UltraCAD Design, Inc

Power Distribution System (PDS) Impedance Calculator


We have understood for decades that power supply systems are quiet when we effectively decouple points in the circuit where there are large, sudden changes in current requirements. This leads to the idea that bypass capacitors, strategically placed, provide the best results. And this, then, logically leads to the conclusion that more capacitors is better!

There is growing interest in the idea that there might be a better, or at least other, way to view the problem. That idea is to design a power conditioning system with an impedance (to ground) function (curve) that is acceptably low at ALL (relevant) frequencies. This leads to the idea that there might be an optimum number of capacitors (rather than maximum) and that their placement might not be of primary importance. Doug Brooks explored this idea and produced his landmark paper in early 2000. The conclusions were startling, at first, but are now well accepted. You can read about that paper here. We have recently written another on Power Distribution Impedance that you can read here.

If we want to evaluate our circuits from this standpoint, the formulas, for any practical problem, are formidable, in fact virtually impossible. Besides, what we all really want is a visual graph of the impedance function. Doug Brooks designed UltraCAD's Bypass Capacitance Impedance Calculator to do this in support of his own research into this concept. The calculator has recently been significantly updated to be much more user friendly and functional. It is now available for anyone to purchase. It is especially helpful in locating and displaying anti-resonant peaks in the impedance function, and for illustrating the unusual and unexpected effects of what happens when a large number of "real world" capacitors are placed in parallel. You can download the calculator here.

It requires a license for use; the license costs $75.00 USD. (There is no longer any freeware functionality to the new version.) If you acquire a license, a license file, named "ucadpds.dat", will be e-mailed to you. This file must be placed in the same folder as the executable.

You can read about the calculator and see a mini operator's manual here. (pdf file)

Information on how to obtain a license is found here.

Some screen shots of the calculator follow:


  Two users of the calculator were kind enough to send us their data as case studies. their data were programmed into the calculator using the custom data input feature.  

  The graphical output of the impedance functions are shown here. It met the users' requirements and they were happy with the results. The results of the analysis were also written out to a data file.  


  The calculator also has a mode where the capacitor (input) values are internally generated. This allows convenient investigation of the general issues of number of caps, values of caps, values if ESR, etc., without having to create an endless number of input data files.  

  This sample output shows the combined effect of two different runs, so that the results can be quickly compared.  
The program has been rewritten in VB.net and should run on all Windows operating systems. All necessary files (except, of course, the license file) are included in the download. See the Help or Operator's Manual for installation guidelines.

Click on logo to return to UltraCAD's Calculator page.