Electronic Design

Capacitance-To-Digital Converter Simplifies Touchpad Design

When it comes to touchpads, most OEMs buy complete subassemblies from outside vendors, essentially subcontracting the electronic design as well as the assembly. The AD7142 capacitance-to-digital converter from Analog Devices may change that, allowing OEMs to reduce bill-of-materials costs while absorbing a small increase in nonrecurring-engineering costs.

Housed in a 5- by 5-mm lead-frame chip-scale package, this 14-channel, 16-bit chip fits touch-pad input devices. Design support for the chip makes it simple for OEMs to develop their own input panels, purchasing only the physical touchpad from their suppliers.

ADI provides application notes and sensor reference designs for a variety of end-user applications for the AD7142. These reference designs can operate with optional host software that enables high-precision sensing capability. ADI also offers a library of sensor sizes, so manufacturers can customize the shape of capacitance sensors to differentiate their products.

In a capacitive touchpad design, sensors consist of electrodes on a two-or four-layer pc board. They can be arranged as buttons, scroll bars, or joypads or as a combination of sensor types. On-chip registers are programmed to control averaging, offsets, and gains for each sensors. An on-chip sequencer controls polling. Capacitance sensing involves an excitation source, a transmitter, and a receiver (see the figure). When a finger or other grounded object interferes with the electric field, some of the field lines are shunted to ground, and the total capacitance measured at the receiver decreases. A delta-sigma converter translates the field lines measured at the receiver into the digital domain.

Preprogrammed threshold levels determine when any detected change in capacitance is due to a button actually being activated. The same thresholds principle determines if other types of sensors, such as sliders or joypads, are activated. The AD7142's environmental compensation eliminates false touches or nonregistered touches. Whenever the approach of a user's finger is sensed, internal calibration is disabled, and the chip is configured to detect a valid contact.

In full-power mode, the chip draws less than 1.0 mA, and shutdown current is less than 2.0 µA. The AD7142 also can flexibly trade off output rate and power.

In production volumes, the AD7142 costs $1.09. An evaluation board costs $199. Versions with I2C-and SPI-compatible interfaces are available.

Analog Devices

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