A symbolic simulator for analog circuits

A symbolic simulator, called ISSAC (interactive symbolic analysis of analog circuits), is described. It derives all AC characteristics for analog integrated circuits, both continuous-time and switched-capacitor, CMOS and bipolar. The program provides the designer with insight into the circuit behavior by allowing symbolic calculation of transfer functions, common-mode rejection ratio, power-supply rejection ratio, impedances, and noise. The program is based on a symbolic Equation solution that exploits sparse matrix techniques, followed by an heuristic approximation. It is written in Common LISP. The input is a file in SPICE or SWAP format. The output is a rational form in the symbolic circuit parameters and the complex frequency variables s or z. All filter elements are treated as symbolic parameters; the op amps are considered to be ideal. The symbolic filter function is obtained in 0.156 CPU seconds on a 15 MIPS (million instructions per second) LISP machine. ISAAC has been used extensively for the analysis of op amps, active filters, and switched-capacitor circuits.<>