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A1140-DS

Worcester, Massachusetts 01615-0036 (508) 853-5000

115 Northeast Cutoff, Box 15036

www.allegromicro.com

Allegro MicroSystems, Inc.

Sensitive Two-Wire Chopper-Stabilized Unipolar Hall Effect Switches

A1140/41/42/43

Chopper Stabilization Technique

When using Hall-effect technology, a limiting factor for switch-

point accuracy is the small signal voltage developed across the

Hall element. This voltage is disproportionally small relative to

the offset that can be produced at the output of the Hall sensor.

This makes it difficult to process the signal while maintaining an

accurate, reliable output over the specified operating temperature

and voltage ranges.

Chopper stabilization is a unique approach used to minimize

Hall offset on the chip. The patented Allegro technique, namely

dynamic quadrature offset cancellation, removes key sources

of the output drift induced by thermal and mechanical stresses.

This offset reduction technique is based on a signal modulation-

demodulation process. The undesired offset signal is separated

from the magnetic field-induced signal in the frequency domain,

through modulation. The subsequent demodulation acts as a

modulation process for the offset, causing the magnetic field-

induced signal to recover its original spectrum at baseband, while

the dc offset becomes a high-frequency signal. The magnetic-

sourced signal then can pass through a low-pass filter, while the

modulated dc offset is suppressed.

The chopper stabilization technique uses a 200 kHz high fre-

quency clock. The chopping occurs on each clock edge, result-

ing in a 400 kHz chop frequency. This high-frequency opera-

tion allows a greater sampling rate, which results in higher

accuracy and faster signal-processing capability. This approach

desensitizes the chip to the effects of thermal and mechani-

cal stresses, and produces devices that have extremely stable

quiescent Hall output voltages and 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 ampli-

fiers in combination with high-density logic integration and

sample-and-hold circuits. This process is illustrated in figure 2.

The repeatability of magnetic field-induced switching is

affected slightly by a chopper technique. However, the Allegro

high-frequency chopping approach minimizes the affect of

jitter and makes it imperceptible in most applications. Applica-

tions that are more likely to be sensitive to such degradation

are those requiring precise sensing of alternating magnetic

fields; for example, speed sensing of ring-magnet targets. For

such applications, Allegro recommends its digital sensor fami-

lies with lower sensitivity to jitter. For more information on

those devices, contact your Allegro sales representative.

Figure 2. Chopper stabilization circuit (dynamic quadrature offset cancellation)

Amp

Regulator

Clock/Logic

Hall Element

Low-Pass

Filter