Dynamic, Self-Calibrating, Peak-Detecting, Differential

Hall Effect Gear Tooth Sensor IC

ATS617LSG

13

Allegro MicroSystems, Inc.

115 Northeast Cutoff

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

1.508.853.5000; www.allegromicro.com

Peak Detecting vs. AC-Coupled Filters High-pass filter-

ing (normal AC coupling) is a commonly used technique for

eliminating circuit offsets. However, AC coupling has errors at

power-on because the filter circuit needs to hold the circuit zero

value even though the circuit may power-on over a large signal.

Such filtering techniques can only perform properly after the

filter has been allowed to settle, which typically takes longer than

1s. Also, high-pass filter solutions cannot easily track rapidly

changing baselines, such as those caused by eccentricities. (The

term baseline refers to a 0 G differential field, where each Hall-

effect element is subject to the same magnetic field strength; see

figure 3.) In contrast, peak detecting designs switch at the change

in slope of the differential signal, and so are baseline-independent

both at power-on and while running.

Peak Detecting vs. Zero-Crossing Reference The usual

differential zero-crossing sensor ICs are susceptible to false

switching due to off-center and tilted installations that result in a

shift of the baseline that changes with air gap. The track-and-hold

peak detection technique ignores baseline shifts versus air gaps

and provides increased immunity to false switching. In addition,

using track-and-hold peak detection techniques, increased air gap

capabilities can be expected because peak detection utilizes the

entire peak-to-peak signal range, as compared to zero-crossing

detectors, which switch at half the peak-to-peak signal.

Power-On Operation The device powers-on in the Off state

(output voltage high), irrespective of the magnetic field condi-

tion. The circuit is then ready to accurately detect the first target

edge that results in a high-to-low transition of the device output.

Undervoltage Lockout (UVLO) When the supply voltage,

device is off and stays off, irrespective of the state of the mag-

netic field. This prevents false signals, which may be caused

by undervoltage conditions (especially during power-up), from

appearing at the output.

Output. The device output is an open-drain stage. An external

pull-up (resistor) must be supplied to a supply voltage of not

Output Polarity. The output of the unit will switch from low

to high as the leading edge of a tooth passes the branded face of

the package in the direction indicated in figure 6. This means that

in such a configuration, the output voltage will be high when the

package is facing a tooth. If the target rotation is in the oppo-

site direction relative to the package, the output polarity will be

opposite as well, with the unit switching from low to high as the

leading edge passes the unit.

of Package

Rotating Target

Branded Face

1

4

Figure 6. This left-to-right (pin 1 to pin 4) direction of target rotation

results in a high output signal when a tooth of the target gear is nearest

the branded face of the package. A right-to-left (pin 4 to pin 1) rotation

inverts the output signal polarity.

Figure 7. The magnetic profile reflects the geometry of the target, allowing the device to present an accurate digital output response.

Target

Mechanical Profile

Target

Magnet ic Profile

IC Out put

Elect rical Profile

Target Motion from

Pin 1 to Pin 4

IC Out put

Elect rical Profile

Target Motion from

Pin 4 to Pin 1

Signature Tooth

B+

B

IN

V+

V

OUT

V+

V

OUT

IC Output

Switch State

On

Off

On

Off

On

Off

On

Off

On

Off

On

Off

On

Off

On

Off