UltraCAD Design, Inc

Bypass Capacitance Impedance Calculator


Special note for Win7 x64 users. See bottom of page.

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, quickly 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.

If we want to evaluate our circuits from this standpoint, the formulas, for any practical problem, are formidable, 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 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.

The freeware version of the calculator allows limited functionality so you can get a feeling for how it performs. The license to use all of its functionality is $75.00 USD.

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.

For support and troubleshooting issues see the bottom of this page.

Some screen shots of the calculator follow:


  A user of the calculator was kind enough to send me his data as a case study. His data was programmed into the calculator using the custom data input feature. The primary front screen of the calculator after it has run is shown here.  

  The graphical output of the impedance function is shown here. It met the user's requirements and he was 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.  
Depending on your system, you may get some error messages that some files are missing. The Calculator is written in Visual Basic, version 6. Right-click, download, and execute the Visual Basic 6 runtime library here.
If you get a message that you need the file comdlg32.ocx, right-click and download the file from here and put it in the Windows/System32 folder.

For Win7 x64 users:  A user has notified us of a special procedure for getting comdlg32.ocx to work with Win7 x64. Here is the procedure:
1. Copy comdlg32.ocx to the c:\Windows\SysWOW64\ folder
2. Open the command prompt as Administrator
    a. Start/Programs/Accessories/Command Prompt
    b. Right-click and select "Run as Administrator"
3. Navigate to c:\Windows\SysWOW64
4. Enter   "regsvr32.exe comdlg32.ocx" (no parentheses but with a space between)     and then return
5. You should receive a message that comdlg32.ocx has been registered successfully.


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