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
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
- 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
- Click the "all"
button under Initial Condition Parameters. OK the Transient Options
- Reconfigure for "conservative"
- 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
HughOriginally posted in cdnusers.org by Hugh