I used a GMR sensor (Model AA003-02) inside a electricly adjustable homogenous magnetic field generated by a long coil. In other words the current is proportional to the magnetic field strength. However, I am not sure how to calculate the magnetic field strength from my graph below. Is there a formula that gives the relationship between the sensor voltage and the magnetic field strength?
These sensors have bridge configuration, so their sensitivity is relative to supply voltage. In the datasheet (page 13) it is written that they are separated into groups by sensitivity. It is measured in "mV/V-Oe" - voltage output at 1V supply and 1 Oersted (0,1 mT) field. Seems that this is what you are looking for, it remains to find out your particular part number.
P.S. just understood that AA003-02 is exactly part No. Thus, coefficient is in the range 2...3,2 mV/V/Oe
