Understanding Smart Sensors - Nomads.usp

56 Understanding Smart Sensorsbudget calculations. Any comparison of competitive data sheet values for linearityshould take into account the method used to determine this parameter.The effect of temperature on offset and span (and sensitivity) is one of themost critical and frequently most troublesome aspects of using semiconductorpressure sensors. Products that have no temperature compensation can be usedwith little effort for a narrow temperature range, such as 25°C ± 15°C or less,and accuracy requirements of a few percentage points. Certain sensors, such asMotorola’s MPX family, can be temperature compensated to 1–3% accuracyover a 0–85°C temperature range without exercising the sensor over temperature.Ease of compensating for temperature is an important factor in evaluatinga pressure sensor.To accurately compare various products, similar units must be used.Full-scale value (F.S.), temperature range, and actual limits (specified as plusor-minuspercentage points F.S., plus-or-minus millivolts, or plus-or-minuspercentage points F.S. per degree Celsius) can vary from one manufacturer toanother. The conversion of those units to millivolts per degree Celsius can providea good comparison. For example, a calibrated pressure sensor, such asMotorola’s MPX2000 family with 40-mV output and ±1% F.S. variation over0–85°C, has a maximum window of 9.4 mV/°C. That type of specification canindicate improved performance over reduced temperature spans. However, auser must know actual curve shapes within the temperature compensation windowto have confidence in that value.Three other terms on sensor data sheets are often a source of confusion:static accuracy, ratiometricity, and overpressure. Frequently, manufacturers lumptogether linearity, pressure hysteresis, and repeatability as static accuracy.Sometimes manufacturers indicate that characteristic, stated simply as the accuracy,as a single parameter on the data sheet.Ratiometricity is one of the new terms that semiconductor manufacturershave brought to pressure sensing. Although output is given at a specific voltagerating, lower or higher voltage supplies can be used (within the maximum ratingof the device) with constant voltage source sensors. Ratiometricity error isthe change in the output resulting from a change to the supply voltage and isusually expressed as a percentage of full-scale output. The output in thatinstance varies as the ratio of source voltage to the manufacturer’s rated voltage.A constant voltage is required for proper operation, a fact that should not beoverlooked when considering the total circuit requirements.The term overpressure takes on a different meaning with semiconductorpressure sensors. Even though the silicon diaphragm is only 0.001 inch(25.4 mm) thick, a device rated at 30 psi can withstand 200–300 psi withoutdamage. Most manufacturers specify overpressure conservatively at two or threetimes the rated pressure.