MSP430FR2633: Touch through thick glass

Hi Tyler,

You are correct about mutual in this case.  The e-field is tight as it bridges between the RX and TX electrodes, so propagating up through the 10mm of glass is not likely.  I will say 10mm is aggressive, even for self capacitance.  The behavior you will see with thick glass, due to its higher dielectric constant, is the e-field from the electrode will have a tendency to spread out as it propagates up through the glass.  Think of a flash light.  If you place sheet of paper in front a few inches away, you get a small circle of light.  Move the paper out a foot or two and now the circle is quite large.  So for a single electrode or button, this isn't a problem, however, this can cause cross talk if you have two or more buttons (electrodes) on your PCB that are close to each other.  By the time the e-fields from each button propagate through the glass, they overlap at the surface.  When you touch the glass over one button, you will see a substantially strong signal on the neighboring button.  Yes, you can address this by setting the touch thresholds accordingly, but I just want to make you aware of this behavior looking at your application.

Yes, for your application you are correct about decreasing the air gap between the glass and using springs is possible.  TI has an reference design  "Touch through glass with sharp LCD reference design"  that might be helpful.

So, with that said, let me ask why are you using 10mm glass? Can you share more about your application?

The product is intended for hazardous locations so the 10mm is defined by the explosion proof enclosure that the device will be installed in. It is almost the exact same enclosure used in the reference design. We are hoping for, more or less, a general purpose, small form-factor computer in a small explosion proof enclosure. The computer needs some mechanism for input. Generally, input is handled by a severely limited number of buttons(either capacitive through-glass as in ref design, or external, sealed, and "safe" mechanical buttons) but we are hoping to get an interaction model much more familiar to a user of a smart phone or tablet.

About crosstalk. It seems like crosstalk should be OK, even desired, to an extent. I would hope that the e-fields spread and overlap at the surface. so that I can use the relative strengths of signals between two electrodes to interpolate a touch that occurs in the overlapping area. Three electrodes could give a 2-dimensional map. Unless the fields simply do not work this way?

Thanks for the feedback.

Hi Tyler,

Yes, you should be able to achieve a simple X-Y coordinate system using a group of buttons, say in a 2x2 or 3x3 pattern, and as you suggest, interpolate the signals between channels to determine finger position.  I would be most curious to hear how well this works for your application.