A3211
and
A3212
Micropower, Ultrasensitive
Hall Effect Switch
FUNCTIONAL DESCRIPTION
Low Average Power.
Internal timing circuitry activates the
sensor for 45
μs
and deactivates it for the remainder of the pe-
riod (45 ms). A short "awake" time allows for stabilization prior
to the sensor sampling and data latching on the falling edge of
the timing pulse. The output during the "sleep" time is latched in
the last sampled state. The supply current is not affected by the
output state.
+V
B
PERIOD
I
DD(EN)
"AWAKE"
"SLEEP"
SAMPLE &
OUTPUT LATCHED
—
HALL
VOLTAGE
I
DD(DIS)
0
Dwg. WH-017-2
+
Dwg. AH-011-2
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 dependencies, 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. A relatively high sam-
pling frequency is used for faster signal processing capability
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.
+V
SAMPLE
& HOLD
X
Dwg. EH-012-1
Allegro MicroSystems, Inc.
115 Northeast Cutoff, Box 15036
Worcester, Massachusetts 01615-0036 (508) 853-5000
www.allegromicro.com
7
ALLEGRO [ ALLEGRO MICROSYSTEMS ]
