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readforce does not work in DC simulation

Qilong

Hi all, 

    Recently I am trying to play with the readforce in DC simulation. My test circuit is a series RC network (R=1 Ohm, C=1pf both from analogLib), with the two terminals connected to gnd. No sources are used. Then I specified the voltage value on the floating plate of the capacitor in a file (with only one line in it: net_flaoting 1). Then I use readforce to read this file in DC simulation, with any other parameters like force, readns empty.  

    From the descriptions in the spectre simulator user guide and the reference, I am expecting a voltage of 0.5V on the floating plate of the cap after the DC run (I used rforce=1 Ohm). However only 0V is shown. On the other hand, if I specified initial state in the cap and use force=dev, the desired 0.5V will be calculated. Comparing this two cases, could I ask why the simulator does not use the statement I wrote in the file? 

    PS: As a crosscheck, I used the same file in a transient simulation with the same network by setting readic=filename.  Then I can see the initial state is calculated quite well. The floating node starts to settle down from the initial voltage of 0.5V (again rforece=1 Ohm), which seems the file is recognisable by spectre.   

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  • I assume you didn't set force=node (or force=all) on the dc analysis. If you had, this would have worked. The reason is that the default behaviour is that forces (or initial conditions) are ignored for a DC analysis, because they can lead to false results in a subsequent small-signal analysis. With transient, there's often a special reason for wanting a force/ic (e.g. to move away from some metastable state), and the transient has the opportunity to move way from any unrealistic force/ic that you've applied. So the default for the transient ic parameter is all, whereas the default for the dc force is none.

    I tried your circuit with force=node and readforce="file.in" and it ended up with 0.5V on net_floating.

    Regards,

    Andrew.

Reply

  • I assume you didn't set force=node (or force=all) on the dc analysis. If you had, this would have worked. The reason is that the default behaviour is that forces (or initial conditions) are ignored for a DC analysis, because they can lead to false results in a subsequent small-signal analysis. With transient, there's often a special reason for wanting a force/ic (e.g. to move away from some metastable state), and the transient has the opportunity to move way from any unrealistic force/ic that you've applied. So the default for the transient ic parameter is all, whereas the default for the dc force is none.

    I tried your circuit with force=node and readforce="file.in" and it ended up with 0.5V on net_floating.

    Regards,

    Andrew.

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