Understanding Smart Sensors - Nomads.usp

Getting Sensor Information Into the MCU 89increasing the effective resolution of the digital output. Second, it performsdecimation, or sample rate reduction, bringing the sampling rate down to theNyquist rate. That minimizes the amount of information for subsequent transmission,storage, or signal processing. Finally, it provides additional antialiasingrejection for the input signal.4.4.2 Performance of A/D ConvertersFrequently the sensor requires high resolution at moderately fast conversionrates. In such cases, signal gain ranging and/or offset zeroing with 8-bit ADCscan be used in a cost-effective manner. Input ranging/zeroing allows for anincrease in dynamic range without compromising the total conversion timeor significantly increasing the silicon area and hence cost. As shown in Figure4.14, 10 bits of signal input can be achieved by range amplification and/oroffset zeroing prior to an 8-bit ADC. Because the ranging/zeroing is typicallydefined by the settling time of the operational amplifier, the total conversiontime remains small in comparison to a full 10-bit A/D conversion. Eight bitsof accuracy is not a limitation depending on the application, because manyprocesses normally operate around a rather small portion of the full-scale signal.A high degree of accuracy is required in the operating range, especially if thesensor is being used as a feedback element. By adjusting the input gain and offset,an 8-bit ADC can be placed at the optimum measurement point but still beable to react to sudden excursions outside the normal range [24].10-bit resolution8-bit resolution4X gainSensoroutput− 6 offsetSensed parameterSensed parameterFigure 4.14 Increasing the accuracy of an 8-bit ADC.