HW-108A is the hall element in the application. At 1V DC bias, it consumes about 2.5 mA with a sensitivity of 4mV/mT. Reducing the bias voltage reduces the power consumption but it also reduces the sensitivity. At this point, to lower the power consumption, micropower amplification takes an important role.
Circuit operation is as follows. The LT1790 micropower reference gives a fixed output of 1.25V. The voltage divider circuitry made up of 7.87kΩ and 100kΩ resistors reduces the voltage and the voltage across the 7.87kΩ becomes 0.09V. LT1782 acts as a buffer. So when this 0.09V is applied as bias voltage across the hall sensor bridge, the current is only 0,23mA. This is about 1/10 of the original value. This means that your batteries will last 10 times longer. On the other hand the sensitivity decreases to 0.4mV/mT.
LT6011 precision micropower amplifier gives the desired high gain back at this point. It is configured as an instrumentation amplifier in a gain of 101. Now the sensitivity becomes 40mV/mT with a total current consumption of 0.6 mA. To have achieved this sensitivity by increasing the bias voltage of HW-108A would require a 25mA from the supply.