Design an electronic scale using a Freescale microcomputer with an 8-bit ADC (Figure 12.66). Let x be in mass to be measured. The input range is 0 to 1 kg and the signals of interest are 0 to 10 Hz. A bonded strain gage bridge will be used to convert mass x into voltage, V1 V2. When the input mass is zero, each arm of the bridge is 100, and the bridge output (V1 V2) is zero. At full scale (x 1 kg), two resistors go to 99 and the other two go to 101. In between 0 and 1 kg, the resistance change is linearly related to the mass:
A REF02 precision reference will provide the constant 5.00 V for the bridge and the ADC.
a) What is the bridge output (V1 V2) at full scale (x = 1kg)? What gain is required to match the full range of 0 x 1kg to the 0 to +5V range of the ADC?
b) Design the preamplifier that has the following characteristics: differential input, gain so that the 0 to +5 V ADC is used, good CMRR, high Zin, low Zout, bandwidth > 200 Hz. Show resistor values, but not pin numbers.
c) The signals of interest are 0 to 10 Hz. There is unwanted noise at 60 Hz. Add a two-pole low-pass filter with a cutoff of 20 Hz.
d) What is the system resolution? Give units.
e) What sampling rate would you choose? Explain your answer.
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