Analog/Mixed-Signal (AMS) Simulation encompasses the integrated simulation of analog circuits (SPICE-level) and digital logic (Verilog/SystemVerilog), essential for verifying data converters, PLLs, charge pumps, and SoCs with substantial analog content.
SPICE Simulation and Variants
- SPICE (Simulation Program with Integrated Circuit Emphasis): Industry-standard circuit simulator solving nonlinear differential equations (Kirchhoff's laws) iteratively using Newton-Raphson.
- Spectre/Spectre RF (Cadence): Advanced SPICE with improved convergence algorithms and noise analysis. Spectre RF adds periodic steady-state (PSS) and periodic AC (PAC) analysis.
- HSPICE (Synopsys): Fast SPICE variant with enhanced models and better convergence for deep-submicron processes. Includes statistical modeling (Monte Carlo).
- Fast SPICE (XSPICE/FSPICE): Acceleration techniques (lookup tables, macro-models) reduce simulation time 10-100x vs standard SPICE. Accuracy trade-off acceptable for estimation.
Verilog-A Behavioral Models
- Verilog-A Language: IEEE 1364.1 standard for analog behavioral modeling. Describes analog systems as differential equations and algebraic relationships.
- Module Definition: Analog module specifies ports (electrical, real-valued), parameters (device characteristics), and statements (branches, voltages, currents).
- Abstraction Levels: Behavioral Verilog-A models abstract detailed physics (gate-level transistor interactions). Enables rapid simulation without transistor-level detail.
- Model Examples: Op-amp models (input impedance, gain, frequency response), filter models (transfer functions), ADC models (quantization, offset, noise).
Fast SPICE and Co-Simulation Approaches
- Co-Simulation Architecture: Verilog/SystemVerilog simulator (Incisive, VCS, Questa) exchanges signal values with SPICE simulator (Spectre, Cadence AMS, Synopsys AMS Designer) at synchronization points.
- Communication Protocol: Simulators connected via socket/shared memory. Signal updates synchronized at specified time intervals (digital clock edges or SPICE time-step granularity).
- Partitioning Strategy: Digital logic (testbench, control) in Verilog, analog circuits (ADC, DAC, PLL) in Spectre. Interface logic (digital-to-analog, analog-to-digital) bridged by behavioral models.
- Simulation Speed: Typical AMS co-simulation 1000x slower than pure digital simulation. Requires careful partitioning to minimize analog simulation load.
Stimulus Generation and Test Methodology
- Testbench Architecture: Verilog testbench generates digital stimulus and clock signals. Stimulus might include ramp voltages, temperature sweeps, noise sources.
- Behavioral Models for Stimuli: Gaussian noise sources, sinusoid generators, ramp functions defined in Verilog-A. Parameters controlled via Verilog test vectors.
- Multiple Domain Stimulus: Analog input stimulus (ADC input voltage), digital control signals (mode select), power supplies (nominal, PVT variation).
Convergence Challenges and Solutions
- Stiff Equations: Some SPICE circuits exhibit widely varying time constants (fast switching + slow integration). Stiff systems difficult for standard numerical solvers.
- Convergence Aids: Time-step control (reduce timestep near discontinuities), initial transient solution, gmin stepping (gradually reduce parasitic conductances).
- Simulation Timeout: Badly converged circuits may run indefinitely. Timeout limits (hours typical) prevent runaway simulations. Identifies problematic netlist regions.
- Co-Sim Synchronization Issues: Mismatched time-steps between Verilog (fast, digital clock) and SPICE (slow, analog detail) cause synchronization errors. Careful scheduling avoids race conditions.
Model Accuracy vs Simulation Speed
- Accuracy Hierarchy: Transistor-level SPICE (highest accuracy, slowest), compact models (BSIM), behavioral Verilog-A (fastest, lowest accuracy).
- PVT Simulation Corners: Process variation (fast/slow corners), temperature range (0-85°C typical), supply variation (±5-10%) all modeled. Multiple corners require separate simulations.
- Statistical Simulation (Monte Carlo): Random process variation sampled (1000-10000 runs). Computes distribution of performance metrics (offset, gain) across population.
Sign-Off Simulation and Qualification
- Pre-Silicon Validation: AMS simulation validates ADC DNL (differential nonlinearity), analog gain, settling behavior before fabrication.
- Post-Silicon Correlation: Measurements on silicon correlated with simulation models. Mismatch indicates model inaccuracy, device physics not captured.
- Production Testing: Sign-off simulations define test limits. Fabricated chips tested against limits derived from corner simulations with guard-banding.