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Both dc and transient solutions are obtained by an iterative process which is terminated when both of the following conditions hold:
1) The nonlinear branch currents converge to within a tolerance of 0.1 percent or 1 picoamp (1.0E-12 Amp), whichever is larger.
2) The node voltages converge to within a tolerance of 0.1 percent or 1 microvolt (1.0E-6 Volt), whichever is larger.
Although the algorithm used in SPICE has been found to be very reliable, in some cases it will fail to converge to a solution. When this failure occurs, the program will print the node voltages at the last iteration and terminate the job. In such cases, the node voltages that are printed are not necessarily correct or even close to the correct solution.
Failure to converge in the dc analysis is usually due to an error in specifying circuit connections, element values, or model parameter values. Regenerative switching circuits or circuits with positive feedback probably will not converge in the dc analysis unless the OFF option is used for some of the devices in the feedback path, or the .NODESET card is used to force the circuit to converge to the desired state.