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Capacitance-to-digital converter eases touch-based switch implementation

Smoothing the design-in for consumer products with reliable, low-cost, no-moving-part switches, the AD7142 capacitance-to-digital converter (CDC) from Analog Devices provides 14 channels and 16 bits of resolution. The IC is designed to compensate and be relatively immune to changes in temperature and humidity, while also consuming little power, making it well-suited for consumer products as well as industrial and automotive applications. The mulltichannel design handles multiple sensor inputs, such as in scroll wheels, scroll bars, multipoint position spots. The user's processor implements the validation algorithm, for which the vendor provides application support, to work with the 30-Hz update rate across the 14 individually programmable inputs.


The CDC has user-programmable parameters, as well as adaptive threshold and sensitivity to adjust to finger size and local conditions, along with continuous, transparent self-calibration, to minimize both false touch and missed touches. Communication between the 32-lead, 5×5 mm lead-frame chip-scale package IC is via an SPI or I2 C interface, depending on which version of the IC is selected. Full-power current for the 2.7 to 3.3 V IC is 1.0 mA; shutdown current is under 2 μA. The complete bill of materials for this $1.09 (OEM volume) device requires just two non-critical capacitors, plus some PCB area; a typical “button” switch is 8 mm diameter but can be as small as 5 mm. The data sheet from Analog Devices, (http://www.analog.com) is available at www.analog.com/AD7142.

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