17 Interface Circuitry and Microsystems

Sensing physical or chemical quantities is a fundamental task in information processing and control systems. A sensing element or transducer converts the quantity to be measured into an electrical signal, such as a voltage, a current, or a resistive or capacitive variation. The data obtained from the transducers then have to be translated into a form understandable by humans, computers, or measurement systems. An electronic circuit called a sensor interface usually performs this task. The functions implemented by a sensor interface can range from simple amplification or filtering to A/D conversion, calibration, digital signal processing, interfacing with other electronic devices or displays, and data transmission (through a bus or, recently, through a wireless connection, such as Bluetooth). Very-large-scale integration (VLSI) technologies have been extensively used to make sensor interface circuits since the appearance of the first integrated circuit (IC) in the early 1960s. Most of the sensor systems realized so far consist of discrete sensors combined with one or more application-specific integrated circuits (ASICs) or commercial components on a printed circuit board (PCB) or hybrid board. Over the past few years, however, progress in silicon planar technologies has allowed minia-turized sensors (microsensors) to be realized by exploiting the sensing properties of IC materials (silicon, polysilicon, aluminum, silicon oxide, and nitride) or additional deposited materials (such as piezoelectric zinc oxide, sensitive polymers, or additional metallization layers). When microsensors are realized using IC technologies and materials, it is possible to integrate the interface circuit and several sensors on the same chip or in the same package, leading to microsystems or micromod-ules. [1-6] The potential advantages of this approach are numerous: The cost of the measurement system is greatly reduced due to batch fabrication of both the sensors and the interface circuits; its size and interconnections are minimized; and its reliability is improved.