ATS612LSG

DYNAMIC, SELF-CALIBRATING, PEAK-DETECTING,

DIFFERENTIAL HALL-EFFECT GEAR TOOTH SENSOR

Page 9 of 15

115 Northeast Cutoff, Box 15036

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

Copyright © 2001, 2003 Allegro MicroSystems, Inc.

Peak Detecting vs. AC-Coupled Filters.

High-pass

filtering (normal AC coupling) is a commonly used

technique for eliminating circuit offsets. 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 filter techniques can only

perform properly after the filter has been allowed to

settle, which is typically greater than one second. Also,

high-pass filter solutions cannot easily track rapidly

changing baselines such as those caused by

eccentricities. Peak detection switches on the change in

slope of the signal and is baseline independent at power

up and during running.

Peak Detecting vs. Zero-Crossing Reference.

The

usual differential zero-crossing sensors are susceptible

to false switching due to off-center and tilted installations

which 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-detecting techniques, increased air

gap capabilities can be expected because a peak

detector utilizes the entire peak-to-peak signal range as

compared to zero-crossing detectors that switch on one-

half the peak-to-peak signal.

NOTE – “Baseline” refers to the zero-gauss differential

field where each Hall-effect element is subject to the

same magnetic field strength.

Power-On Operation.

The device will power on in the

OFF state (output high) irrespective of the magnetic field

condition. 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.

Under-Voltage Lockout.

When the supply voltage is

device is OFF and stays OFF irrespective of the state of

the magnetic field. This prevents false signals, which

may be caused by under-voltage conditions (especially

during turn on), 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) to a supply voltage of not more than 24 V must

be supplied.

Output Polarity.

The output of the unit will switch from

HIGH to LOW as the leading edge of the tooth passes

the unit in the direction indicated in figure 3 which means

that in this configuration, the output voltage will be high

when the unit is facing a tooth. If rotation is in the

opposite direction, the output polarity will be opposite as

well, with the unit switching LOW to HIGH as the leading

edge passes the unit.

Figure 3