Understanding Smart Sensors - Nomads.usp

90 Understanding Smart SensorsADC resolution can also be increased with circuit techniques. For example,a circuit that uses both ADC and DAC channels of an MCU and a twostageamplifier has been designed that increases the effective resolution of an8-bit ADC to 12 bits for pressure measurements [27]. Standard componentswere used for the MCU and amplifier circuit. An applied pressure in the midrangeof a 200-kPa sensor originally measured with an error of 0.3 kPa was subsequentlyreduced to 0.1 kPa using that approach.Another consideration in choosing an ADC is specification of therequirements within the noise sources of the system. Those limitations may bedue to a less-than-optimum printed circuit layout, unstable power sources,nearby high-energy fields, and/or use of devices without good signal-to-noiseration and power-supply CMRR. It is important to first assess the noise floorfor the system and then compare that to the desired resolution of the signal. Ifthe combined system’s design and noise floor conditions create a minimumrequired signal that must be greater than 5–10 mV, only 8–9 bits of resolutioncan be determined in a 5V system. Using a higher resolution ADC will not provideany additional data unless the conversion speed of the ADC and processorthroughput allows multiple sampling with data correction. Furthermore, theprocess must also meet the desired bandwidth, power consumption, and systemresponse [28]. Oversampling and averaging are, therefore, desired because it isnot possible to separate the noise from the signal using a single A/D sample/conversion.4.4.3 Implications of A/D Accuracy and ErrorsAn amplified pressure signal supplied to an 8-bit ADC is an example of thecombined capability of the amplifier and the ADC. The resulting A/D conversionis related to the pressure input by the following equation:( ) ( )count = V − V ⋅255 / V − V(4.2)FS Offset RH RLWhere V FS –V Offset is the sensor’s full-scale span voltage, 255 is the maximumnumber of counts from the 8-bit converter, and V RH –V RL is 5V, basedon using the same 5V supply as the MCU. Using the same reference voltage forthe ADC and the sensor minimizes the number of additional components butdoes sacrifice resolution. For those instances where greater resolution isrequired, separate lower voltage references should be provided for the ADCand/or the sensor.For a sensor with a 0.25–4.75V output, the maximum number of countsavailable at the output register will be