A3425 Просмотр технического описания (PDF) - Allegro MicroSystems

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A3425

Ultra-Sensitive Dual-Channel Quadrature Hall-Effect Bipolar Switch

Allegro MicroSystems 

A3425

Ultra-Sensitive Dual-Channel Quadrature

Hall-Effect Bipolar Switch

Functional Description

Chopper-Stabilized Technique

When using Hall effect technology, a limiting factor

for switchpoint accuracy is the small signal voltage

developed across the Hall element. This voltage is dis-

proportionally small relative to the offset that can be

produced at the output of the Hall device. This makes

it difficult to process the signal and maintain an accu-

rate, reliable output over the specified temperature and

voltage range.

Chopper stabilization is a unique approach used to

minimize Hall offset on the chip. The patented Allegro

technique, dynamic quadrature offset cancellation,

removes key sources of the output drift induced by

thermal and mechanical stress. This offset reduction

technique is based on a signal modulation-demodula-

tion process. The undesired offset signal is separated

from the magnetically induced signal in the frequency

domain through modulation. The subsequent demodu-

lation acts as a modulation process for the offset,

causing the magnetically-induced signal to recover its

original spectrum at the baseband level, 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 element chopping

occurs on each clock edge, resulting in a 340 kHz

chop frequency. This high-frequency operation allows

for a greater sampling rate, which produces higher

accuracy and faster signal processing capability. This

approach desensitizes the chip to the effects of ther-

mal and mechanical stress. The disadvantage to this

approach is that jitter, also known as 360° repeatability,

can be induced on the output signal. The sample-and-

hold process, used by the demodulator to store and

recover the signal, can slightly degrade the signal-

to-noise ratio. This is because the process generates

replicas of the noise spectrum at the baseband, causing

a decrease in jitter performance. However, the improve-

ment in switchpoint performance, resulting from the

reduction of the effects of thermal and mechanical

stress, outweighs the degradation in the signal-to-noise

ratio.

This technique produces devices that have an

extremely stable quiescent Hall element output volt-

age, are immune to thermal stress, and have precise

recoverability after temperature cycling. This tech-

nique 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. This

process is illustrated in the following diagram.

Regulator

Amp

Low-

Pass

Filter

Chopper stabilization circuit (dynamic quadrature offset cancellation)

Allegro MicroSystems, Inc.

7

115 Northeast Cutoff, Box 15036

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

www.allegromicro.com

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