A1122LLHLT-T2 datasheet(13/18 Pages) ALLEGRO

13 / 18 page

Chopper-Stabilized Precision Hall-Effect Switches

A1120, A1121, A1122,

A1123, and A1125

Allegro MicroSystems, LLC

115 Northeast Cutoff

Worcester, Massachusetts 01615-0036 U.S.A.

1.508.853.5000; www.allegromicro.com

Amp

Regulator

Clock/Logic

Hall Element

Low-Pass

Filter

Chopper Stabilization 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 disproportionally small relative

to the offset that can be produced at the output of the Hall ele-

ment. This makes it difficult to process the signal while main-

taining an accurate, reliable output over the specified operating

temperature and voltage ranges.

Chopperstabilizationisauniqueapproachusedtominimize

Halloffsetonthechip.ThepatentedAllegrotechnique,namely

DynamicQuadratureOffsetCancellation,removeskeysources

of the output drift induced by thermal and mechanical stresses.

Thisoffsetreductiontechniqueisbasedonasignalmodulation-

demodulation process. The undesired offset signal is separated

fromthemagneticfield-inducedsignalinthefrequencydomain,

throughmodulation.Thesubsequentdemodulationactsasa

modulation process for the offset, causing the magnetic field

induced signal to recover its original spectrum at baseband, while

thedcoffsetbecomesahigh-frequencysignal.Themagnetic

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

modulated DC offset is suppressed. This configuration is illus-

trated in figure 2.

Thechopperstabilizationtechniqueusesa400kHzhighfre-

quencyclock.Fordemodulationprocess,asampleandhold

techniqueisused,wherethesamplingisperformedattwicethe

chopperfrequency(800kHz).Thishigh-frequencyoperation

allows a greater sampling rate, which results in higher accuracy

and faster signal-processing capability. This approach desensi-

tizes the chip to the effects of thermal and mechanical stresses,

andproducesdevicesthathaveextremelystablequiescentHall

output voltages and precise recoverability after temperature

cycling.Thistechniqueismadepossiblethroughtheuseofa

BiCMOS process, which allows the use of low-offset, low-noise

amplifiers in combination with high-density logic integration and

sample-and-hold circuits.

The repeatability of magnetic field-induced switching is affected

slightlybyachoppertechnique.However,theAllegrohigh

frequencychoppingapproachminimizestheaffectofjitterand

makes it imperceptible in most applications. Applications that are

morelikelytobesensitivetosuchdegradationarethoserequiring

precise sensing of alternating magnetic fields; for example, speed

sensing of ring-magnet targets. For such applications, Allegro

recommends its digital device families with lower sensitivity

tojitter.Formoreinformationonthosedevices,contactyour

Allegro sales representative.

Figure 2. Model of chopper stabilization technique

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