Sensitive Two-Wire Field-Programmable

Chopper-Stabilized Unipolar Hall Effect Switches

A1180, A1181,

8

Allegro MicroSystems, Inc.

115 Northeast Cutoff, Box 15036

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

www.allegromicro.com

Figure 2. Chopper stabilization circuit (dynamic quadrature offset cancellation)

Amp

Regulator

Clock/Logic

Hall Element

Low-Pass

Filter

Chopper Stabilization Technique

A limiting factor for switchpoint accuracy when using Hall

effect technology is the small signal voltage developed across

the Hall element. This voltage is proportionally small relative to

the offset that can be produced at the output of the Hall sensor

device. This makes it difficult to process the signal and maintain

an accurate, reliable output over the specified temperature and

voltage range.

Chopper stabilization is a unique approach used to minimize

Hall offset on the chip. The Allegro patented technique, dynamic

quadrature offset cancellation, removes key sources of the output

drift induced by temperature and package 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 demodulation acts as a modulation

process for the offset causing the magnetically induced signal

to recover its original spectrum at base band 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 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 (400KHz). The sampling demodulation

process produces higher accuracy and faster signal processing

capability. Using this chopper stabilization approach, the chip is

desensitized to the effects of temperature and stress. This tech-

nique produces devices that have an extremely stable quiescent

Hall output voltage, is immune to thermal stress, and has precise

recoverability after temperature cycling. This technique 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.

The repeatability of switching with a magnetic field is slightly

affected using a chopper technique. The Allegro high frequency

chopping approach minimizes the affect of jitter and makes it

imperceptible in most applications. Applications that may notice

the degradation are those that require the precise sensing of alter-

nating magnetic fields such as ring magnet speed sensing. For

those applications, Allegro recommends the “low jitter” family

of digital sensors.