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Tip of the Week: Rapid transient setup for switched-cap circuits

When simulating switched-capacitor circuits (such as ADC subrange amplifiers), it often takes several clock cycles for the switched-capacitor circuitry to pump up to the proper common-mode operating voltage. This can cause simulation times to drag out considerably while you wait for the circuit to reach a usable operating point.

The solution is to run a "dummy" loose-tolerance transient sim (which will run much more rapidly), saving the final op point, then use this saved op point to preload your "real", conservative sim's. Then your sim's will start out on the very first (or second) clock step at the proper operating point, saving much time per simulation.

Step by step procedure as follows:

Set up your simulation as usual, but set the transient accuracy to "liberal".
Set the transient Stop Time to run enough clock cycles to let the common-mode voltages settle.
Run the sim and allow it to finish (so that it writes the final op points).
Prior to running the next sim, open the Transient Options form, go to the "Initial Condition Parameters", and fill in the "readic" field with the name of the final operating point file from the last sim, which is found in the "writefinal" field under "State File Parameters". (The default value in 5.1.41 is "spectre.fc".)
Click the "all" button under Initial Condition Parameters. OK the Transient Options form.
Reconfigure for "conservative" accuracy.
Run your conservative transient sim as usual. You will see that the simulation starts up very close to the correct settled operating condition.

Taking this even a step further, you can use different file names in the "writefinal" and "readic" fields to go along with your different simulation corners. For each corner, give the file a unique, descriptive name. Then as part of your Artist State setup for different corners, you can have the correct name filled in to the "readic" field, so that the corner-specific final operating point is read as your starting point. This will keep you from losing time when, for example, you change the supply voltage and the initial condition no longer is as accurate as before.

- Hugh

Originally posted in cdnusers.org by Hugh

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