Explorer pHAT analog inputs sensing .5 volts when not connected

Hi all. I am very new to the Pi world and started experimenting with an 3B plus, analog temperature sensor (Velleman VMA320) and Explorer pHAT. I have an above basic understanding of electronics, components and test equipment. I read a few things online and tried hooking everything up but the temperature calculated was consistantly about 10 degrees F too high. The sensor pins are hooked up to +5v, Gnd and Analog 1 on the Explorer board. I checked all my logic, formulas, Vin, Vout, checked sensor resistance, swapped out the sensor, and checked the Steinhart-Hart numbers but the temp was just not coming out correct. I then pulled in the data from Analog input 1, printed it out and and I found that I was getting a reading of about .55 volts when nothing was hooked up to the Explorer. All the Analog Inputs are registering about .55 volts when nothing is connected. When I adjusted the formulas to subtract .55 from the value of Analog Input 1 my temperature was calculated correctly.

Since I am new to anything Pi and Explorer pHAT related is this normal operation of should I be seeing 0 vols on the Analog inputs? Is there a way to zero out the detected voltage so my numbers are accurate?

Any tips or suggestions would be greatly appreciated. Thanks.