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Guest

Mon Oct 17, 2005 1:06 pm   



Hey,
I simulated a circuit in Hspice such that the rise time was within
10ns. I designed the layout of the circuit in Magic and extracted to
spice and again simulated in HSpice. There is a difference in the
result of the two simulations. Should there be a diff in the first
place, if yes why should there be a diff?

Thanks,

Haran

PS: In both the cases I added an external load capacitance of 1fF.

Gerry Vandevalk
Guest

Tue Oct 18, 2005 8:20 pm   



What do you mean by different? (i.e. How close?)

Have you exactly created the same circuit. (or close ?)
Are the netlists identical? If not, then the results will be different.

Is the model information of the devices filled in to the same extent?
( i.e. Source & Drain Areas and perimeters, Resistor end effects, etc.

As Hspice (or any analog simulator) is essentially solving a sequence of
equations to an acceptable tolerance using (many) floating point
calculations,
the exact answer arrived at will be (possibly) slightly different with each
difference in the netlist.

It is possible to get (slightly) different answers by just re-ordering the
identical netlist as the tool
does not have to converge to exactly the same value on startup and still
meet the default (or your own)
convergence criterion.

Now, having said all of that, most circuits should approx. simulate the
same, when not exposing the simulators
to ill conditioned inputs. ( Try simulating a series of ideal diodes and
watch it give the simulator fits ... )

-- Gerry




<nkharan_at_gmail.com> wrote in message
news:1129558018.350759.176280_at_g43g2000cwa.googlegroups.com...
Quote:
Hey,
I simulated a circuit in Hspice such that the rise time was within
10ns. I designed the layout of the circuit in Magic and extracted to
spice and again simulated in HSpice. There is a difference in the
result of the two simulations. Should there be a diff in the first
place, if yes why should there be a diff?

Thanks,

Haran

PS: In both the cases I added an external load capacitance of 1fF.


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