Understanding Smart Sensors - Nomads.usp

The Nature of Semiconductor Sensor Output 53ActiveelementS−EtcheddiaphragmboundaryS+VoltagetapsTransversevoltagestrain guideresistor14231. Ground2. +Vout3. Vs4. −VoutFigure 3.2 Shear stress strain gauge.Excitation current is passed longitudinally through the resistor (pins 1and 3), and the pressure that stresses the diaphragm is applied at a right angle tothe current flow. The stress establishes a transverse electric field in the resistorthat is sensed as an output voltage at pins 2 and 4, which are taps located at themidpoint of the resistor. The single-element shear stress strain gauge can beviewed as the mechanical analog of a Hall-effect device.Using a single element eliminates the need to closely match the fourstress- and temperature-sensitive resistors on the Wheatstone-bridge designswhile greatly simplifying the additional circuitry necessary to accomplish calibrationand temperature compensation. The offset does not depend onmatched resistors but on how well the transverse voltage taps are aligned. Thatalignment is accomplished in a single photolithography step, making it easy tocontrol, and is only positive, simplifying schemes to zero the offset. The temperaturecoefficient of the offset is small (nominally ±15 mV/°C), because multipleresistors and temperature coefficients do not have to be matched. By usingproper doping levels, the temperature dependence of full-scale span (the differencebetween full-scale output and offset) can be carefully controlled and