High-Temperature Chopper-Stabilized Precision

Hall-Effect Switch for 5 V Applications

APS11205

9

Allegro MicroSystems, LLC

115 Northeast Cutoff

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

1.508.853.5000; www.allegromicro.com

Amp

Clock/Logic

Hall Element

Low-Pass

Filter

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 sensor. 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 proven approach used to minimize Hall offset.

The Allegro 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-demodulation process. Figure 4: Model of

Chopper Stabilization Circuit (Dynamic Offset Cancellation)

illustrates how it is implemented.

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

innovative chopper stabilization technique uses a high-frequency

clock. The high-frequency operation allows a greater sampling

rate that produces higher accuracy, reduced jitter, and faster sig-

nal processing. Additionally, filtering is more effective and results

in a lower noise analog signal at the sensor output. Devices such

as the A11205 that uses this approach 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 and sample-and-hold circuits

Figure 4: Model of Chopper Stabilization Circuit

(Dynamic Offset Cancellation)

CHOPPER STABILIZATION