296178-AN, Rev. 1
MCO-0000768

APPLICATION INFORMATIONTARGET DESIGN FOR HALL TRANSMISSION SENSORS
By Dr. Dominik Weiland, Syed Bilal Ali, and Eric Burdette
Allegro MicroSystems
such as tooth and valley width are varied. The information
herein can be used as a general guide for coarsely
predicting performance for a set of gear geometries;
validation should always be done through applications
analysis. For support, contact your local Allegro sales
representative.

INTRODUCTION
Ferromagnetic targets come in all shapes and sizes.
Depending on the type of application, the teeth can beof almost any form, including straight, stamped, spiral,or helical. Therefore, when designing a system with amagnetic sensor and ferromagnetic target, a common
question is “what is the perfect target geometry?” In
short, there is no perfect target geometry, and unlessmany of the free parameters that influence the magnetic
interaction are specified, predicting performance for aset of gear geometries cannot be easily addressed.

FREE DESIGN PARAMETERS
Multiple factors that influence Hall IC performance witha ferromagnetic target include:• Tooth shape (i.e. rectangular or triangular, andradius of edge)

However, it is possible to discuss the relative effects ofvarying one or two of the system parameters if the others
are defined. In this application note, the magnet designs
are fixed by using Allegro IC packages with integrated
magnet and pole piece (concentrator). Specifically, two
direction-detection transmission speed sensor ICs—
the ATS692LSH and ATS19520LSN—are considered.

• Tooth arc length• Valley arc width• Tooth height• Target face width (thickness)• Properties of integrated magnet (shape, material)

Both straight-toothed targets and stamped targets are
considered, as shown in Table 1 and Table 2 respectively.
Targets considered contain only features of uniform
geometry.

Target Material
The magnetic permeability of the target material is notdiscussed in this application note. For that information,refer to application note AN296132: Impact of Magnetic
Relative Permeability of Ferromagnetic Target on BackBiased Sensor Output, available at allegromicro.com.

Data and graphs will show some of the properties of thedesired signal shape and amplitude when parameters

Table 1: Parameter definitions for Allegro Reference Target 60-0.
SymbolNominal
Value

Units

Outer Diameter

Do

120

mm

Target Thickness

F

6

mm

Circular Tooth Lengtht

Circular Valley Widthtv

Tooth Heightht

Parameter

3.14

mm

3

degrees

3.14

mm

3

degrees3

mm

Symbol Key

t

Do

tv

Air Gap
Branded Face of Sensor

Fht296178-AN, Rev. 1
MCO-0000768

APPLICATION INFORMATION

Table 2: Parameter definitions for stamped target.
Parameter

SymbolNominal Value

Units

Diameter

D

100

mm

Target Thickness

F

3

mm

Tooth Width

t

2.6

mm

Valley Width

tv

2.6

mm

Tooth Height

ht

3

mm

Symbol Key

SENSORSDATA

Two of Allegro’s transmission-specific sensors—
ATS692LSH and ATS19520LSN—have been simulated,allowing for measurement of speed and direction ofrotating targets.

For each investigated parameter set, the differential
magnetic signals sensed by the Hall IC were analyzed fora range of angular positions and air gaps. The sensors
under discussion contain a total of three Hall elementsoriented in a row, a back-biasing magnet, and a polepiece to concentrate the field lines, as shown in Figure 2.

Table 3: Details of sensors used in simulations
ATS692LSH

ATS19520LSN

Hall-Element
Spacing

1.75 mm

1.5 mmPackage CodeSHSNSH Package

SN Package

Figure 1: Sensor IC packages used in the simulation

Figure 2: Definition of sensor channels.

955 PERIMETER ROAD • MANCHESTER, NH 03103 • USA+1-603-626-2300 • FAX: +1-603-641-5336 • ALLEGROMICRO.COM

2

296178-AN, Rev. 1
MCO-0000768

APPLICATION INFORMATION

From the three Hall elements, two differential channels arecalculated by subtracting the magnetic field measured by
E2 from the field measured by E3 and E1, respectively.
These differential channels are called Channel A andChannel B. An example of this data is shown in Figure 3.
Note that the Hall elements in the simulated devices are
only sensitive to the component of the magnetic field thatis normal to the face of the IC. To determine the maximum
air gap for a given sensor/target combination, the signal
amplitude at each air gap is extracted from the curves
shown in Figure 3. These are plotted in Figure 4, andthe method for determining minimum and maximum air
gap is shown.

Figure 4: An example plot of peak-to-peak field vs. air gap. The maximum
air gap is determined as the point where the interpolated field is equal to theminimum field. For back-biased sensors, the Allegro datasheet references
the maximum air gap in mm with the Allegro Reference Target 60-0. Basedon simulations or measurements, Allegro can determine the maximum airgap, across all boundary conditions identified in the datasheet, for bespoke
target shapes. Contact an Allegro representative for support in evaluating the
expected maximum air gap for a specific target design. Note that the y-axis
uses a logarithmic scale.

