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A1181 Datasheet(PDF) 5 Page - Allegro MicroSystems

Part No. A1181
Description  Two-Wire, Programmable, Chopper-Stabilized, Unipolar Hall-Effect Switch
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Maker  ALLEGRO [Allegro MicroSystems]
Homepage  http://www.allegromicro.com

A1181 Datasheet(HTML) 5 Page - Allegro MicroSystems

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Rev. 1.8 25 April 2003
Page 5
115 Northeast Cutoff, Box 15036
Worcester, Massachus etts 01615-0036 (508) 853-5000
Copyright © 1993, 1995 Allegro MicroSystems, Inc.
Functional Description
Chopper-Stabilization Technique. A limiting factor for switch point 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 unique approach used to minimize Hall offset on the chip. The Allegro patented 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. 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 original
spectrum at base band 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.
The chopper stabilization technique uses a 170 kHz high frequency clock. The Hall plate chopping occurs on each clock
edge resulting in a 340 kHz chop frequency. The high frequency operation allows for a greater sampling, which produces
higher accuracy and faster signal processing capability. Using this chopper stabilization approach, the chip is de-
sensitized to the effects of temperature and stress. This technique produces devices that have an extremely stable
quiescent Hall output voltage, is immune to thermal stress, and has precise recoverability after temperature cycling. This
technique is made possible through the use of a BiCMOS process which allows the use of low offset and low noise
amplifiers in combination with high-density logic integration and sample and hold circuits.
The repeatability of switching with a magnetic field is slightly affected using a chopper technique. Allegro’s high frequency
chopping approach minimizes the affect of jitter and makes it imperceptible in most applications. Applications that may
notice the degradation are those that require the precise sensing of alternating magnetic fields such as ring magnet speed
sensing. For those applications, Allegro recommends the “low jitter” family of digital sensors.
Hall Element
Concept of Dynamic Quadrature Offset Cancellation

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