Capacitive sensor signal-conditioning IC from ZMD delivers 14-bit resolution at low power

The ZMD31210 can be configured to interface with capacitive sensors from 2 to 260 pF with sensitivity as low as 125 atto-Farads (aF) per digital bit. The part accommodates both single and differential input sensor configurations. All calibration is digital, and completed in one pass; eliminating the cost of laser trimming and speeding production of fully-calibrated sensor modules.

Capacitive sensors are widely found in many MEMS sensing elements. Examples include pressure sensors for hydraulic control systems, humidity sensors and liquid level gauges. Capacitive sensors are often favored for their small size and lower power consumption over resistive bridge sensors.

Capacitive sensors do not touch or make direct contact with the system being sensed, which makes them a preferred option for industrial linear and angular position sensors and contactless potentiometers, where harsh or explosive environments make other sensor technologies impractical.

In close cooperation with key customers, we developed a digital technique to correct not only first-order, but second-order nonlinearity errors difficult or impossible to correct with a pure analog signal path”, says Thilo von Selchow, CEO at ZMD. Second-order correction is especially useful in humidity and pressure applications, but it can of course correct for sensor imperfections in almost any situation.”

The ZMD31210 connects to micro-controllers but can also be utilized in stand-alone designs for transducer and switch applications. The mixed signal design of cLite offers 14-bit capacitive-to-digital conversion and full 14-bit compensation of sensor offset, sensitivity and temperature via an internal digital signal processor running a correction algorithm. Calibration coefficients are stored in an on-board non-volatile EEPROM. The system interface offers I2C, SPI, PDM or alarm outputs.

The standard supply voltage is 2.7 to 5.5V. At this voltage, accuracy is 0.25% over the -20 to +85°C range, and 0.5% from -40 to +125°C. The ZMD31210 offers a 'Vcore' pin option for operation from 1.8V to 3.6V, with an accuracy of 1% from -20 to +85°C. Depending upon configuration, current consumption can be as low as 60μA.

The Sleep Mode feature reduces consumption to less than 1μA, making the device ideal for battery-based and other power-sensitive applications.

Programming and single-pass calibration of the capacitive sensor and the ZMD31210 is available through a standard PC environment using the ZMD31210KIT development tools. The package includes TSSOP14 samples, a development board, USB cable and calibration software.

The ZMD31210 is targeted for production in Q4/2009. Samples and kits are available. The devices are available in TSSOP14 packages.