9

A3245-DS, Rev. 1

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

115 Northeast Cutoff, Box 15036

www.allegromicro.com

Allegro MicroSystems, Inc.

Chopper-Stabilized Omnipolar Hall Effect Switches

A3245

Figure 2. Chopper Stabilization Circuit (Dynamic Quadrature Offset Cancellation)

Amp

Regulator

Clock/Logic

Hall Element

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 sensor. This makes it difficult to process the signal while

maintaining an accurate, reliable output over the specified oper-

ating 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 mag-

netic-field-induced signal then can pass through a low-pass filter,

while the modulated dc offset is suppressed. This configuration

is illustrated in figure 2.

The chopper stabilization technique uses a 200 kHz high-fre-

quency clock. For demodulation process, a sample and hold

technique is used, where the sampling is performed at twice the

chopper frequency (400 kHz). This high-frequency operation

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,

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

amplifiers in combination with high-density logic integration and

sample-and-hold circuits.

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. Applications 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 applica-

tions, Allegro recommends its digital sensor families with lower

sensitivity to jitter. For more information on those devices,

contact your Allegro sales representative.