Understanding Smart Sensors - Nomads.usp

The Nature of Semiconductor Sensor Output 55Nevertheless, that factor must be considered to avoid problems in volumeproduction.The term typical specification generally has several implications in thesemiconductor industry. Usually, it indicates a parameter that has been characterizedduring the design phase and represents the mean value for the manufacturingprocess. Certain manufacturers also add that a ±3 sigma value is the totalspread that users can expect on typical specified parameters. That spread cannotbe assumed, however, because typical means that limits are not normallyattached to the manufacturer’s quality assurance program.In many cases, typical is used to indicate that the measurement processcontributes more to the inaccuracy of the reading than to the actual variationfrom unit to unit. That practice shows why users must evaluate the parameterspecified as typical. Pressure hysteresis in silicon diaphragms, for example, isessentially nonexistent and should not cause great concern if specified as typical.On the other hand, the variation over temperature of span and offset compensationcould be significant and requires further investigation if listed as atypical specification.Many parameters on sensor data sheets are specified as the plus-or-minuspercentage of full-scale (output or span). That designation is the key to understandingother specifications. Full-scale output is the output at rated pressureincluding the zero offset. Full-scale span is the difference between the outputwith no pressure applied (offset) and the output at rated pressure. For a givendevice, for example, Motorola’s MPX100, a 0.25% full-scale span rating onlinearity can range from 0.113 mV for a low-output device (45 mV) to 0.225mV for a device with the highest output (90 mV).Sensitivity is the span divided by the operating pressure range. The sensitivitiesfor devices made by various manufacturers are frequently compared.The pressure ranges and the supply voltage must be considered for an accuratecomparison. The procedure often reveals interesting information that mayaffect a design decision, especially if the linearity at a given sensitivity is alsotaken into account.The linearity or deviation from a straight-line relationship is expressed asa percentage of full-scale output (or span) and is also subject to different testmethods. The least-squares, best-fit technique is the commonly accepted choicefor establishing linearity. However, it requires several measurement points toprovide acceptable results.A simpler measurement is the end-point method, which requires onlythree points: zero, midscale, and full scale. The method lends itself to highvolumetesting. Because it always yields a value that is about double the magnitudeof a least-squares method, it is frequently more appropriate for error