Dynamic, Self-Calibrating, Peak-Detecting, Differential
Hall Effect Gear Tooth Sensor IC
ATS617LSG
13
Allegro MicroSystems, LLC
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,
V
CC
, is below the minimum operating voltage, V
CC(UV)
, the
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
more than V
CC
(max).
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
Magnetic Profile
IC Output
Electrical Profile
Target Motion from
Pin 1 to Pin 4
IC Output
Electrical 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 OffOn OffOn OffOn Off
