Two-Wire True Zero Speed Miniature Differential

Peak-Detecting Gear Tooth Sensor with Continuous Calibration

ATS642LSH

13

Allegro MicroSystems, LLC

115 Northeast Cutoff

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

1.508.853.5000; www.allegromicro.com

Sensor Operation

Each operating mode is described in detail below.

Power-On

period of time is required to power the various portions of the

IC. During this period, the ATS642 will power-on in the high

could induce a change in the output state. Such an event could be

caused by thermal transients, but would require a static applied

magnetic field, proper signal polarity, and particular direction

and magnitude of internal signal drift.

Initial Offset Adjust

The sensor intially cancels the effects of chip, magnet, and

installation offsets. Once offsets have been cancelled, the digital

tracking DAC is ready to track the signal and provide output

switching. The period of time required for both Power-On and

Initial Offset Adjust is defined as the Power-On Time.

Calibration Mode

The calibration mode allows the sensor to automatically select

the proper signal gain and continue to adjust for offsets. The

AGC is active, and selects the optimal signal gain based on the

the AGC DAC, the Offset DAC is also adjusted to ensure the

internal analog signal is properly centered.

During this mode, the tracking DAC is active and output switch-

ing occurs, but the duty cycle is not guaranteed to be within

specification.

Running Mode

the device moves to Running mode. During Running mode, the

sensor tracks the input signal and gives an output edge for every

peak of the signal. AOA remains active to compensate for any

offset drift over time.

The ATS642 incorporates a novel algorithm for adjusting the

signal gain during Running mode. This algorithm is designed

magnetic signal “seen” during the calibration period is not repre-

sentative of the amplitude of the magnetic signal for the installed

sensor air gap (see figure 9).

Sensor Electrical

Output, I

OUT

Internal Differential

Signal, V

PROC

1

2

3

4

5

Figure 9: Operation of Running Mode Gain Adjust.

Position 1. The device is initially powered-on. Self-calibration occurs.

Position 2. Small amplitude oscillation of the target sends an erroneously small differential signal to the sensor. The ampli-

Position 3. The calibration period completes on the third rising output edge, and the device enters Running mode.

Position 4. True target rotation occurs and the correct magnetic signal is generated for the installation air gap. The estab-

lished signal gain is too large for the target’s rotational magnetic signal at the given air gap.

Position 5. Running Mode Calibration corrects the signal gain to an optimal level for the installation air gap.