Figure 4. Chopper stabilization technique

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 sen-

sor IC. 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. Allegro

employs a patented technique to remove key sources of the out-

put drift induced by thermal and mechanical stresses. This offset

reduction technique is based on a signal modulation-demodula-

tion 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 base band, 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. In addition to the removal of the thermal and

stress related offset, this novel technique also reduces the amount

of thermal noise in the Hall sensor IC while completely removing

the modulated residue resulting from the chopper operation. The

chopper stabilization technique uses a high-frequency sampling

clock. For the demodulation process, a sample-and-hold tech-

nique is used. This high-frequency operation 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 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.

Amp

Regulator

Clock/Logic

Hall Element

Tuned

Filter

Anti-Aliasing

LP Filter

Hall Effect Latch for High Temperature Operation

A1225, A1227

and

A1229

10

Allegro MicroSystems, Inc.

115 Northeast Cutoff

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

1.508.853.5000; www.allegromicro.com