Touch sensors allow for variations in parasitic capacitance
By Margery Conner, Technical Editor -- EDN, June 23, 2009
Capacitive-touch sensors can detect a finger touch by measuring capacitive changes on the touch pad. Currently, the most popular form of capacitive-touch sensors relies on a microprocessor and requires programming before use. SMSC’s new CAP10xx capacitive-sensor family takes an alternative hardware-based approach that uses dedicated registers. You set these registers during a three-step tuning process that determines the system sensitivity, sets the sensor threshold to zero for the “no-touch” value, and then verifies any change in values for no-touch states. This process takes place at power-on and can adapt to system changes, such as dirt buildup on switches and manufacturing variability, including changing parasitic capacitance, in PCBs (printed-circuit boards). The sensors’ tolerance of changing PCB parameters allows you to use the sensors with products from multiple vendors and manufacturing processes.
Although 2 kV is a common ESD (electrostatic-discharge) rating for capacitive switches, the CAP10xx family offers an 8-kV rating, which is an advantage in consumer electronics, such as LCD monitors. The chips also provide built-in noise filtering at the input pins, which wireless circuits or dc/dc-power regulators on the same board can cause. The chips in the family vary in options such as the number of sensor inputs, slider-input capability, and LED-driver circuitry. All chips have an I2C (inter-integrated-circuit)-bus interface; some provide a SPI (serial-peripheral interface) or a BC-Link interface. Chip packages range from a 10-pin, 3×3-mm DFN to a 32-pin, 5×5-mm QFN. Production prices range from 75 cents to $2 (10,000).
