Ultra-Sensitive Dual-Channel Quadrature

Hall-Effect Bipolar Switch

A1230

11

Allegro MicroSystems, LLC

115 Northeast Cutoff, Box 15036

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

www.allegromicro.com

FUNCTIONAL DESCRIPTION

A limiting factor for switchpoint accuracy when using Hall effect

technology is the small signal voltage developed across the Hall

plate. This voltage is proportionally small relative to the offset

that can be produced at the output of the Hall IC. 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 baseband 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.

Allegro’s new innovative chopper-stabilization technique uses a

high frequency clock. This chopper-stabilization approach de-

sensitizes the IC to temperature and stress. The high-frequency

operation also allows a greater sampling rate that produces higher

accuracy and faster signal processing capability. Additionally,

filtering is more effective and results in a lower noise analog

signal at the input to the Schmitt trigger. Therefore, this high-

frequency chopping technique reduces jitter, also known as 360°

repeatability, can be induced on the output signal. The sample-

and-hold process, used by the demodulator to store and recover

the signal, can slightly degrade the signal to noise ratio. This is

because the process generates replicas of the noise spectrum at the

baseband, causing a decrease in jitter performance. However, the

improvement in switchpoint performance, resulting from the reduc-

tion of the effects of thermal and mechanical stress, outweighs the

degradation in the signal to noise ratio.

This technique produces devices that have an extremely stable

quiescent Hall output voltage, are immune to thermal stress,

and have 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.

Figure 1: Chopper Stabilization Circuit (Dynamic Quadrature Offset Cancellation)

Amp

Regulator

Low-

Pass

Filter

Chopper-Stabilized Technique