Chopper Stabilized Precision Hall Effect Switches

A1120 and

A1125

6

Allegro MicroSystems, Inc.

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.

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. This configuration is illus-

trated in figure 2.

The chopper stabilization technique uses a 400 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 (800 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 applications, Allegro

recommends its digital device families with lower sensitivity

to jitter. For more information on those devices, contact your

Allegro sales representative.

Figure 2. Model of chopper stabilization technique