Differential Field (G)For direction detection ICs, a second critical parameter toevaluate is magnetic signal shape, in order that direction
detection and vibration detection function properly under
all conditions. This shape assessment is commonly
distilled to a Switch Point Separation specification on
the datasheet. At each switch point crossing, the verticaldifference of the normalized magnetic channels (example
shown in Figure 5) must meet a minimum criterion.

Requirement

Target Angle (degrees)
Figure 3: An example of the simulated differential channels of a sensor over
one tooth-valley period of target rotation for the Allegro 60-0 target shown in
Table 1. The dotted lines are channel A and the solid lines are channel B. Eachchannel is plotted at 7 different air gaps.

955 PERIMETER ROAD • MANCHESTER, NH 03103 • USA+1-603-626-2300 • FAX: +1-603-641-5336 • ALLEGROMICRO.COM

Figure 5: Definition of switch point separation

3296178-AN, Rev. 1
MCO-0000768

The data shown in the following sections is calculated fora worst-case temperature of 150°C, and the maximum
air gap values are presented as a percentage of the
maximum air gap measured for this particular sensor forthis set of simulations. The trends shown in this report
should be helpful for guiding steel target design. For
more detailed application support and target-specificsimulations, contact an Allegro sales representative.

MILLED TARGETS
The data presented here is from magnetic simulations,performed with the system shown in Table 1 and Figure 6.
The outer diameter of the target was held constantat 120 mm for all simulations in this section, since
past analyses have shown that target diameter is not(independently) a major factor.

Figure 6: An example image from the simulation software Ansys Maxwell. The
steel target is blue, the magnet is turquoise, and the concentrator is violet.

APPLICATION INFORMATIONASSESSMENT 1 – TOOTH LENGTH AND
VALLEY WIDTHFor optimum device performance, it is vital to match thedimensions of the teeth and valleys on the target withthe spacing of the Hall elements dHall in the sensor. Asa first approximation, a target pitch TCycle (Valley width+ Tooth length) ofTCycle = 4 × dHall

is recommended, which should then be further optimized
as discussed in subsequent sections.
One of the most important parameters for target designis the valley width—typically the maximum air gap ishigher when the spacing between teeth is larger. The
normalized maximum air gaps for three different sensors
as a function of the tooth length and valley width, which
are defined as distances along the circumference of a
120 mm outer diameter target, are shown in Figure 7.
The maximum air gap is much more strongly affectedby increasing the valley width than by increasing thetooth length.
Note that the points marked with X’s have a switch point
separation below 20%, and thus would result in unreliabledirection detection and performance outside datasheet
specification for the evaluated ICs. For both sensors, itappears that a tooth length greater than 4 mm resultsin unacceptable switch point separation. Increasing thevalley width greatly improves the maximum air gap up
to a valley width of approximately 8 mm, above which
the performance reaches a plateau.

955 PERIMETER ROAD • MANCHESTER, NH 03103 • USA+1-603-626-2300 • FAX: +1-603-641-5336 • ALLEGROMICRO.COM

4

296178-AN, Rev. 1
MCO-0000768

APPLICATION INFORMATION

Tooth Length (mm)Tooth Length (mm)Figure 7: Maximum air gap as a function of tooth length aand valley width for three different Allegro back-biased sensors for a target with an outer diameter of 120 mm.

ASSESSMENT 2 – TOOTH SHAPE
Figure 8 and Figure 9 show examples of targets withvaried tooth shape, retaining a constant valley width asmeasured at the root diameter, while changing angle ofthe tooth flank. Simulated maximum air gap is shown in
Figure 10. The tooth angle here is defined relative to thetarget radius. For relatively small teeth and valleys, in thiscase 3.1 mm each, simulations show that increasing thisangle initially improves maximum air gap performance
up to an optimum angle of 10˚. This improvement can beunderstood to result from the increased space between
teeth and is roughly equivalent to decreasing the tooth/valley ratio (see section above).

Figure 8: Example image of ferromagnetic target (blue) meshed for simulation
with a tooth angle of 0°.

955 PERIMETER ROAD • MANCHESTER, NH 03103 • USA+1-603-626-2300 • FAX: +1-603-641-5336 • ALLEGROMICRO.COM

5

296178-AN, Rev. 1
MCO-0000768

APPLICATION INFORMATION

Figure 9: Example image of ferromagnetic target (blue) meshed for simulation
with a tooth angle of 20° while retaining root valley width shown in Figure 8.
Figure 11: Example image of ferromagnetic target (blue) meshed for simulation with a tooth angle of 0°.

