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.
Figure 1. A self-capacitive touch sensing system measures the capacitance from a wire to earth ground. Your finger provides a parallel path and increases the effective capacitance.
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.
Figure 2. A mutual capacitive touch sensing system measures the capacitance between the x-axis and y-axis wires. It’ is more accurate for multi-touch but takes longer to do.
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).
Figure 3. A self- touch sensing IC integrated circuit connects to both the x-axis and y-axis wires on the display. These are clear wires formed from ITO (indium tin-oxide).