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ECE2801_hw5_C08_sol

ECE2801_hw5_C08_sol - ECE2801Homework#5ADC12&SPISolutions...

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ECE2801 Homework #5 – ADC12 & SPI --Solutions 1)  The analog output of a pressure sensor is connected to the input of an analog- to-digital converter.  The pressure sensor has a sensitivity of 0.89 mV/kPa  (kiloPascal) and 1.0 KPa = .145 psi.  Atmospheric pressure at sea level is  approximately 14.7 psi which  is equal to 1 atmosphere (1atm).  (25 pts)     a)  If the sensor's full scale analog output range is 0-2.5V, what is the minimum           number of bits that the ADC must have to be able to detect changes of            0.25KPa  across the full range (Do  not  assume you must use the MSP430's           built-in ADC12). Resolution in KPa = 0.25kPa (0.25kPa)( 0.89 mV/kPa) = 0.2225 mV = Resolution (voltage) Resolution of ADC = FSR/(2 k ) where k = number of bits in ADC 0.0002225 V = (2.5V – 0)/(2 k );    (2 k ) = 11235.955 But 11235.955 is not a power of 2!!    >> 2 13  = 8192 < 11235.955 < 2 14  = 16384.  Need at least a 14 bit ADC        to achieve the desired resolution      b) What is the dynamic range (in dB) of the ADC chosen in part (a) ? DR = 20*log 10 (2 14 ) =  20*log 10 (16384)  = 84.29 dB      c)  What is the pressure in kPa associated with the full-scale reading of 2.5V if  the minimum detectable pressure is actually 0.14 kPa (i.e. 0V actually  corresponds to 0.14 kPa instead of 0 kPa, a perfect vacuum)? There is an offset of 0.14KPa in the readings.   ( 2.5V) / (0.00089 V/kPa) + 0.14KPa = 2809.129 kPa      d)  What is the dynamic range of the  pressure   sensor  in dB? The definition of Dynamic Range  = 20* log 10 (largest meas./smallest meas.)             DR sens  = 20*log 10 (2809.129 kPa/ 0.14KPa) = 86.049 dB
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The reason the DR of the sensor is slightly greater than that of the ADC is that  the ADC is not capable of measuring 0.14KPa.        e)  Assume that the output of the ADC from part (a)  is read into the  unsigned int adcPressure .  Write a C function that converts  adcPressure  to atmospheres.
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