Speaking of variation -- and isn't everyone these days -- something strikes me in reading about all the powerful and elegant features of corners management and statistical analysis. After all the simulations are run and the results are presented, unless you've managed to hit a bullseye on the first design you tried (good luck with that), you're probably going to have to change something in the circuit in order to turn all those lights green. (Of course, you'll only see that lovely green coloring --or the not-so-desirable yellow and red coloring--if you're using specifications in ADE XL).
We'll talk in a later article about the features that will help you figure out what circuit values to change. Today I want to focus on how to change them.
You could use the brute force method -- select devices on the schematic, edit their values, save the schematic, then resimulate. It's fairly tedious and you have no record of what combinations you've tried. I'll bet you're jotting down notes on little pieces of paper to keep track, aren't you?
Or you could get a bit fancier and edit the schematic to assign design variables to critical device properties. That allows you to change those variable values from within ADE XL using all its different analysis types. A very useful approach, but you still have to edit the schematic to set everything up and then edit it again once you've settled on the correct device values.
Which brings me to the subject of this article -- an Extremely Useful Feature in ADE XL which is also, sadly, one of the least known. ADE XL allows you to change the values of device instance parameters without having to edit the schematic. You can then easily run sweeps and corner simulations, do sensitivity analysis and circuit optimization without touching your schematics. And ADE XL's simulation history enables you to keep track of what experiments you've tried and what the results were. And yes, it takes care of triggering the callbacks at the appropriate time so the netlist is accurate.
Let's take a look at how to do this...
Parameters are created using the Variables and Parameters Assistant (VPA). Because the parameters are associated with device instance properties, you need open the VPA from a schematic. Start with an ADE XL setup, then use either File->Open... or RMB->Open Design in Tab on the test name in the Data View Assistant (and possibly descend the hierarchy) to get to the schematic containing the devices whose values you want to vary. Now open the Variables and Parameters Assistant.
Devices you select in the schematic will show up in the top half of the VPA with their editable CDF properties. Select one or more of these properties -- for example, length and finger width -- and RMB->Create Parameter. At the bottom of the VPA (and in the Parameters section of the Data View Assistant), you'll see something that looks like "M7/l" and "M7/fw". If you hover over them, you can see that it keeps track of the library/cellname/view of the schematic they come from (in case you parameterize "M7" in multiple blocks). These can now be treated just like any other variables.
You can change their values. You can assign them a list of values (1u,2u,3u) or a range of values (1u:1u:10u) to perform a sweep. You can access them in the Corners UI to change their values in corners simulations. You can use them in Sensitivity Analysis to better understand their impact on each circuit specification. You can use Global or Local Optimization to vary the values in order to find a design that meets all your specs across corners.
The VPA also has 2 buttons at the top that let you create matching and ratio relationships between parameters. Simply select the devices in the schematic, expand one of them in the top part of the VPA, select the properties you want to be matched amongst the devices, and click the "Match Parameters" button at the top (sort of looks like 2 blue boxes with a red equals sign between them). In the bottom section of the VPA, you will see that one device will have a value (e.g. M7/fw = 2u) and the other(s) will be matched to it (e.g. M6/fw = M7/fw@ ). Now you only have to vary the value on one device and the others will vary with it.
Similarly, the "Ratio Matched Parameters" button (looks like "1:n") will create a ratioed relationship based on the lowest value.
Once you start using these handy parameterization techniques, you can quickly find yourself awash in results from all your "what-if" simulations. Next time, I'll talk about some convenient ways of managing all these different sets of parameter values.
For now, we'll skip ahead to the ultimate goal of this whole exercise -- you've found a circuit design that meets all your specifications across all the required corner and statistical conditions. Hooray! Now, it's time to edit your schematic to capture those golden device values. But you don't have to manually edit the properties of each transistor to do that. Just look at the ADE XL Result panel and find those good simulation results (it's easy--they're the ones that are all green!). Now RMB->click on the grey line just above those results and select Backannotate. The values will be written back to the appropriate devices on the schematic, including any matching or ratioing relationships you had made, and triggering any relevant callbacks.
For More Information
Details about how to create and work with device instance parameterization can be found in the ADE XL User Guide, the ADE XL Quick Start and this video.