OMG! couldn't you write all this in one mesage?
the sensor you use is probably not a range finder, it's only a proximity sensor, so it's output is allready DIGITAL, as said before.. the output would be either approx 5v either 0V. and after reading the datasheet:
**broken link removed**
this sensor is ineed active Low, meaning it will give you 0V when it receives any ammount of 38Khz pulsed IR...
again the output IS digital, you you don't need a ADC!
in replie to your last message, well neither an ADC nor a comparator will give you analog output, because the sensor doesn't support this from the geginng..
you need to build you'r own sensor using a photodiode, phototransistor or LDR...
good luck