Understanding Smart Sensors - Nomads.usp

The Nature of Semiconductor Sensor Output 57Obviously, no damage should occur when the sensor operates within theoverpressure rating. However, readings made above the normal range areanother matter. Normally, linearity starts to degrade and could fall outside thespecified rating if the sensor is operated above its rated pressure. If linearitywere only marginally acceptable to begin with, overscale readings could causetrouble. However, that is not the case for many applications.The normal mindset for making pressure readings with a pressure gauge isto avoid operating at full scale for best accuracy. Full-scale operation can alsopin the gauge needle, resulting in miscalibration or damage. However, operatinga semiconductor sensor at full scale usually provides better accuracy than amidscale reading. That is especially true for units measured for endpoint linearity,because output is accurately measured to specification at full scale. Slightexcursions over the pressure rating do not significantly degrade linearity. However,if the output saturates above the rated pressure or the digital gauge’s operatingrange is exceeded, a higher pressure input will not produce a higheroutput reading.3.2.4 Static Versus Dynamic OperationThe protective gels or coatings that protect the active surface from the pressuremedia degrade response time for semiconductor pressure sensors. Also, isolatingthe semiconductor by stainless steel diaphragms and oil-filled chambers candecrease response time. However, response time within 1 ms is typicallyachieved when the sensor is exposed to a full-scale pressure excursion.Measuring systems that operate at 6,000 rpm require sensors to operate atfrequencies above 100 Hz. Frequently, a higher frequency range or higher frequencysignal components are also of interest, as in acceleration and vibrationsensors. However, lower frequency sensors tend to have lower noise floors. Thelower noise floor increases the sensor’s dynamic range and may be more importantto the application than higher frequency capability [6].3.3 Other Sensing TechnologiesPiezoresistive sensing is the most common micromachined sensor because itsoutput signal is predictable and an easy-to-signal condition. Other techniquesare gaining in popularity, especially as sensing techniques for smart sensors arebased on the capability of electronics circuitry to handle the signal conditioning.A brief review focusing on the output of current semiconductor sensingtechniques follows.