Figure 10: Normalized maximum air gap as a function of tooth angle startingfrom the nominal condition (OD = 120 mm, 3.1 mm tooth, 3.1 mm valley,
4 mm tooth height) and decreasing the circular tooth length in order toincrease the tooth angle.

Figure 12: Example image of ferromagnetic target (blue) meshed for simulation
with a tooth angle of 14° while retaining root valley width shown in Figure 11.

Thus, simulating the same set of tooth angles butwith a larger valley width, as shown in Figure 11 andFigure 12, results in a different trend (Figure 13), withthe maximum air gap falling monotonically as the toothangle is increased.

Figure 13: Normalized maximum air gap as a function of tooth angle startingfrom OD = 120 mm, 4.2 mm tooth, 11.5 mm valley and decreasing the circular tooth length in order to increase the tooth angle.

955 PERIMETER ROAD • MANCHESTER, NH 03103 • USA+1-603-626-2300 • FAX: +1-603-641-5336 • ALLEGROMICRO.COM

6

296178-AN, Rev. 1
MCO-0000768

APPLICATION INFORMATION

Alternatively, the tooth angle can be varied by keeping
the circular tooth length constant and reducing circularvalley width, as shown below in Figure 14 and Figure 15.
In this case, there is a weak, monotonic reduction in
the maximum air gap. This effect is like the reduction inmaximum air gap when the valley width is decreased(see section above). This effect has a weak impact;
however, it is recommended to use targets with a toothangle near zero in this case.

Figure 16: Normalized maximum air gap as a function of tooth shape startingfrom the nominal condition (OD = 120 mm, 3.1 mm tooth length, 3.1 mm
valley width) and reducing the root valley width in order to increase the toothangle.
Figure 14: Example images of ferromagnetic targets (blue) meshed for simulation with a tooth angle of 4.3° while keeping circular tooth length constant.

Figure 15: Example images of ferromagnetic targets (blue) meshed for simulation with a tooth angle of 16.7° while keeping circular tooth length constant.

ASSESSMENT 3 – TOOTH HEIGHTAs the differential magnetic signals measured by the Hall
elements in the sensor are generated by modulation of
the magnetic field of the integrated permanent magnet,
the amount of material in front of the sensor has asignificant impact on its performance. Modifying thetooth height of the target effectively varies the relative
air gap changes between teeth and valleys. As shownin Figure 18, the normalized maximum air gap reaches
a plateau where no further improvement can typically beachieved by increasing tooth height. The tooth height at
which the plateau is reached depends on the tooth widthand valley width of the teeth. In general, a tooth heightof 4 mm or larger is recommended. When working withsmall feature targets (t <4.2 mm), a tooth height of 3 mmor larger may be appropriate as well.

Figure 17: Example images of ferromagnetic targets (blue) meshed for simulation with varying tooth height.

955 PERIMETER ROAD • MANCHESTER, NH 03103 • USA+1-603-626-2300 • FAX: +1-603-641-5336 • ALLEGROMICRO.COM

7

296178-AN, Rev. 1
MCO-0000768

APPLICATION INFORMATION

Figure 18: Normalized maximum air gap as a function of tooth height withconstant OD = 120 mm and various tooth widths. Note that tooth width andvalley width are assumed to be equal.

ASSESSMENT 4 – TARGET THICKNESSWhen the thickness of the ferromagnetic target drops
below the thickness of the back-bias magnet (in thevertical direction in Figure 19 below), maximum air gap
starts to be affected. For example, the thickness of themagnet used for the ATS19520 is 3.8 mm, while for theATS692 the magnet is 3.6 mm thick. For all sensors(see Figure 20), increasing the target thickness above
approximately 4 mm has no effect on the maximum airgap, while decreasing the target thickness below 4 mmstarts to have a significant negative impact. The above
conclusions assume that the sensor is mounted perfectly
central over the target. When designing the target for an
application, manufacturing related placement tolerances
should be considered as well. It is recommended tomaintain a worst-case distance between the center ofthe sensor and the edge of the target of no less than
2 mm to avoid negatively impacting the maximum air
gap capability.

Figure 19: Example images of ferromagnetic targets (blue) meshed for simulation with varying target thickness.

955 PERIMETER ROAD • MANCHESTER, NH 03103 • USA+1-603-626-2300 • FAX: +1-603-641-5336 • ALLEGROMICRO.COM

8

296178-AN, Rev. 1
MCO-0000768

APPLICATION INFORMATION

Figure 20: Normalized maximum air gap as a function of target thickness at the nominal condition(OD = 120 mm, 3˚ tooth-length, 3˚ valley-length).

