked example — partitioning an ADC front-end

  1. Place the ADC, reference, and input filter in a compact region with ground stitching around the area.
  2. Route digital signals away and use guard traces and ground vias to shield analog inputs.

Troubleshooting

  • If noise persists, measure with the MCU powered down to isolate digital coupling paths.
  • Check for EMI from power supplies and clock sources coupling into analog traces.

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