The strength of Earth’s magnetic field ranges from 0.25 G to 0.65 G. The
AD22151 has a sensitivity of 0.4mV/G. If you want 0.65 G to map to full scale,
then a sensitivity of 2.5V per 0.65G or 3.85V/G would work. This would require
a gain of 9600 on the AD22151.