Issue link: https://resources.pcb.cadence.com/i/1541046
19. Keeping Analog, Digital, and Power Domains Isolated How To Implement 1. Identify and define functional domains: f Classify all circuit sections as "Analog," "Digital," or "Power" on the schematic and in layout. f Physically separate these blocks on the board, using silkscreen or mechanical layers to define domain boundaries. 2. Establish domain-specific ground and power regions: f If possible, provide dedicated analog (AGND), digital (DGND), and power (PGND) planes or copper pours. Use a solid, unbroken plane for each. f Join grounds at a single star-point or bridge, typically under or near the relevant ADC, DAC, or mixed-signal IC, following the manufac- turer's recommendations. f Never route traces over a split between planes. This forces the return current to detour, creating noise and EMI. 3. Route signals to minimize cross-domain coupling: f Keep analog and digital signals physically separated, and do not run parallel traces from different domains close together. f When crossing domains is necessary, route at right angles and over a solid ground plane; avoid running sensitive analog lines near noisy digital clocks, buses, or switching nodes. f Use guard traces (grounded copper routed parallel to sensitive lines) where isolation is most critical. 4. Filter and decouple domain interfaces: f Decouple each domain's supply locally, using separate capacitors and ferrite beads for analog, digital, and power regions. f Use RC or LC filters on power lines entering the analog domain, and isolate power for noise-sensitive blocks with LDOs or dedicated supplies. f Add series resistors, low-pass RC filters, or opto/transformer isolation for digital signals crossing into analog/RF domains. 5. Shield and protect sensitive areas: f For ultra-low-noise or RF circuits, consider a copper shield can, with the shield grounded to the local analog ground via multiple vias. f Place analog/RF domains farthest from high-current power or motor control blocks, and closest to the relevant connectors or antennas. 6. Test and validate isolation: f During bring-up, measure analog and digital ground potentials to check for voltage differences. f Probe for coupled noise or crosstalk using an oscilloscope, spectrum analyzer, or EMI pre-compliance tools. Common Pitfalls, Their Impact, and How to Avoid Them Common Pitfall How to Avoid it Crossing split planes with sensitive traces Forces noisy return currents into the analog domain, ruining performance – Use grounded guard traces and keep traces short for better return path. Overlapping analog and digital traces Creates direct coupling paths for noise – Route perpendicularly on adjacent layers and use ground planes for isolation. Using a single, fragmented ground pour for all domains Results in ground loops and unpre- dictable noise paths – Partition the board by function and route signals within their specific zones. Omitting filtering at power and domain boundaries Leaves analog/RF blocks vulnerable to digital or power noise – Implement robust grounding strategies.