User Guide


Getting Started

Installing CapExt

To install or upgrade CapExt, download and run CapExt_(version_number)_setup.exe. CapExt will automatically notify you when an update is available.

Install license file

If CapExt could not detect a license file, or the previous license has expired, the license manager will be displayed on start-up. If you have a valid license or trial license, select "browse" and choose the file.

If you are already running CapExt under a valid license, but want to change the license file, for example to change user or to go from a trial license to a full license, with CapExt open, go to About->Open License Manager to open the license manager again.

Graphical overview of CapExt

Quickstart examples

CapExt comes with pre-built example projects with matching Gerber files which you can use to quickly familiarize you with how CapExt works, and how to extract the capacitances from your stackup in CapExt. The examples are located in the folder where you installed CapExt, typically C:\Program Files\CapExt\example project\

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CapExt projects

Create a new project

To start a new project, go to File->New Stackup. You will then be presented with the New Stackup Wizard.

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It is also possible to create a project without going through the New Stackup Wizard, see PCB Stackup Pane.

Each layer can be either a normal flat layer, or a drill/via layer. The checkbox next to each Gerber file can be toggled to switch between drill/via and normal layer. Drill/via layers are layers representing the vias between layers. If there is a via from a conductor on one layer to a conductor on another layer, the software will detect this and treat both conductors + any connected vias as a single large conductor.

The last column for each layer is the detected layer dimensions, in either inches or mm depending on the current unit setting. See Change units to metric(mm) or imperial(inches).

All settings in the New Stackup Wizard can be modified later in the PCB Stackup Pane.

Saving a project

To save a project go to File->Save or File->Save As.

Loading a project

To load a project go to File->Open.

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3D/2D view

Navigating in the 3D/2D view

To navigate through the 3D/2D view, simply drag with the left mouse button to drag the board around, scroll with the mouse-wheel to zoom in or out, and drag with the right button to rotate the view.

Navigating with the keyboard

You can zoom in and out with the W/S keys, and you can use the arrow keys to move the view.

Change from the 3D to 2D view

You can freely choose between the 3D view, which is good to verify that the stackup is correct, and the 2D view which is more similar to the familiar view in most PCB CAD software.

To switch between 3D and 2D, go to View->Toggle 3D/2D

Reset view

To reset the 3D/2D view to the standard zoom and position, press View->Reset 3D view.

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PCB Stackup Pane


Click to magnify.

Modifying a stackup

All modifications to the current stackup is done in the PCB Stackup Pane.

After modifying the stackup, press Apply Changes. This will update the 3D view of the stackup, and extract all the electrodes from the stackup.

Electrodes which are connected to electrodes on different layers through vias are treated as a single electrode.

Since pressing Apply Changes will change the configuration of electrodes and dielectrics on the board, all electrode names and any simulation results will be lost.

Changing board dimensions or DPI

The dimensions of the layout can be changed in 3 ways:

Adding or removing a layer

Pressing Add Layer will add a new layer. A layer is built up of a dielectric layer between two electric layers. The dielectric layer has a permittivity and a thickness, while the electric layers have not thickness or permittivity. The Gerber files set for the electric layers will act as the normal metallic print on a PCB, while the Gerber files for the dielectric layers are treated as drill files. If a hole connects to a metallic part on either electric layer, the hole will be treated as a 3D plated through via.

Pressing Delete Layer+Above will remove the currently selected layer and the layer above. Delete Layer+Below will remove the currently selected layer and the layer below. The reason for this is that a single dielectric layer must always be surrounded by two electric layers, and two electric layers must always have a dielectric inbetween. The electric and dielectric layers does not need to have associated Gerber files, and can be empty.

Moving a layer

Selecting any layer and pressing the up or down buttons will move the layer either up or down. When moving a layer, it will switch places with the next layer in the chosen direction so that every dielectric layer is always surrounded by two electric layers.

Changing layer properties

For dielectric layers, the thickness and permittivity can easily be changed by simply selecting the thickness or permittivity for the layer, and typing in a new value.

Adding or removing a Gerber file

To add or change a Gerber file for either an electric or dielectric layer, press the name of the layer, for example "layer 1, dielectric", press the browse button which appears to the far right when the name is selected, and select the new Gerber file.

To remove a Gerber file, right click on the layer, and select Clear layer contents.

Mirroring a layer

If a layer is mirrored, right click the layer and choose either Mirror layer around x-axis or Mirror layer around y-axis.

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Simulation Options Pane


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Selecting electrodes for simulation

All electrodes detected are given a number and are shown as a list in the Simulation Options Pane. All the electrodes will be a part of the simulation, irregardless if they are selected or not, but only electrodes selected for simulation will have its self capacitance and mutual capacitance to all other electrodes on the board calculated. Therefore, if you want to know the self capacitance of a conductor, or find potential mutual capacitance coupling which might inject noise to a conductor, it is important to select that conductor for simulation.

