Combining Self- & Mutual-Capacitive Sensing for Distinct User Advantages

DN Staff

January 31, 2014

1 Min Read
Combining Self- & Mutual-Capacitive Sensing for Distinct User Advantages

Capacitive touch systems are clearly superior to resistive touch systems. Resistive touch systems break down and wear out due to their moving parts. The majority of resistive touch systems also can't effectively distinguish multi-touch interaction with a user. Legacy capacitive touch systems used self-capacitance sensing (Figure 1). They don't wear out, and they can support multi-touch gestures as long as you don't rotate your touch points or get them too close together.

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After the iPhone popularized pinch and rotate gestures in 2005, system designers have used mutual capacitive sensing to determine multiple touch points and gestures (Figure 2). The drawback of mutual capacitive sensing is that it takes longer to do the measurement and, hence, uses more power. If you use a dual-architecture chip that can do both schemes, you can provide both lower power and good multi-touch accuracy. While self-capacitive systems are less affected when there's a drop of water on the screen, mutual systems can be significantly affected by moisture. To get the best touch screen, you benefit from both sensing schemes.

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Legacy capacitive touch screens rely on self-capacitance sensing. Any wire in space will have a capacitive coupling to earth ground. In one instantiation a self-capacitance touch IC will dump a fixed charge on all the wires that run in the X-direction. That charge reacts against the capacitance to earth and creates a voltage. The touch chip will then measure that voltage. If your finger is touching the display, those wires will have an additional capacitive path to earth ground. Now the effective capacitance of that wire is increased, and the resultant voltage for the fixed charge injection on the wire will be less (Figure 3).

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