Analog circuit simulation usually means simulation of analog circuits or very detailed simulation of digital circuits. The most widely known and used circuit simulation program is simulation program with integrated circuit emphasis (SPICE). Before simulating, a circuit must be coded into a netlist. In SPICE, the circuits are represented by a system of ordinary differential equations. These equations are then solved using several different numerical techniques. For analog circuits, three methods of analysis are commonly used: DC, AC, and transient analysis. In circuit simulation, noise analysis, is an extension of AC analysis. Verilog-A is a new language designed for simulation of analog circuits at various levels. Mathematical equations can be entered directly as well as normal SPICE-type circuit elements.