Self-Calibrating, Zero-Speed Differential
Gear Tooth Sensor IC with Continuous Update
ATS643LSH
9
Allegro MicroSystems, Inc.
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
Hall Technology. The ATS643 contains a single-chip differ-
ential Hall effect sensor IC, a samarium cobalt pellet, and a flat 
ferrous pole piece (concentrator). As shown in figure 1, the Hall 
IC supports two Hall elements, which sense the magnetic profile 
of the ferrous gear target simultaneously, but at different points 
(spaced at a 2.2 mm pitch), generating a differential internal 
analog voltage (V
PROC
) that is processed for precise switching of 
the digital output signal.
The Hall IC is self-calibrating and also possesses a tempera-
ture compensated amplifier and offset cancellation circuitry. Its 
voltage regulator provides supply noise rejection throughout the 
operating voltage range. Changes in temperature do not greatly 
affect this device due to the stable amplifier design and the offset 
rejection circuitry. The Hall transducers and signal processing 
electronics are integrated on the same silicon substrate, using a 
proprietary BiCMOS process. 
Target Profiling During Operation. When proper power is 
applied to the IC, it is capable of providing digital information 
that is representative of the mechanical features of a rotating 
gear. The waveform diagram in figure 3 presents the automatic 
translation of the mechanical profile, through the magnetic 
profile that it induces, to the digital output signal of the ATS643. 
No additional optimization is needed and minimal processing 
circuitry is required. This ease of use reduces design time and 
Functional Description
Target (Gear)
Back-biasing
Rare-earth Pellet 
South Pole
North Pole
Case
(Pin 1 Side)
(Pin n >1 Side)
Hall IC
Pole Piece
Element Pitch
(Concentrator)
Dual-Element
Hall Effect Device
Hall Element 1
Hall Element 2
of Package
Rotating Target
Branded Face
1
4
incremental assembly costs for most applications.
Determining Output Signal Polarity. In figure 3, the top 
panel, labeled Mechanical Position, represents the mechanical 
features of the target gear and orientation to the device. The bot-
tom panel, labeled IC Output Signal, displays the square wave-
form corresponding to the digital output signal that results from 
a rotating gear configured as shown in figure 2. That direction of 
rotation (of the gear side adjacent to the package face) is: perpen-
dicular to the leads, across the face of the device, from the pin 1 
side to the pin 4 side. This results in the IC output switching from 
low, I
CC(Low)
, to high, I
CC(High)
, as the leading edge of a tooth (a 
rising mechanical edge, as detected by the IC) passes the package 
face. In this configuration, the device output current switches to 
its high polarity when a tooth is the target feature nearest to the 
package. If the direction of rotation is reversed, so that the gear 
rotates from the pin 4 side to the pin 1 side, then the output polar-
ity inverts. That is, the output signal goes high when a falling 
edge is detected, and a valley is the nearest to the package. Note, 
however, that the polarity of I
OUT
 depends on the position of the 
sense resistor, R
SENSE
 (see Operating Characteristics table).
Continuous Update of Switchpoints. Switchpoints are the 
threshold levels of the differential internal analog signal, V
PROC
, 
at which the device changes output signal polarity. The value of 
V
PROC 
is directly proportional to the magnetic flux density, B, 
Figure 1. Relative motion of the target is detected by the dual Hall ele-
ments mounted on the Hall IC.
Figure 2. 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 face of the package (see figure 3). A right-to-left (pin 4 to pin 1) rota-
tion inverts the output signal polarity.
Figure 3. The magnetic profile reflects the geometry of the target, allow-
ing the ATS643 to present an accurate digital output response.
B
OP(#1)
B
RP(#1)
B
OP(#2)
On
Off
Off
On
IC Internal Switch State
Package Orientation to Target
IC Internal Differential Analog Signal, V
PROC
Mechanical Position (Target movement pin 1 to pin 4)
IC Output Signal, I
OUT
Target
(Gear) 
Package
Package Branded Face
Pin 1 
Side
Pin 4 
Side
+t
Target Magnetic Profile
+B
+t
+t
This tooth 
sensed 
earlier
This tooth 
sensed 
later