Understanding Smart Sensors - Nomads.usp

82 Understanding Smart SensorsOnce the circuit is calibrated, the inherent power-supply rejection schemeoperates automatically without additional software overhead or MCU processingtime.4.3 Separate Versus Integrated Signal ConditioningThere is a worldwide effort [17, 18] in both academia and industry to integratevarious sensors with electronics because of the potential advantages. Thoseadvantages include improved sensitivity, differential amplification for cancelingparasitic effects such as temperature and pressure, temperature compensationcircuits, multiplexing circuits, and A/D-converting circuits [19]. The combinationdoes not inherently offer lower cost or improved performance. However,selection of the proper sensor, application (especially high-volume), and designcriteria can provide a sensor that is lower cost and more reliable than a thickfilm,multichip version.Semiconductor sensors, like other semiconductor devices, are subject tothe paradox that (1) increased integration is inevitable with semiconductortechnology for performance and cost reduction and (2) all devices cannot beintegrated and obtain improved performance or cost reduction. Proper systempartitioning is required to avoid components that are difficult to integrate, suchas high-value capacitors, and components that can be obtained more cost effectivelyby other approaches. Combining the sensor and additional circuitry canbe beneficial in some applications and undesirable in others.Precision outputs for integrated sensors can be obtained by laser trimmingof zero-TCR thin-film metals at the wafer, die, or package level. Siliconresistors created by alloying silicon and aluminum can also be trimmed forimproved accuracy. One trim process has achieved essentially continuousadjustment by computer-controlled current pulses applied to the resistorseither at the wafer level or after packaging [20].Integrated electronic techniques are especially useful when only a smalladjustment must be made. Active elements, such as op amps and transistors, areused to produce an amplified signal that can be easily interfaced to the rest of asensing system. Improved precision can be obtained by characterizing devicesover temperature and pressure (or acceleration or force) and storing correctionalgorithms in memory. Semiconductor sensing technology allows the integrationof both the sensing element(s) and signal conditioning circuitry into thesame monolithic (silicon) structure. A number of factors determine which technologya manufacturer would choose when approaching a new design. Theintegration of passive elements and active elements with a piezoresistive pressuresensor is used here to demonstrate common approaches.