Cadence PSPICE SCHEMATIC Instrukcja Użytkownika Pobierz pdf (Strona 23)

Output

1Vdc

10n

10k

Figure 30: Filter formed by a resistor and capacitor, fed by a constant voltage.

everything should have settled down to a steady value by the end of the simulation.

There’s one more step needed because we must specify the initial conditions for the capac-

itor: How much charge does it hold at the start of the simulation? Assume that we have left the

circuit for a long time before connecting the battery, so that the capacitor has discharged. We

must therefore set its initial charge to be zero. Select the capacitor and choose Edit > Proper-

ties. . . to open the Property Editor, shown in ﬁgure 32 on the following page. Enter zero for IC

(initial condition) and close the window.

Now run the simulation. You should get a plot in the usual way but with time along the

horizontal axis as in ﬁgure 33 on the next page. The curve may be polygonal (obviously made

of segments of straight lines) if there are too few points. In this case enter a suitable value in

the Maximum step size box of the Simulation Settings (ﬁgure 31) so that Spice calculate the

voltage more often. Usually Spice chooses an suitable interval automatically.

One way of ﬁnding the time-constant from a graph like this is to extrapolate the initial decay

Figure 31: Settings for a transient simulation.

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Cadence PSPICE SCHEMATIC Instrukcja Użytkownika Strona 23