DC Analysis (dc)

Description

DC analysis finds the DC operating-point or DC transfer curves of the circuit. To generate transfer curves, specify a parameter and a sweep range. The swept parameter can be circuit temperature, a device instance parameter, a device model parameter, a netlist parameter, or a subcircuit parameter for a particular subcircuit instance. You can:

sweep the circuit temperature by giving the parameter name as param=temp without a dev, mod or sub parameter.

sweep a top-level netlist parameter by giving the parameter name without a dev, mod or sub parameter.

sweep a subcircuit parameter for a particular subcircuit instance by specifying the subcircuit instance name with the sub parameter, and the subcircuit parameter name with the param parameter.

After the analysis is complete, the modified parameter returns to its original value.

Syntax

Name dc parameter=value ...

Parameters

Sweep interval parameters

Sweep variable parameters

State-file parameters

Output parameters

Convergence parameters

Annotation parameters

Emir output parameters

You can define sweep limits by specifying the end points or the center value and span of the sweep. Steps can be linear or logarithmic, and you can specify the number of steps or the size of each step. In addition, you can specify a step size parameter (step, lin, log, or dec) and determine whether the sweep is linear or logarithmic. If you do not specify a step size parameter, the sweep is linear when the ratio of stop to start values is less than 10 and logarithmic when this ratio is 10 or greater. If you specify the oppoint parameter, Spectre computes and prints the linearized model for each nonlinear component.

Nodesets help find the DC or the initial transient solution. You can specify nodesets in the circuit description file with nodeset statements or in a separate file by using the readns parameter. When nodesets are specified, Spectre computes an initial guess of the solution by performing DC analysis, while forcing the specified values on to nodes by using a voltage source in series with a resistor whose resistance is rforce. Spectre then removes these voltage sources and resistors and computes the required solution from this initial guess.

Nodesets have two important uses. First, if a circuit has two or more solutions, nodesets can bias the simulator towards computing the required solution. Second, this is a convergence aid. By estimating the solution of the largest possible number of nodes, you might be able to eliminate a convergence problem or significantly speed up convergence.

When you simulate the same circuit multiple times, it is recommended that you use both write and readns parameters and assign the same file name to both parameters. DC analysis then converges quickly even if the circuit has changed since the last simulation, and the nodeset file is automatically updated.

You can set the 'force' parameter and specify the values to force the DC analysis. The values used to force signals are specified by using the force file, the ic statement, or the ic parameter on the capacitors and inductors. The force parameter controls the interaction of various methods of setting the force values. The effects of individual settings are as follows:

force=none: All initial conditions are ignored.

force=node: The ic statements are used, and the ic parameter on the capacitors and inductors are ignored.

force=dev: The ic parameters on the capacitors and inductors are used, and the ic statements are ignored.

force=all: Both the ic statements and the ic parameters are used, with the ic parameters overriding the ic statements.

If you specify a force file with the readforce parameter, force values read from the file are used, and any ic statements are ignored.

After you specify the force conditions, Spectre performs DC analysis with the specified nodes forced to the given value by using a voltage source in series with a resistor whose resistance is rforce (see options).