Chopper-Stabilized, Precision Hall-Effect Switch
A3240
7
Allegro MicroSystems, Inc.
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
FUNCTIONAL DESCRIPTION
Chopper-Stabilized Technique. The Hall element can be
considered as a resistor array similar to a Wheatstone bridge. A
large portion of the offset is a result of the mismatching of these
resistors. These devices use a proprietary dynamic offset cancel-
lation technique, with an internal high-frequency clock to reduce
the residual offset voltage of the Hall element that is normally
caused by device overmolding, temperature de pen den cies, and
thermal stress. The chopper-stabilizing technique cancels the
mismatching of the resistor circuit by changing the direction of
the current flowing through the Hall plate using CMOS switches
and Hall voltage measurement taps, while maintaing the Hall-
voltage signal that is induced by the external magnetic flux. The
signal is then captured by a sample-and-hold circuit and further
processed using low-offset bipolar circuitry. This technique
produces devices that have an extremely stable quiescent Hall
output voltage, are immune to thermal stress, and have precise
recoverability after temperature cycling. This technique will
also slightly degrade the device output repeatability. A relatively
high sampling frequency is used in order that faster signals can
be processed.
More detailed descriptions of the circuit operation can be
found in: Technical Paper STP 97-10, Monolithic Magnetic Hall
Sensor Using Dynamic Quadrature Offset Cancellation and
Technical Paper STP 99-1, Chopper-Stabilized Amplifiers With A
Track-and-Hold Signal Demodulator.
Operation. The output of these devices switch es low (turns
on) when a magnetic field (south pole) perpendicular to the Hall
sensor exceeds the operate point threshold (B
OP
). After turn-on,
the output is capable of sinking 25 mA and the output voltage is
V
OUT(SAT)
. When the magnetic field is reduced below the release
point (B
RP
), the device output goes high (turns off). The dif fer-
ence in the magnetic operate and release points is the hysteresis
(B
hys
) of the device. This built-in hysteresis allows clean switch-
ing of the output even in the presence of external mechanical
vibration and electrical noise.
Applications. It is strongly recommended that an external
bypass capacitor be con nect ed (in close proximity to the Hall
sensor) between the supply and ground of the device to reduce
both external noise and noise generated by the chopper-sta bi li za-
tion technique.
The sim plest form of magnet that will op er ate these devices
is a ring magnet. Oth er meth ods of operation, such as linear
magnets, are possible. Extensive ap pli ca tions in for ma tion on
magnets and Hall-effect sensors is also available in Application
Note 27701, or at
www.allegromicro.com
Dwg. EH-012
REG
SAMPLE
& HOLD
X
+V
HALL
VOLTAGE
B
+
—
Dwg. AH-011-2
+B
0
OUTPUT VOLTAGE
FLUX DENSITY
Dwg. GH-007-2
0
RP
B
V
OUT(SAT)
CC
V
OP
B
+V
SUPPLY
OUTPUT
Dwg. EH-013
0.1 MF
SUPPLY
1
V
CC
3
2
X
PTCT
