Chopper Stabilized, Precision Hall Effect Latches

for Consumer and Industrial Applications

A3291

5

Allegro MicroSystems, Inc.

115 Northeast Cutoff

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

1.508.853.5000; www.allegromicro.com

Functional Description

Amp

Regulator

Low-

Pass

Filter

Figure 2. Chopper stabilization circuit (dynamic quadrature offset

cancellation)

Chopper-Stabilized Technique

The Hall element can be considered as a resistor array similar

to a Wheatstone bridge. A basic circuit is shown in figure 1,

demonstrating the effect of the magnetic field flux density, B,

impinging on the Hall element. When using Hall effect tech-

nology, a limiting factor for switchpoint accuracy is the small

voltage is disproportionally small relative to the offset that can

be produced at the output of the Hall device, caused by device

overmolding, temperature dependencies, and thermal stress.

A large portion of the offset is a result of the mismatching of

these resistors. The A3290 and A3291 use a proprietary dynamic

offset cancellation technique, with an internal high-frequency

clock, to reduce the ressidual offset. The chopper-stabilizing

technique cancels the mismatching of the resistor circuit by

changing the direction of the current flowing through the Hall

element. To do so, CMOS switches and Hall voltage measure-

induced by the external magnetic flux.

The signal is then captured by a sample-and-hold circuit and fur-

ther processed using low-offset bipolar circuitry. 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 will also

slightly degrade the device output repeatability. A relatively high

sampling frequency is used in order to process faster signals.

More detailed descriptions of the circuit operation can be found

on the Allegro Web site, including: Technical Paper STP 97-10,

Monolithic Magnetic Hall Sensing Using Dynamic Quadrature

Offset Cancellation, and Technical Paper STP 99-1, Chopper-

Stabilized Amplifiers with a Track-and-Hold Signal Demodula-

tor.

Operation

The outputs of the A3290 and A3291 switch low (turn on) when

a magnetic field perpendicular to the Hall element transitions

illustrated in figure 3. After turn-on, the output is capable of

B+

V+

Hysteresis of

Switching Due to

ΔB

Figure 3. Output voltage responds to sensed magnetic flux density.

Figure 1. Hall element, basic circuit operation

B