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Measuring Self-Capacitance

Posted 18.02.2014, 13:56 GMT-5 Low-Frequency Electromagnetics, Mesh Version 3.5a 4 Replies

Chris Reilly
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I would like to determine the self capacitance of a single electrode on a PCB. I have successfully built the model and solved for the capacitance, but I'm not sure of my answer. The process I used is as follows:

1. Create 3D AC/DC->Electrostatics model
2. Use rectangles to create PCB and electrode
3. Create two bounding spheres (r=0.2mm and r=0.3mm) centered at 0,0,0 that encompass the PCB/electrode
4. Select both spheres and create a union
5. Select the sphere union and electrode and take the difference
6. Set the relative permittivity of the PCB (4.8)
7. Set the sphere union as an infinite boundary (spherical)
8. Edit the boundaries so that the infinite boundary is ground and the electrode is port with forced voltage
9. Mesh and refine once
10. Solve and use point analysis to determine capacitance

With a 3mmx3mm pad, I am getting a self capacitance of about 172fF, which makes sense. However, when I increase the spherical boundary size to double check my answer I am getting a different value. Is this simply due to the mesh size changing, or am I doing something wrong?

Is this the best way to measure self capacitance of a single electrode? Also, any tips for optimizing the mesh in this scenario? The computer I'm using can only handle one refinement after initializing the mesh before it runs out of memory.

Thanks,
Chris
4 Replies Last Post 24.02.2014, 12:41 GMT-5
Chris Reilly
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Posted: 1 decade ago 24.02.2014, 09:47 GMT-5
Anyone have any answers? It doesn't need to be version specific (3.5a), I'm more concerned whether I am using the correct process.
Anyone have any answers? It doesn't need to be version specific (3.5a), I'm more concerned whether I am using the correct process.
Edgar J. Kaiser Certified Consultant
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Posted: 1 decade ago 24.02.2014, 11:57 GMT-5
Chris,

what you call self-capacitance is a capacitancy of the electrode with respect to some concept of ground and the environment of the electrode. I rather call it stray capacitance.
So if you change the geometry this capacitance will change. Difficult to say if you are choosing the right concept. It depends a lot on the practical application behind the model.

Cheers
Edgar

--
Edgar J. Kaiser
emPhys Physical Technology
www.emphys.com
Chris, what you call self-capacitance is a capacitancy of the electrode with respect to some concept of ground and the environment of the electrode. I rather call it stray capacitance. So if you change the geometry this capacitance will change. Difficult to say if you are choosing the right concept. It depends a lot on the practical application behind the model. Cheers Edgar -- Edgar J. Kaiser emPhys Physical Technology http://www.emphys.com
Chris Reilly
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Posted: 1 decade ago 24.02.2014, 12:16 GMT-5
Edgar,

Thanks for the response.

A little background: I designing a copper sensor electrode on a PCB that will be used to detect the proximity of a conductor using a capacitance to digital (CDC) converter. The CDC I am using detects changes in self-capacitance. With that said, I would like to use Comsol to model the scenario and determine the self-capacitance of the electrode so that I can predict the change seen by the CDC converter when a conductor is present (eventually I hope to model the conductor near the electrode).

My original idea was to create some infinite boundary to measure the self-capacitance of the square electrode, but as you mentioned a change in boundary size changes the capacitance. How would you reccommend finding the self-capacitance of an electrode?

Thanks,
Chris
Edgar, Thanks for the response. A little background: I designing a copper sensor electrode on a PCB that will be used to detect the proximity of a conductor using a capacitance to digital (CDC) converter. The CDC I am using detects changes in self-capacitance. With that said, I would like to use Comsol to model the scenario and determine the self-capacitance of the electrode so that I can predict the change seen by the CDC converter when a conductor is present (eventually I hope to model the conductor near the electrode). My original idea was to create some infinite boundary to measure the self-capacitance of the square electrode, but as you mentioned a change in boundary size changes the capacitance. How would you reccommend finding the self-capacitance of an electrode? Thanks, Chris
Edgar J. Kaiser Certified Consultant
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Posted: 1 decade ago 24.02.2014, 12:41 GMT-5

Chris,

I would suggest to set up the model according to the measurement circuit. This circuit is probably using some ground concept, in most cases a housing or a backplane of a PCB. So the stray capacitance of your electrode is dominated by this ground environment, not by the infinite boundary.
This stray capacitance is then changed when some object gets close to the electrode.

Cheers
Edgar

--
Edgar J. Kaiser
emPhys Physical Technology
www.emphys.com
Chris, I would suggest to set up the model according to the measurement circuit. This circuit is probably using some ground concept, in most cases a housing or a backplane of a PCB. So the stray capacitance of your electrode is dominated by this ground environment, not by the infinite boundary. This stray capacitance is then changed when some object gets close to the electrode. Cheers Edgar -- Edgar J. Kaiser emPhys Physical Technology http://www.emphys.com

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