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Capacitance-to-digital converter targets touch controls

Norwood, Mass. — Advanced sensing technology in Analog Devices, Inc.'s programmable 14-channel capacitance-to-digital converter (CDC) for handheld consumer applications is said to enable touch controls that are paper-thin yet highly reliable.

Using ADI's advanced capacitance sensor core, the AD7142 CDC has the capability to automatically adjust to rapidly changing climatic conditions, making it especially suitable for mobile applications. Other battery-powered applications include MP3 players, digital cameras, television remote controls, and gaming devices.

“Capacitance sensors, which enable finger-driven screen navigation, are fast replacing mechanical input devices to improve the look and feel of screen controls,” said Pat O'Doherty, ADI's product line director, Precision Signal Processing.

The 16-bit, 240-kHz AD7142 converter offers advantages over competing alternatives, including higher immunity to environmental factors such as temperature and humidity that would degrade performance in other capacitance sensors, O'Doherty said.

Full power mode is less than 1.0 milliamp, and shutdown current is less than 2.0 microamps. The CDC's power consumption is 50 percent lower than similar devices, O'Doherty said. The part also offers the capability to flexibly trade-off output rate and power. ADI's AD7142 is available in a choice of SPI or I(2)C compatible interfaces.

Other features include a 30-Hz update rate at maximum sequence length and better than one femto farad resolution. With 14 inputs, the AD7142 can be programmed for a variety of sensor configurations, such as finger-driven scroll bars, eight-way position sensors, and scroll wheels that drive pop-up menus, making it easier to browse through large files of music, pictures and video, O'Doherty said.

The AD7142 CDC is programmable, and includes adaptive threshold and sensitivity algorithms that allow the chip to adjust to the user's finger size, according to O'Doherty.

The CDC allows the end-user to adjust individual sensor sensitivity levels to suit their style and touch. The AD7142 has a unique automatic environmental compensation feature that performs calibration digitally on-chip, ensuring error-free sensor performance at all times, regardless of climate conditions, O'Doherty said. “The device performs this continuous calibration transparently to the user, so there will be no false touches or non-registered touches on the external sensors,” he said.

Unlike competitors' module offerings, the AD7142's single chip, IC-based solution provides a library of sensor sizes, allowing manufacturers to customize the shape of capacitance sensors, O'Doherty said. In addition, ADI offers sensor reference designs for a variety of end user applications.

 
 
See related block diagram

An evaluation board can be purchased for $199.00. The AD7142 is sampling now, with full production expected in February. The device is offered in a 32-lead 5 x 5 mm LFCSP and operates from a 2.7 V to 3.3 V supply. High-volume pricing is $1.09. Click here for the AD7142 data sheet.

Analog Devices , 1-800-262-5643, www.analog.com.

ADI's latest devices complement the CDCs and impedance-to-digital converters (IDCs) that the company rolled out in March. Those CDCs (AD7745, AD7746 and AD7747), and IDCs (AD5933 and AD5934) targeted high precision sensor designs in industrial, automotive, and medical applications.

ADI's CDC (AD7142), on the other hand, targets the consumer space. Even though all the CDCs are based on ADI's sigma-delta architecture, they are very different devices.

Click here for more details on ADI’s CDCs and IDCs that came out in March at eeProductCenter.

The AD7142 provides 14 inputs to implement a variety of sensors for consumer devices. Also, the AD7142 is programmable. The register programming capability of the AD7142 gives customers a great deal of capability for tuning their sensors, said Kevin O'Connell, ADI's marketing manager for Precision Signal Processing. “Fourteen inputs can translate to as much as 30 buttons or sensors — since the rows and columns of buttons can be matrixed into the inputs of the AD7142,” he explained.

The AD7142 CDC can interface to external capacitance sensors implementing functions such as capacitive buttons, scroll bars, or joypads. When used to implement a slider switch, which is common on digital cameras and cell phones, the user gets an eight-way switch for added functionality, instead of the typical four-way switch, O'Connell said. “The four-way version only moves up, down, right and left, whereas the eight-way version also moves in a 45-degree angle for faster more accurate movement, which would be especially useful when controlling a cursor or scrolling through a 1,000-song list in an MP3 player,” he said.

Although the primary applications for the AD7142 CDC are cell phones, digital cameras, and MP3 players, the CDCs may also be used in TVs, remotes, gaming devices and white goods (like dishwashers!), O'Connell. “The sensors can be placed behind plastic so there are no moving parts. Avoiding dirt, spills, etc. will make them more durable so joysticks won't break and keys won't stick on cell phones,” he said.

 
 
See related illustration

The illustration above shows the top transmitter (TX) layer in a printed-circuit board and the receiving (RX) layer on the bottom. Inside, the chip clock generates a 240-kHz square wave, which goes from the TX layer to the RX layer. Capacitance is set up between the two layers. As a grounded finger comes in proximity of the plastic, some capacitance fringe fields get shunted. This causes a change in current to the A/D converter or CDC. It measures the change in current due to the change in capacitance caused by finger motion.

Because the high-resolution (16-bit) sigma-delta core has good sensitivity, it can measure upwards of 6 mm above the sensor (4 to 5 mm above plastic). Typically, 2 mm is the sweet spot for plastic thickness, O'Connell said.

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