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a small explanation of the problem...

the pressure sensor tht i use is MPX100 .the differential amplifier is ina 121..the o/p at diff amp at atmospheric pressure is 4.68v and when i exert pressure(by blowin on the sensor) is about 4.71 volts...the capacitor is to get only the change i.e. 0.1 volts..i amplify this and after the two stages the o/p is 2-3 volts..there was some stray signal wen connected to the adc so i put a peak detector to get the peak signal i.e(the signal i create wen i blow)after this step i gave it to the ADC and monitored the MSB at the output..now wen i exert good amount of pressure the msb changes from 0 to 1..
PROBLEM: I FED THE MSB TO THE P3.0 PORT OF THE MICROCONTROLLER AND WEN I MEASURE THE VOLTAGE LEVELS AT THE PINS IT IS ALWAYS TIED TO ZERO..EVEN WEN I EXERT GREAT PRESSURE...THE MICROCONTROLLER IS WORKING FINE I HAV TESTED IT WITH OTHER INPUTS..
WHAT COULD BE THE PROBLEM??SUGGESTIONS PLEASE..