Basic Hall effect sensors

The Hall element is the basic magnetic field sensor.
It requires signal conditioning to make the output
usable for most applications. The signal conditioning
electronics needed are an amplifier stage and temperature
compensation. Voltage regulation is needed
when operating from an unregulated supply. Figure
2-4 illustrates a basic Hall effect sensor.
If the Hall voltage is measured when no magnetic
field is present, the output is zero (see Figure 2-1).
However, if voltage at each output terminal is measured
with respect to ground, a non-zero voltage will
appear. This is the common mode voltage (CMV),
and is the same at each output terminal. It is the potential
difference that is zero. The amplifier shown in
Figure 2-4 must be a differential amplifier so as to
amplify only the potential difference – the Hall voltage.
The Hall voltage is a low-level signal on the order of
30 microvolts in the presence of a one gauss magnetic
field. This low-level output requires an amplifier with
low noise, high input impedance and moderate gain.

A differential amplifier with these characteristics can be readily
integrated with the Hall element using standard bipolar transistor
technology. Temperature compensation is also easily integrated.

As was shown by equation 2-1, the Hall voltage is a function of the
input current. The purpose of the regulator in Figure 2-4 is to hold
this current constant so that the output of the sensor only reflects
the intensity of the magnetic field. As many systems have a
regulated supply available, some Hall effect sensors may not
include an internal regulator.

Source ( pdf )
http://content.honeywell.com/sensing/prodinfo/solidstate/

technical/hallbook.pdf