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.