Field-Programmable, Chopper-Stabilized,

Unipolar Hall-Effect Switches

A3251

8

Allegro MicroSystems, Inc.

115 Northeast Cutoff, Box 15036

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

www.allegromicro.com

Functional Description

Chopper-Stabilized Technique

The Hall sensor is based on a Hall element, a small sheet of

semiconductor material in which a constant bias current flows

when a constant voltage source is applied. The output takes the

form of a voltage measured across the width of the Hall element,

and has negligible value in the absence of a magnetic field.

When a magnetic field is applied with flux lines at right angles

to the current in the Hall element, a small signal voltage directly

proportional to the strength of the magnetic field occurs at the

output of the Hall element.

This small signal voltage is disproportionally small relative to

the offset produced at the input of the device. This makes it very

difficult to process the signal and maintain an accurate, reliable

output over the specified temperature and voltage range. There-

fore, it is important to reduce any distortion of the signal that

could be amplified when the signal is processed.

Chopper stabilization is a unique approach used to minimize

input offset on the Hall IC. This technique removes a key

source of output drift due to temperature and mechanical stress,

and produces a 3X reduction in offset in comparison to other,

conventional methods.

This offset reduction chopping technique is based on a sig-

nal modulation-demodulation process. The undesired offset

signal is separated from the magnetically-induced signal in the

frequency domain. The offset (and any low-frequency noise)

component of the signal can be seen as signal distortion added

after the signal modulation process has taken place. Therefore,

the dc offset is not modulated and remains a low-frequency

component. Consequently, the signal demodulation process acts

as a modulation process for the offset, causing the magnetically-

induced signal to recover its original spectrum at baseband while

the dc offset becomes a high-frequency signal. Then, the signal

passes using a low-pass filter, while the modulated dc offset is

suppressed.

The advantage of this approach is significant offset reduction,

which desensitizes the Hall IC against the effects of temperature

and mechanical stress. The disadvantage is that this technique

features a demodulator that uses a sample-and-hold block to

store and recover the signal. This sampling process can slightly

degrade the SNR (signal-to-noise ratio) by producing replicas of

the noise spectrum at the baseband. This degradation is a function

of the ratio between the white noise spectrum and the sampling

frequency. The effect of the degradation of the SNR is higher

jitter, also known as signal repeatability. However, the jitter in a

continuous-time device can be 5X that of the A3250/A3251.

Chopper stabilization circuit (dynamic quadrature offset cancellation)

Amp

Regulator