Micropower Ultrasensitive 3D Hall-Effect Switch
A1266
13
Allegro MicroSystems, LLC
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
A limiting factor for switchpoint accuracy when using Hall-effect
technology is the small signal voltage developed across the Hall
plate. This voltage is proportionally small relative to the offset
that can be produced at the output of the Hall sensor. This makes
it difficult to process the signal and maintain an accurate, reliable
output over the specified temperature and voltage range. Chopper
stabilization is a proven approach used to minimize Hall offset.
The Allegro technique, dynamic quadrature offset cancellation,
removes key sources of the output drift induced by temperature
and package stress. This offset reduction technique is based on a
signal modulation-demodulation process. Figure 6 illustrates how
it is implemented.
The undesired offset signal is separated from the magnetically
induced signal in the frequency domain through modulation. The
subsequent demodulation acts as a modulation process for the
offset, causing the magnetically induced signal to recover its orig-
inal spectrum at baseband while the DC offset becomes a high-
frequency signal. Then, using a low-pass filter, the signal passes
while the modulated DC offset is suppressed. Allegro’s innova-
tive chopper-stabilization technique uses a high-frequency clock.
The high-frequency operation allows a greater sampling rate
that produces higher accuracy, reduced jitter, and faster signal
processing. Additionally, filtering is more effective and results in
a lower noise analog signal at the sensor output. Devices such as
the A1266 that use this approach have an extremely stable quies-
cent Hall output voltage, are immune to thermal stress, and have
precise recoverability after temperature cycling. This technique
is made possible through the use of a BiCMOS process, which
allows the use of low-offset, low-noise amplifiers in combination
with high-density logic and sample-and-hold circuits.
Chopper Stabilization
Amp.
Multiplexer
Low-Pass
Filter
Sample, Hold &
Averaging
VDD
Figure 6: Model of Chopper-Stabilization Circuit (Dynamic Offset Cancellation)
