This atmospheric pressure sensor works by measuring the differential pressure between your input (the port on the top of the chip) and the atmospheric pressure around it. The sensor can measure up to a 5.8 PSI (40kpa) difference with an accuracy of better than +/-0.25%. The sensor is a Wheatstone Bridge and the output is very tiny analog voltage ranging from 50 – 100mV so an amplifier circuit is required before it can be connected to a microcontroller.