Dynamic, Self-Calibrating, Peak-Detecting, Differential

Hall Effect Gear Tooth Sensor IC

ATS617LSG

11

Allegro MicroSystems, Inc.

115 Northeast Cutoff

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

1.508.853.5000; www.allegromicro.com

Assembly Description The ATS617 gear-tooth sensor IC is a

Hall IC/rare-earth pellet configuration that is fully optimized to

provide detection of gear tooth edges. This device is packaged in

a molded miniature plastic body that has been optimized for size,

ease of assembly, and manufacturability. High operating tem-

perature materials are used in all aspects of construction.

After proper power is applied to the component, the chip is

capable of instantly providing digital information that is repre-

sentative of the profile of a rotating gear. No additional optimi-

zation or processing circuitry is required. This ease of use should

reduce design time and incremental assembly costs for most

applications.

Hall Technology The ATS617 contains a single-chip dif-

ferential Hall effect sensor IC, a samarium cobalt pellet, and a

flat ferrous pole piece (figure 2). The Hall IC consists of 2 Hall

elements (spaced 2.2 mm apart) located so as to measure the

magnetic gradient created by the passing of a ferrous object. The

two elements measure the magnetic gradient and convert it to an

analog voltage that is then processed in order to provide a digital

output signal.

The Hall IC is self-calibrating and also possesses a tempera-

ture compensated amplifier and offset cancellation circuitry. Its

voltage regulator provides supply noise rejection throughout the

operating voltage range. Changes in temperature do not greatly

affect this device due to the stable amplifier design and the offset

rejection circuitry. The Hall transducers and signal processing

electronics are integrated on the same silicon substrate, using a

proprietary BiCMOS process.

Internal Electronics The processing circuit uses a patented

peak detection scheme to eliminate magnet and system offsets.

This technique allows dynamic coupling and filtering of offsets

without the power-up and settling time disadvantages of classical

high-pass filtering schemes. The peak signal of every tooth and

valley is detected by the filter and is used to provide an instant

reference for the operate and release point comparator. In this

manner, the thresholds are adapted and referenced to individual

signal peaks and valleys, providing immunity to zero line varia-

tion from installation inaccuracies (tilt, rotation, and off-center

placement), as well as for variations caused by target and shaft

eccentricities.

The ATS617 also includes self-calibration circuitry that is

engaged at power on. The signal amplitude is measured, and

then the device gain is normalized. In this manner switchpoint

drift versus air gap is minimized, and excellent timing accuracy

can be achieved.

The AGC (Automatic Gain Control) circuitry, in conjunction

with a unique hysteresis circuit, also eliminates the effect of gear

edge overshoot as well as increases the immunity to false switch-

ing caused by gear tooth anomalies at close air gaps. The AGC

circuit sets the gain of the device after power-on.

Functional Description

Target (Gear)

Back-Biasing

Rare-Earth Pellet

South Pole

North Pole

Case

(Pin 1 Side)

(Pin n >1 Side)

Hall IC

Pole Piece

Element Pitch

(Concentrator)

Dual-Element

Hall Effect Device

Hall Element 1

Hall Element 2

Figure 2. Relative motion of the target is detected by the dual Hall ele-

ments mounted on the Hall IC.

Figure 3. The peaks in the resulting differential signal are used to set the

Device Output

Differential

Input Signal

V

PROC

V+

V–

0