Cadence Ebooks

26. Routing Differential Pairs with Consistent Geometry and Return Paths Differential pairs (used in interfaces like USB, Ethernet, HDMI, PCIe, LVDS, and DDR) rely on equal and opposite signals traveling together, with their noise immunity and low EMI performance dependent on tightly matched impedance, trace lengths, and return paths. Even small mismatches in length, spacing, or return current continuity can degrade signal integrity, increase jitter, cause data errors, or violate interface specs. Differential routing discipline is critical for high-speed data reliability. When And Where To Apply Apply these practices to all differential pairs, especially in high-speed, high-frequency, or low-jitter systems: USB, PCIe, HDMI, DisplayPort, MIPI, LVDS, CAN, and memory data lines, as well as precision analog (e.g., differential amplifiers, ADC inputs). Differential pair routing in OrCAD X

• Cover
• I. Project Planning, Requirements, and Documentation
• 1.Establishing a Clean Design Rule Set
• 2. Preventing Floating or Unused Inputs and Outputs
• 3. Finalizing the PCB Stackup Early
• 4. Validating Symbol-to-Footprint Mapping
• 5. Using Descriptive Net and Component Naming
• 6. Confirming Component Availability and Avoiding Obsolete Parts
• 7. Archiving and Documenting the Complete Design for Future Reuse and Support
• II. Placement, Floorplanning, and Mechanical Integration
• 8. Including Mechanical References Early in the PCB Layout
• 9. Prioritizing Placement of Fixed and Critical Components
• 10. Planning BGA and Fine-Pitch Fanout Before Surrounding Placement
• 11. Grouping by Functional Blocks and Signal Flow
• 12. Respecting Assembly Spacing and Rework Access
• 13. Keeping Height and Keep-In/Keep-Out Constraints Visible
• 14. Reviewing Silkscreen and Markings for Readability and Clarity
• 15. Confirming Board Edge Clearances for Copper, Vias, and Components
• 16. Reviewing and Optimizing Via Types, Sizes, and Counts
• III. Power, Grounding, and Thermal
• 17. Using a Continuous Ground Plane
• 18. Optimizing Regulator and Power Converter Current Loops
• 19. Keeping Analog, Digital, and Power Domains Isolated
• 20. Preventing Ground Loops and Unintentional Return Paths
• 21. Pouring Copper with Electrical Intent, Not Just for Fill
• 22. Building Ground Via Fences and Edge Stitching
• 23. Managing Split Planes and Power Islands Correctly
• 24. Planning for Thermal Management: Heat Sinks, Vias, and Copper Pours
• IV. Signal Integrity and High-Speed Routing
• 25. Minimizing Crosstalk by Managing Parallelism and Trace Spacing
• 26. Routing Differential Pairs with Consistent Geometry and Return Paths
• 27. Avoiding Right-Angle and Acute Trace Corners
• 28. Minimizing Layer Transitions on Critical Nets
• 29. Eliminating Stubs and Dead-End Tees
• 30. Treating Length and Skew Matching as Numeric Constraints
• 31. Isolating Clocks, Resets, and Sensitive References from Noise Sources
• 32. Using Guard Traces and Shielding for Ultra-Sensitive or High-Speed Nets
• 33. Optimizing Decoupling and Bulk Capacitor Placement
• 34. Specifying Controlled Impedance and Reference Plane Requirements
• V. Manufacturability, Test, Reliability
• 35. Performing Design Rule Checks (DRC) and Electrical Rule Checks (ERC) Iteratively
• 36. Designing for Effective Test Point Access and Coverage
• 37. Verifying All Pad, Hole, and Footprint Sizes for Manufacturing
• 38. Specifying and Checking Solder Mask and Paste Mask Clearances
• 39. Reviewing Design for Manufacturability (DFM) and Assembly Constraints
• 40. Designing for Reliability in Harsh Mechanical, Thermal, and Environmental Conditions