Capacitive touch screens have become the human-machine interface (HMI) of choice. They’re easy to use, aesthetically pleasing and, thanks to smartphones and tablets, very familiar. They’re also durable and reliable since they can be protected by an “overlay,” usually a rigid layer of safety glass.
Capacitive sensing relies on the fact that humans are basically walking capacitors. Human touch introduces a capacitive change on the order of hundreds of femtofarads into a circuit. The “signal” is the change in capacitance measured from a baseline capacitance equal to the combined parasitic capacitances of various components of the circuit. The term sensitivity is often used to describe the signal strength—a more sensitive solution has a stronger signal. Sensitivity, which can be increased by decreasing the parasitic capacitance, directly affects resolution, defined as the smallest change in capacitance detectable by the system.
Noise, on the other hand, is something that does not change the capacitance but introduces a false count as a result of power supply switching, electrostatic discharge, electrically fast transients, radiated noise, or some other type of electrical noise that couples into the system.
Engineers seeking to implement capacitive touch need to evaluate the resolution and noise-filtering capabilities of various solutions. And, with the proliferation of battery-powered devices, power consumption has become a particularly critical selection criterion. Finally, engineers need to evaluate the design tools associated with a particular solution for ease of use.
The Texas Instruments (TI) solution is based on its proprietary CapTIvate™ touch technology, implemented as a peripheral on its MSP430™ family of low-power microcontrollers — integrating capacitive touch as a self-contained analog front-end with a full-featured FRAM MCU, allows developers to build true single-chip systems. And, recognizing that not all engineers are capacitive-sensing experts, TI has created the CapTIvate Design Center, for developing applications with an easy-to-use GUI.