eldo_user

Monte Carlo Analysis
Tolerance Setting Using DEV, DEVX or LOT
In the above example, both c1 and c2 use the model cmod. Their nominal values are both 10pF.
The dev declaration placed immediately after the cap keyword indicates that during a Monte
Carlo analysis, their values may vary independently of each other by at most ±10%. So, c1
could have a value of 9.9pF while c2 has the value of 10.1pF during a simulation run.
To use a Gaussian distribution, the DEV parameter is changed to DEV/GAUSS. Using the same
example, changing the distribution to Gaussian would alter the .MODEL card as follows:
c1 4 0 cmod 10p
c2 6 8 cmod 10p
.model cmod cap dev/gauss=10%
Both c1 and c2 may vary independently of each other with a Gaussian distribution, where the
standard deviation is 10 percent of the nominal value, 10pF.
The DEV tolerance is appropriate for situations where the variation of parameters is uncorrelated.
The devices on a printed circuit board are such an example.
The DEVX specification forces Eldo to use a new random value for each instance of a
subcircuit. The difference compared with DEV is that even if a parameter is used several times
in the same subcircuit, only one value will be used for that particular instance.
Note
DEVX can only be used in a .PARAM statement and not in a .MODEL statement. It is
impossible to have DEV and DEVX specified on the same parameter. If DEV and DEVX
are specified on the same parameter, the last specification will be retained.
The LOT tolerance setting causes devices which use the same .MODEL statement to vary
together with each other, as illustrated in the following example:
c1 4 0 cmod 10p
c2 6 8 cmod 10p
.model cmod cap lot=10%
This is the same as the previous example with the exception that the tolerance has been changed
from DEV to LOT. Now c1 and c2 will always have the same value. They may be both equal to
9.9pF during one run and 10.1pF during another run, but c1 will not have a value of 9.9pF
while c2 has the value of 10.1pF during the same run.
The LOT tolerance is appropriate for situations where there is a variation of the parameters
track. Devices in an integrated circuit are such examples.
When an instance parameter is used in the expression of a subcircuit parameter that varies with
LOT, one draw is made for all the parameter instantiations inside the subcircuit (LOT type) and
one draw is made for each instance of the subcircuit (DEVX type). In this particular case, the
LOT variation is equivalent to a DEVX variation. An example of this is provided below.
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Eldo® User's Manual, 15.3