7

30Dec03, Rev. 1.42

Worcester, Massachusetts 01615-0036 (508) 853-5000

115 Northeast Cutoff, Box 15036

www.allegromicro.com

Allegro MicroSystems, Inc.

Self-Calibrating Gear Tooth Sensor with 9-Bit Signal Capture

ATS672LSB

Start-Up Detection. The ATS672 generates a digital output

transition when the first rising or falling mechanical edge is

detected after being powered-on, indicating when the first sig-

nificant motion in the target is detected.

Undervoltage Lockout. When the supply voltage falls below

then stays off, regardless of the state of the magnetic field, until

lockout feature prevents false signals, caused by undervoltage

conditions, from propagating to the output of the sensor.

Power Supply Protection. The device contains an on-chip

that need to operate from an unregulated power supply, transient

protection must be added externally. For applications using a

regulated line, EMI/RFI protection may still be required.Contact

Allegro Microsystems for information on the circuitry needed

for compliance with various EMC specifications.

Automatic Gain Control (AGC). The patented self-calibrat-

ing circuitry is unique. Each time the device is powered-on, the

device starts measuring the peak-to-peak magnetic gradient. The

gain of the sensor is automatically adjusted, keeping the internal

electrical signal amplitude constant over the air gap range, AG,

of the device. This feature provides consistent operational char-

acteristics independent of variances in the air gap.

Switch Points. Switch points are the levels of magnetic flux

density, B, which trigger switch turn-on and turn-off. When B

the trigger provides a clean transition from off to on. When the

from on to off.

In the ATS672, switch points are established dynamically as a

percentage of the amplitude of the normalized magnetic signal.

Two DACs track the peaks of the normalized magnetic signal,

and the switching thresholds are established at fixed percentages

of the two DAC values. The values of the thresholds have been

carefully selected to provide the most accurate and consistent

switching where the signal is steepest and least affected by air

gap variation.

The figure below graphically demonstrates the establishment of

the switching threshold levels.

The low hysteresis of 10% provides high performance over the

full AG, and immunity to false switching due to noise, vibration,

backlash, or other transient events.

Mechanical Profile

Magnetic Gradient

(No amplification)

Electrical Signal

Response, with AGC

Ferrous Target

Automatic Gain Control (AGC). The AGC function corrects for

variances in the air gap. Differences in the air gap affect the

magnetic gradient, but AGC prevents that from affecting device

performance.

100%

Valley Signal

Tooth Signal

t

V