User`s guide

4 DC Analyses

To calculate the DC response of an analog circuit, PSpice A/D

removes time from the circuit. This is done by treating all

capacitors as open circuits, all inductors as shorts, and using

only the DC values of voltage and current sources. A similar

approach is used for digital devices: all propagation delays are

set to zero, and all stimulus generators are set to their time-zero

values.

In order to solve the circuit equations, PSpice A/D uses an

iterative algorithm. For analog devices, the equations are

continuous, and for digital devices, the equations are Boolean. If

PSpice A/D cannot get a self-consistent result after a certain

number of iterations, the analog/digital devices are forced to the

X value, and more iterations are done. Since X as input to a

digital component gives X as output, the Boolean equations can

always be solved this way.

If a digital node cannot be driven by known values during the

DC iterations (for instance, the output of a flip-flop with the

clock line held low), then its DC state will be X. Depending on

the circuit, some, none, or all of the digital nodes may have the

state X when the bias point is calculated.