Cadence Design Tools
Cadence Tools | ECEN 5837 (Mixed-Signal IC Design)
AMS C35 Inverter Example
Part II: Parametric Sweep and Corner Analysis
This tutorial builds on Part I to introduce parametric sweep and corner
analysis simulations.
1.
Complete
Part I to create the inv1 and inv1_sim Cells and setup the basic simulation. Remove the input node from the
simulation outputs (leaving the output node and supply current).
2.
Parametric
Sweep: This is useful for automating generation of multiple simulations to test
the effects of parameter variations on your design (e.g. device, supply, or
load variations) or to search out suitable solutions.
a. In the Simulator window,
select Tools à Parametric
Analysis ...
b.
Variable
Name: Wn
c.
Setup
the sweep as below, with Step Control: Linear Steps, Step Size: 1u
d.
Select
Analysis
à Start. This will run 5 separate simulations
and display the results in the waveform window. Click on the Switch Axis Mode icon to separate the current
and voltage waveforms as below:
3.
Corner
Analysis: The simulations so far have used the typical models from the AMS C35
device library. It is also necessary to analyze the performance of your design
under more realistic variations in the process. Corner analysis is a worst-case
approach, where you can simulate over multiple corners of process, power
supply, and temperature. To setup the inverter example for corner simulations:
a. Go to the main Cadence CIW
window (text window that opens when you start Cadence), select HIT-Kit utilities à Corner Analysis
b.
In
the pop-up window, enter into Circuit Name: inv1_sim
c.
Click
Add
Corner at
the bottom 5 times (for 5 process corners), fill out as shown below. Note:
temperature and supply variations can be simulated as well. We will ignore
these for this tutorial (but you are encouraged to come back and adjust these
values as well). The process corners stand for: worst-case speed (ws), power
(wp), one (wo), and zero (wz) and typical mean (tm). These are general
estimates on what is worst-case in many applications (very slow devices (ws),
very fast devices (wp, high power), etc.) for variations in threshold, k’,
oxide, etc. Note that these do not necessarily give you a true worst-case for
YOUR design and specifications, but do give a good indication. The Monte Carlo
statistical model gives better results if you perform sufficient simulations
with statistical variations of the device parameters modeled directly.
d.
When
the form is complete, click Save Template File, then OK. Then go back to the simulation window.
e.
Select:
Tools
--> Corners...
f.
The
corner analysis window should pop-up and already be filled out as below. You
can delete the row for Vsupply since we are not using it here.
g.
Click
on Run in the corner analysis
window. This will auto-generate simulations at each process corner selected
(similar to parameter sweep). The simulation result is shown below:
h.
For
additional information on using corner analysis in the AMS HIT-Kit, see: AMS Design Documents,
Design guidelines, Corner analysis with AMS HIT-Kit.
4.
You
can also perform statistical Monte Carlo simulations (rather than worst-case
corners). Again, see AMS Design Documents.