Dynamic Self-Calibrating Peak-Detecting Differential

Hall Effect Gear Tooth Sensor IC

ATS616LSG

11

Allegro MicroSystems, Inc.

115 Northeast Cutoff

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

1.508.853.5000; www.allegromicro.com

(normal AC coupling) is a commonly used technique for elimi-

nating 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 chang-

ing 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 dif-

ferential zero-crossing sensor ICs are susceptible to false switch-

ing 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-cross-

ing 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 power-up time of the circuit is no greater than 500 μs.

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).

off and stays off, irrespective of the state of the magnetic field.

This prevents false signals, which may be caused by undervolt-

age conditions (especially during power-up), from appearing at

the output.

Output.

The device output is an open-collector stage capable of

sinking up to 20 mA. An external pull-up (resistor) must be sup-

plied to a supply voltage of not more than 24 V.

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