Understanding Smart Sensors - Nomads.usp

54 Understanding Smart Sensorstherefore compensated without the requirement of characterizing each deviceover the temperature range.A more efficient single-element transducer design was developed recentlyto produce a higher output signal from the same level of mechanical stress [4].Figure 3.3(a) shows the original X-ducer design and its location relative tothe diaphragm edge. The design provided a tighter offset distribution, whichwas a significant improvement over Wheatstone-bridge designs used in the earlydays of micromachined pressure sensors. However, improvements in processingand design can achieve a tighter distribution without using the X-ducerdesign. As a result, the “picture frame” design shown in Figure 3.3(b) wasdeveloped. The picture frame design provides about 40% more span than theX-ducer. That more efficient design allows both better linearity and highersignal-to-noise ratio. That improvement in output signal allows a lower overallgain to be used, enhancing accuracy.3.2.3 Semiconductor Sensor DefinitionsThe batch-processing techniques used by semiconductor manufacturers areideally suited for making high-volume, low-cost sensors. One limitation ofbatch-processed parts, however, is that certain parameters are not preciselyspecified but only listed as “typical” on the manufacturer’s data sheet. In manycases, meeting fixed limits on all parameters can drive costs up and offset thebenefits of high-volume batch processing [5].Use of typical specifications is not necessarily a drawback, however. Uniformityin each wafer lot is a specific strength of semiconductor processing.Piezoresistive sensing element(a)(b)Figure 3.3 Piezoresistor design comparison of (a) original single element to (b) new design.(Courtesy of Motorola, Inc.)