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A3295 Datasheet(PDF) 5 Page - Allegro MicroSystems

Part # A3295
Description  Chopper Stabilized, Precision Hall Effect Switch
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Manufacturer  ALLEGRO [Allegro MicroSystems]
Direct Link  http://www.allegromicro.com
Logo ALLEGRO - Allegro MicroSystems

A3295 Datasheet(HTML) 5 Page - Allegro MicroSystems

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Chopper Stabilized, Precision Hall Effect Switch
for Consumer and Industrial Applications
A3295
5
Allegro MicroSystems, Inc.
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
Functional Description
Amp
Regulator
Low-
Pass
Filter
Figure 2. Chopper stabilization circuit (dynamic quadrature offset
cancellation)
Chopper-Stabilized Technique
The Hall element can be considered as a resistor array similar
to a Wheatstone bridge. A basic circuit is shown in figure 1,
demonstrating the effect of the magnetic field flux density, B,
impinging on the Hall element. When using Hall effect tech-
nology, a limiting factor for switchpoint accuracy is the small
signal voltage, VHALL, developed across the Hall element. This
voltage is disproportionally small relative to the offset that can
be produced at the output of the Hall device, caused by device
overmolding, temperature dependencies, and thermal stress.
A large portion of the offset is a result of the mismatching of
these resistors. The A3295 uses a proprietary dynamic offset
cancellation technique, with an internal high-frequency clock, to
reduce the residual offset, see figure 2. The chopper-stabilizing
technique cancels the mismatching of the resistor circuit by
changing the direction of the current flowing through the Hall
element. To do so, CMOS switches and Hall voltage measure-
ment taps are used, while maintaining VHALL signal that is
induced by the external magnetic flux.
The signal is then captured by a sample-and-hold circuit and fur-
ther 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 to process faster signals.
More detailed descriptions of the circuit operation can be found
on the Allegro Web site, including: Technical Paper STP 97-10,
Monolithic Magnetic Hall Sensing Using Dynamic Quadrature
Offset Cancellation, and Technical Paper STP 99-1, Chopper-
Stabilized Amplifiers with a Track-and-Hold Signal Demodula-
tor.
Operation
The output of the A3295 switches low (turns on) when a mag-
netic field perpendicular to the Hall element transitions through
and exceeds the Operate Point threshold, BOP. This is illus-
trated in figure 3. After turn-on, the output is capable of sinking
25 mA, and the output voltage reaches VOUT(SAT).
BHYS
VOUT(off)
VOUT(on)(sat)
B+
V+
Hysteresis of
ΔVOUT
Switching Due to
ΔB
Figure 3. Output voltage responds to magnetic flux density.
Figure 1. Hall element, basic circuit operation
+V CC
B
–VHALL
+VHALL


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