When mixing analog and digital circuits on printed circuit boards, managing ground planes appropriately minimizes noise injection ensuring proper function. A common technique is creating separate analog and digital ground domains tied to a central star point or points.
This article provides electronics hardware engineers a comprehensive guide on leveraging star grounding configurations to achieve critical isolation between sensitive analog and noisy digital return paths. By covering fundamental concepts, implementation considerations, printed circuit board layout recommendations, a real-world design example, and helpful FAQs, PCB designers can confidently utilize star grounds to solve mixed-signal stability problems.
What is Star Grounding?
Star grounding involves routing all ground returns from a section of circuitry to a single, localized node point before connecting to the main ground plane. This star point becomes the single tie-in location for that region’s currents flowing to the shared system ground reference.
For analog and digital separation, individual star points are created for groupings of analog and digital circuitry. Traces transport respective return currents from each zone to its localized star point before interconnecting peripherally to the board’s primary ground plane.
Star grounding routes return paths to localized star points
Star grounding enables optimal high frequency isolation between critical circuit zones on dense, mixed signal printed circuit boards.
Why Use Star Grounding?
Star grounding optimizes grounding performance by:
Lowering Inductance:
Shorter traces between components and a single localized star point minimize overall inductive loop area versus sharing long ground lines resulting in lower path inductance.
Containing Noise:
Converging return paths into an star point limits opportunity for shared noisy ground coupling by containing and channeling electrical fluctuations toward the star points instead of peripherally across the entire ground plane.
Improving Filtering:
The discrete inductances and capacitances formed between the star points and main ground provide passive filtering of noise as currents flow through these components.
Star grounding aims to lower inductance while isolating noise
Overall star grounding enhances signal integrity, especially critical for precision analog performance.
Design Considerations
Several key factors guide proper star ground implementation:
Zone Partitioning
Carefully grouping circuits functions into separate analog, digital, or mixed zones facilitates creating effective star grounds for the respective sections.
Star Point Locations
Central points within each zone avoid long return traces reducing inductance. Multiple points per zone can further minimize loop areas.
Return Path Routing
Use fat traces or polygons routed directly between each circuit and corresponding zone star point without extending across zones.
Single Point Interconnections
Tie zones star points together in single locations, often connecting analog zones to cleaner digital star points first before linking to the broader ground plane to avoid injected noise from spreading across domains.
Passive Filtering
Adding capacitors from star points to main ground aids filtering. Ferrites on long domain interconnects also suppress noise leakage.
Implementing Star Grounds
Best practices for executing star grounding include:
1. Partition Circuit Zones
Group analog, digital, and mixed signal functions into separate physical zones. Analog is most sensitive.
2. Place Star Points
Locate singular tie-in points central to each defined zone.
3. Route Return Paths
Use thick traces or polygon pours to route component returns directly to respective nearby star point.
4. Connect Star Points
Join star points together in single locations from cleaner domains progressively towards noisier zones.
This methodology yields low inductance isolated grounds.
PCB Layout Recommendations
Star grounding success depends on printed circuit board layout:
Distribute Ground Vias
Spread ground vias uniformly along the periphery of each zone to enable multiple interconnect locations and lower inductance to the shared ground plane underneath.
Careful Trace Routing
Avoid intertwining ground returns between zones. Interzone traces should optimally cross zones perpendicularly minimizing overlap.
Ground Plane Voids
Punch voids in the primary ground plane underneath analog zones to limit injected noise coupled through a common return path underneath devices. Some digital area voids also help force return currents into intended star points.
Robust Power Distribution
Clean power routing feeding zones prevents unwanted fluctuations passed through ground returns. Strategically placed local decoupling supports robust functionality.
Post Layout Verification
Perform signal integrity analysis and ensure ground noise voltages don’t exceed threshold requirements for proper circuit operation.
Real World Star Grounding Examples
Examining a high speed mixed signal printed circuit board shows star grounding best practices:
Design Goals
- 16 layer board with dedicated power planes
- High speed FPGA, DDR Memory, and PCIE zones dominated left side
- Right side mixed analog, audio, and intermediate frequency (IF) circuits
- Critical RF circuits housed centrally below FPGAs
Implementation
- Careful zonal partitioning on left and right board sides
- Localized star points placed within each zone type
- Short fat traces swiftly route returns to each zone’s star point
- Sparse interconnections tie star points from cleaner domains to noisier ones
- Extensive ground voids underneath analog zones
This combination enables the high performance mixed signal PCB containing sensitive RF, analog and noisy digital circuits in close proximity.
FAQs About Star Grounding
Q: Are star ground points connected to power planes?
No, star points only collect ground returns. Connections to power planes risks unwanted coupling. Separate power returns route to respective power pins or planes.
Q: Is a single star ground point used in small zones?
Yes, when zones host less circuitry, convergence to a single optimized star point is simplest. Larger zones benefit from multiple discrete points targeting device groupings.
Q: How are ground islands used with star grounding?
Raised ground islands underneath devices served by a star point provide contiguous return paths between components and the star point through vias minimizing inductance.
Q: Does adding capacitance impact filtering?
Additional capacitors from zone star points can improve filtering but necessitates considering series resonance. Capacitance between closely spaced stars enables charge transfer equalizing potential differences.
Q: What causes star grounding to be ineffective?
Failure to properly define circuit zones, overlapping space between zones, poor return path routing, inadequate board layer stack planning, and excessive interconnects between star points reduces noise isolation. Careful planning is vital.
Conclusion
In summary, star grounding requiring carefully routing ground returns from circuits to single localized nodes leverages inherent trace inductances to channel electrical fluctuations minimizing injection across sensitive zones. By partitioning mixed signal PCBs into analog, digital and mixed function domains with dedicated star points tied together progressively from cleaner to noisier sections, robust high frequency isolation can be achieved through simple, low-cost layout approaches.
Engineering teams specializing in dense, mixed signal printed circuit boards combining demanding RF, analog, and high speed digital circuits can leverage these proven star grounding best practices to fulfill complex performance requirements critical for cutting edge technologies.
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