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MSP430 Capsense overlay material

Sai Jagini
Expert 1205 points
Other Parts Discussed in Thread: MSP430G2553

HI,

My customer is building an application on MSP430G2553 for touchpanel. He wants to use an acrylic Overlay material thickness 4~5mm (max). Please confirm whether it will work. What is the maximum limitation of overlay thickness or is there any thumb rule.

Please help us in selecting the PCB ( and its thickness) and overlay material (and its thickness).

  • 0
    TI__Mastermind 22835 points

    Hi Sai,

    The overlay material and thickness on touch applications have a huge impact on sensitivity of the touch sensor.  The Sensor Design Guide actually has an entire section devoted to different overlay materials, thickness, and the impact on the sensitivity of the sensor of each.

    Offhand, I can tell you 4-5mm is VERY thick for an overlay and will have a significant impact on the sensitivity.  Typically, the overlay is around 0.5 to 1 mm.

    Mike

  • 0
    Guru 227245 points

    Capacitance is pad size (the smaller one) divided by distance. Capacitance change is therefore influenced by distance change. The largest change happens when a short distance becomes even shorter. Going from 5mm distance to 3mm has much more effect than going from 10mm to 5mm.
    If a 5mm acrylic layer limits the minimum distance, this severely limits the capacitance change.

    In other words, if with 0.1mm coating (not an uncommon value), the variable part of the capacitance is assumed to be 1 higher than if not pressed, it will only be 0.02 if the minimum distance is 5mm.

    You can partly make up for this by keeping the base capacitance as low as possible (large distance to the next pad, and definitely no GND layer on the other side of the PCB etc.). But the best way to keep the touch sensors sensitive is to keep the distance as small as possible.
    Larger pads won’t help, unless the thumb is larger too :)

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