Any electrodes selected for simulation will be highlighted in green in the 3D view.

Simulation iterations/Simulation accuracy

The more simulation iterations run for each electrode, the more accurate the result will be. The simulation will continue until either the number of iterations has been met or until the calculated self capacitance error is below the set value.

The total number of simulations is an accumulated number, each electrode remembers how many iterations it has gone through, so if you run a million iterations for an electrode, then select a second electrode, and set the number of iterations to 1000, no iterations will be run for the first electrode, and it will not loose its accuracy, only the second electrode will run for 1000 iterations. If you later set the number of iterations to 2 000 000, the first electrode will be simulated for 1 000 000 iterations, while the second electrode will be simulated for 1 999 000 iterations.

Starting a simulation

To start a simulation, press Run Simulation.

Each electrode selected for simulation will be simulated in sequence. A progress-bar will be shown while the simulation is in progress. You can abort the simulation at any time while still keeping the results obtained so far. If you abort the simulation for an electrode, the next electrode in the queue will be simulated.

Renaming electrodes

To rename an electrode, for example to mark VCC/VDD/GND or specific pins or touch electrodes, either right click the electrode and select Rename or left click the electrode name and type in a new name.

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Simulation Results Pane


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For each electrode selected for simulation, there are two results:

If a result is displayed as 0, then the capacitive coupling is too small compared to the calculated error, and more simulation iterations must be performed to get a more accurate result.

Hovering over an electrode in the list will color the corresponding electrode in the 3D view dark blue. Hovering over a mutual capacitance result will color the electrode selected for simulation dark blue and the electrode to which the mutual capacitance results belongs will be colored light blue.

By selecting any capacitance on the list and expanding the list, you can press any electrode in the 3D view to find the corresponding result in the mutual capacitance list.

Sorting results

To sort results either by name or by capacitance, press Sort by name or Sort by capacitance. Both the list of self-capacitance and the mutual capacitances for each electrode will be sorted.


Due to the algorithm used by CapExt, it is possible to get a very accurate estimate of how far the current result is from the accurate physical result. In the results pane, this is represented as an error value. If the current estimated result is smaller than the error estimate, the error estimate is colored red to signify that more simulation iterations should be run if a more accurate result is needed.

The error displayed is displayed as a standard deviation, meaning that there is a 68% probability that the real result is within [result] ± [error], and there is a 95% probability that the result is within [result] ± 2x[error].

Export as CSV

To export the results as a csv file that can be imported into for example Microsoft® Excel, press the Export as csv button. All results available in the simulation results view with the corresponding error estimates will be exported.

What is coupling w/ infinity?

Even for a single conductor alone in an infinite empty space, there is some capacitance, see for example "Does an isolated charged sphere have capacitance?" at HyperPhysics. To model this coupling, a ground box surrounds the stackup in CapExt. The ground-box is extremely large compared to the stackup dimensions to model a ground at infinity. The coupling to this ground box is called "Coupling w/ infinity" in the simulations results.

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Touch Simulations


Capacitive touch is simulated with a floating (not connected to any other potentials) metallic tube as a model for the human finger. While conducting the simulation, CapExt calculates the capacitances with and without the finger probe present, and returns both capacitance matrices.

Configuring the touch probe

Activating and modifying the parameters of the touch probe is done through the PCB Stackup tab:

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Remember to press Apply Changes after reconfiguring the touch probe.

Interpreting the results

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In the results above, we can see that the mutual capacitive coupling between X2 and Y3 is ~2.2pF before the touch event, and ~1.9pF after, with an uncertainty of ~0.046pF.

When simulating capacitive touch, you are usually interested in the change in capacitance rather than the absolute value of capacitance. In order to ensure high quality of your results, you should check that the reported uncertainty is much less than the difference in capacitance due to the touch. If that is not the case, increasing the number of iterations in steps of 10x until the uncertainty is within acceptable limits is recommended.

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Change units to metric(mm) or imperial(inches)

The current unit setting can always be seen in the info-bar in the lower left corner of the application. To switch between Units in [mm] and Units in [inches], go to View->Units.

Show/Hide layers

Individual layers can be hidden by going to View->Show/Hide Layers. This will open a dialog where you can individually turn layers on or off. This is useful if an electrode is below another electrode, making it both hard to see and hard to select.

To show all layers again, select View->Show All Layers.

Batch simulations and scripting

CapExt has extensive batching and scripting functionality. Common use cases are:

The scripting interface is invoked by calling CapExt with a script as a parameter. For information on the different commands available, pass the parameter "--h" to capext from the command line.

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