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Save/Plot Operating Points

by Dr. Rishi Todani & Dr. Ashis Kumar Mal

By default, Cadence does not save the operating points of a schematic to keep simulation data small in size. For example, consider a single NMOS with Vgs and Vds as shown. If we need to plot some MOS parameter like gm, gds etc while performing dc sweep, then things become little difficult the usual way. We can however achieve this by forcing cadence to save all the operating points. The following steps may be undertaken to save/plot operating points during a DC sweep analysis.

Draw the schematic and start ADE (Analog Design Environment) window

In ADE, change the default waveform viewer. To do this, in ADE menu, click SESSION > OPTIONs

under waveform tool, select AWD. Click OK

IN ADE, setup the DC sweep analysis for which some MOS parameter is to be plotted. let us consider that we sweep Vds between the supply rails. The ADE Window may look something like this

 

 

 

 

 

 

 

 

 

 

 

 

We now create a model file which forces cadence to save the operating points. Let us consider that the instance name for the NMOS transistor is M0 (M zero). we create a simple *.scs file with the text

save M0:oppoint

we can save the file in any readable location and add the model file under the model library section at ADE WINDOW > SETUP > MODEL LIBRARIES

once the *.scs file is added, we perform netlist and run. When the simulation is over, we open the result browser by clicking ADE WINDOW > TOOLS > RESULT BROWSER

A small window with the title “browse project hierarchy” opens, simply click on OK

The result browser now opens. In this window click SCHEMATIC > PSF > RUN 1 > DC – DC > M0 

Here M0 is the instance name of the device whose operating point u wish to see or plot. Under M0, the parameters are found. If we right click on a parameter, say “gm” then its plotted versus the dc sweep variable. it is shown below

Thus the operating points for gm while performing dc sweep over vds is saved and plotted successfully. if we left click on gm in the result browser, its expression is taken into the calculator. From here, we can bring it to the outputs in ADE for frequent or future use.