MEMSdesign2 - A Generic Interface Circuit for Capacitive...

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A Generic Interface Circuit for Capacitive Sensors in Low-Power Multi-Parameter Microsystems Navid Yazdi * , Andrew Mason, Khalil Najafi, and Kensall D. Wise Center for Integrated Microsystems University of Michigan Ann Arbor, MI 48109-2122 ABSTRACT This paper presents a generic low-power sensor interface circuit compatible with smart microsystems and a wide range of capacitive transducers. The interface chip is highly programmable, can communicate with an external microcontroller using a nine- line sensor bus standard, contains a switched-capacitor readout circuit, supports sensor self-test, and includes a temperature sensor. The circuit can interface with up to six external sensors and contains three internal programmable reference capacitors in the range of 0.15-8pF. The chip measures 3.2mm x 3.2mm in a standard 3μm single-metal double-poly p-well process, dissipates less than 2.2mW from a single 5V supply, and can resolve input capacitance variations of less than 1fF in 10Hz bandwidth. * Currently with Electrical Engineering Dept., Arizona State University, Tempe, AZ 85287.
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I. I NTRODUCTION As the Micro-Electro-Mechanical Systems (MEMS) industry continues to experience accelerating growth, there has been an obvious trend toward combining MEMS transducers with increasingly sophisticated circuits. One step in this evolution is to integrate circuitry and transducers to form “smart” sensors, yet the low cost and wide availability of present signal processing electronics make it possible to go one step further and form entire smart microsystems. These microsystems, which combine multiple microsensors and/or microactuators with signal processing circuitry on a common substrate, are capable of gathering electronic data from the physical world, processing and acting on the information, and transferring the data to other electronic systems which gain intelligence from this process [1]. It is precisely these capabilities that ensure microsystems will have a pervasive impact on the future of the microelectronics industry in application fields ranging from automotive systems to health care. A significant subset of such microsystems will be small, portable, autonomous units [2,3] that can be distributed to collect data over a long period of time. Because these portable microsystems will be battery powered, one of their key constraints is the overall power consumption, which must be minimized without sacrificing performance. A low-power transducer front-end is vital to this effort, yet most of the devices being manufactured today are piezoresistive and consume large amounts of power. Capacitive microsensors provide an attractive alternative since they dissipate no power and can be read out using relatively low-power circuit techniques while offering high sensitivity and self-test capabilities [4,5]. However, the various capacitive transducers that have been reported display a wide range of base capacitances and sensitivities depending on the
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technology and structure of each device [6]. This variation among transducers places a
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MEMSdesign2 - A Generic Interface Circuit for Capacitive...

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