Analog Layout Basics

Analog Layout — the manual art of physically arranging analog circuit components (transistors, resistors, capacitors) with extreme care for matching, noise isolation, and parasitic minimization.

Why Analog Layout Is Different

• Digital: Automated (PnR tools). Hundreds of millions of cells
• Analog: Mostly manual. Hundreds to thousands of devices. Each placement decision matters
• Analog circuits are sensitive to microvolt-level offsets and picofarad parasitics

Key Techniques

• Common-Centroid Layout: Interleave matched transistor pairs (ABBA pattern) so process gradients affect both equally. Critical for differential pairs and current mirrors
• Dummy devices: Place inactive dummy transistors at array edges to ensure uniform etch environment
• Guard rings: Surround sensitive analog blocks with substrate/well contacts to shield from noise
• Symmetry: Signal paths for differential circuits must be geometrically symmetric (equal wire lengths, equal parasitics)

Parasitic Awareness

• Wire resistance: Can cause IR drop errors in precision circuits
• Capacitance: Stray capacitance affects frequency response and stability
• Substrate coupling: Digital switching noise couples into analog through the shared substrate

Best Practices

• Separate analog and digital power supplies
• Deep N-well isolation for sensitive analog blocks
• Keep digital switching far from analog circuits

Analog layout is one of the few remaining areas where human expertise dominates over automation — a skilled analog layout engineer is worth their weight in gold.