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The medical industry needed an implantable pressure- sensing system that had to meet very harsh requirements. Its must-haves included ultra-miniature size, wireless operation, the lowest power consumption for battery operation, precision low drift and temperature stability, and isolation from media such as blood, tissue, and saline solutions.

Such were the very difficult challenges that Tronics Microsystems met with a two-chip microelectromechanical system (MEMS) solution. In fact, the customer’s requirements were so tough that no one could meet them until Tronics stepped in. The system was discussed at this year’s Sensors Expo (see “MEMS Motion Sensors Lead The Way At Sensors Expo 2008” at www.electronicdesign.com, ED Online 19242).

SENSOR, ASIC TEAM UP
The solution consisted of a capacitive pressure sensor that measures about 1 mm2 and a companion high-resolution, signal-conditioning ASIC that has a wireless interface (Fig. 1). According to Tronics’ Ariel Cao, U.S. director for business development, “This solution is the smallest form-factor design for implantation in the body and other medical devices like leads and catheters.” Both of the chips are attached to a header that’s part of a hermetically sealed biocompatible titanium cylinder (Fig. 2).

The packaged pressure sensor fits within medical French catheter sizes of 11 by 21 and operates over a typical pressure range of 500 to 1000 mm of mercury (Hg) (700 to 1300 mbars) with an accuracy of ±0.75 Hg (1 mbar). It features low drift of ±1 mm Hg/year (1.2 mbars/year), samples at 100-Hz rates, and operates from 35°C to 42°C.

The device’s sensing element consists of reliable singlecrystal membranes with a customizable range and form factor to fit into square medical structures as well as needles. Three different pressure ranges are available to cover 0 to 200 bars of pressure.

A PIVOTAL ASIC
Key to the design’s success, the ASIC chip includes a highresolution, 16-bit, sigma-delta modulator that directly converts capacitance changes to digital signals and provides compensation for nonlinearities and temperature changes (Fig. 3). Compensation calibration is performed with nine coefficients using matrix-based correction. This chip can also be customized to meet different form-factor requirements.

The ASIC features an integrated temperature gauge with 0.1°C resolution and 0.3°C accuracy from 0°C to 80°C. An EEPROM stores nonlinearity and temperature coefficients and produces 64 bits of unique ID information. The ASIC can be interfaced to standard I²C and serial-parallel interface (SPI) ports over a single wire.

The chip also includes the RF transceiver circuitry and power- supply management circuitry. RF data is transmitted using Manchester and one-out-of-four protocols via an ISO15693- based RF transponder interface.

To meet long-term biocompatibility requirements, the chip uses a titanium package. Incorporation of non-ferromagnetic materials helps with magnetic-resonant-imaging (MRI) procedure compliance. The packaged sensor is compatible with common sterilization procedures.

For further information, visit Tronics Microsystems’ Web site at www.tronics.com.