Cell phones, multimedia players, digital cameras, and other rapidly shrinking mobile devices increasingly require new user interfaces to replace mechanical switches. Analog Devices’ new capacitance-to-digital converter (CDC) is a highly integrated touch controller that allows designers to quickly and easily add responsive user controls, such as scroll wheels and touch pads, to portable electronic devices.

 

As the newest addition to Analog Devices’ family of CapTouch controllers for consumer applications, the AD7147 provides highly accurate measurement and responsiveness, and features the addition of an active shield feature for improved noise performance. Requiring only a single PCB (printed-circuit board) for full implementation, the new CDC is ideal for ultra-thin mobile electronic devices.

 

 

ADI has a number of single- and dual-channel capacitance-to-digital converters, useful for numerous applications, but this is the third of their multichannel converters, following on the 14-channel AD7142 and 8-channel AD7143. Like the AD7142 there are two versions; the AD7147 has an SPI-compatible interface while the AD7142-1 has an I2C-compatible interface. The register map is also compatible with that of the AD7142.

 

The biggest difference with the new 13-channel AD7147 is the dramatic increase in the capacitance input range from ±2 pF to ±8 pF. ADI has also added a shield in the IC that greatly reduces the sensitivity to stray capacitance. This removes one of the very last hurdles to an excruciatingly thin keypad by allowing the electronics to be away from the sensors -- instead of actually being on a PCB under the sensors.

 

The remainder of the features and performance of the part seems to be the same as previous devices with a nominal 3.3 V rail and total power of about 3.6 mW with a low-pair mode of 25 µA and a full shutdown of 2 µA. On-chip algorithms compensate for finger size, temperature and humidity. The 16-bit delta-sigma ADC is driven by an input switch matrix. Typical times of between-sensor readings are typically 35 ms to 45 ms in most applications.

 

The shield in the AD7147 makes for implementation of thinner keypads, the increase in sensitivity makes for easier implementations of wheels and scroll bars. What's not to like?

 

The AD7147 and AD7147-1 are in production in LFCSP-24 with pricing at $1.30 in 1000-piece lots.