eldo_user

Setting Up An Analysis
DC Convergence Troubleshooting
.OPTION ITL1=200
• Use ramping and related options to change DC bias incrementally to full bias, for
example:
.OPTION GRAMP=9
• Add large shunt resistors and the keyword OFF to diodes, and so on, for example:
D5 5 6 DMOD OFF
RD5 5 6 100MEG
For large circuits, it is advisable to save the entire DCOP data so that DC analysis will have to
be done once and the results reused again and again as long as the design does not change.
Follow these steps to do this:
For an initial simulation (DCOP):
• Use the suggestions above until a successful convergence is achieved.
• Make sure you have the following commands in the simulation file:
.OP
.SAVE /filename.iic DC
For subsequent simulations (AC, Transient), DC analysis will be skipped thus saving valuable
CPU time. Do the following:
• Remove the commands .OP and .SAVE from the simulation file.
• Add a command to use the initial conditions captured for the whole circuit:
.USE /filename.iic IC
where IC is a keyword for Initial Conditions.
• Modify the .TRAN statement to make use of the initial conditions:
.TRAN tprint tstop [tstart [Hmax]] UIC
where UIC is a keyword for Use Initial Conditions. This makes a big difference
especially when DCOP data is available.
• Modify the .AC statement in a similar way, by adding UIC at the end.
These steps will solve most convergence problems without going into the guesswork of tuning
simulator options. For troublesome designs, more advanced steps will need to be taken.
When initial conditions are used (UIC parameter) Eldo performs a logical initialization prior to
the Transient run (that is, no DC analysis to be performed). Specify option NOINIT to disable
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Eldo® User's Manual, 15.3