STAMPED TARGETS
This type of target is typically produced by stamping holes
into a sheet of metal. The holes can be interpreted as
valleys of infinite depth and the areas between the holes
as teeth resulting in a magnetic system comparable tomilled targets. The data presented here are from magnetic
simulations, performed with the system shown in Figure 23.
In applications using this type of target, it is common to haveferromagnetic material, either from another rotating disc ora fixed piece of the assembly located behind the rotatingtarget. Note that having such an adjacent ferromagnetic
material is similar to a valley in the target of Figure 1. Anexample of such an assembly is shown in Figure 21 andFigure 22. The diameter of the target was held constantfor all simulations in this application note, since the target
diameter is not (independently) a significant factor.
Figure 22: Definition of valley width and tooth width

Figure 21: An example image from the simulation software Ansys Maxwell.
The steel target is blue, the magnet is turquoise, and the lead frame is red.
Shown in olive is a ferromagnetic plate situated behind the target.

Figure 23: Definition of target thickness and valley clearance

955 PERIMETER ROAD • MANCHESTER, NH 03103 • USA+1-603-626-2300 • FAX: +1-603-641-5336 • ALLEGROMICRO.COM

9

296178-AN, Rev. 1
MCO-0000768

APPLICATION INFORMATIONASSESSMENT 1 – ARC LENGTH(TOOTH AND VALLEY WIDTH)
A critical design parameter is the dimensioning of thestamped holes used in the target. Large tooth widths
typically increase the maximum air gap, with no additional
effect above 3.5 mm tooth width. For teeth with a tooth
width smaller than 3.5 mm, the valley width becomes
the dominating factor for maximum air gap capability,
plateauing for valley widths larger than 6 mm.
Tooth widths of around 3 mm to 3.5 mm and valley
widths larger than 6 mm allow for target designs without
too much impact of small changes to either parameter.
Table 4: Normalized maximum air gap vs. tooth width and valley width.
Tooth

Valley

Width
[mm]

Width
[mm]

ATS692

ATS19520

1.75

4.5

79

85

2.1

4

72

80

70

80

2.6

3.8

1.75 to 3.5

>6

> 3.5

>3

Figure 24: Normalized maximum air gap vs. clearance for a target thicknessof 3 mm, a valley width of 2.6 mm and a tooth width of 2.6 mm.

Maximum Air Gap Normalized [%]95-100ASSESSMENT 2 – CLEARANCE
For small-feature targets—in this case both tooth andvalley widths used were 2.6 mm—the depth of thevalley, typically caused by a fixed or rotating plate
mounted behind the target, does not have a significant
influence on the maximum air gap, assuming a thicknessof the stamped target of 3 mm and larger, as shownin Figure 24. For thicknesses smaller than 3 mm, an
impact on maximum air gap is expected, as discussed
in the next section. Comparing this to the tooth height for
milled targets discussed in earlier sections, a stamped
target with a thickness of 3 mm and a tooth height of
2 mm is approximately equivalent to a milled target witha tooth height of 5 mm. An impact on maximum air gap
is expected once the sum of tooth height and target
thickness drop below 4 mm.

ASSESSMENT 3 – TARGET THICKNESSThe thickness of the ferromagnetic material used asbase material for stamped targets has an impact on
device performance in cases where very thin materials
are used. When choosing a target thickness of morethan 1.5 mm, no significant impact on the maximum air
gap is expected. However, thin targets typically are more
prone to effects such as warping and run-out which can
result in a change of air gap between sensor and target
for different positions on the target. This can negatively
impact the performance of the sensors with this particulartarget; therefore, the thickness of the target should be
carefully chosen as to avoid such effects.

Figure 25: Normalized maximum air gap vs. target thickness.

955 PERIMETER ROAD • MANCHESTER, NH 03103 • USA+1-603-626-2300 • FAX: +1-603-641-5336 • ALLEGROMICRO.COM

10

296178-AN, Rev. 1
MCO-0000768

CONCLUSIONSThis application note has detailed the impact of multiple
target design parameters on the maximum air gap
achievable with Allegro’s ATS692LSH and ATS19520LSN
transmission sensors, allowing for measurement ofspeed and direction of rotating targets. Two types oftargets, representing the majority of targets used intoday’s transmission applications, namely milled andstamped targets, have been analyzed. In following therecommendations in this application note, target design
decisions can be supported. Prior to the start of production

APPLICATION INFORMATION

tooling, it is highly recommended to perform in-application
verification testing. For further support in evaluating
the performance of Allegro’s sensor components withspecific targets, especially for applications requiring highperformance or large air gaps, contact your local Allegro
sales representative.
The conclusions drawn throughout the document are
qualitatively applicable to other Allegro back-biased
sensors of the same Hall-element spacing and sensor
package, the latter defining the integrated permanent
magnet.

955 PERIMETER ROAD • MANCHESTER, NH 03103 • USA+1-603-626-2300 • FAX: +1-603-641-5336 • ALLEGROMICRO.COM

11

296178-AN, Rev. 1
MCO-0000768

APPLICATION INFORMATION

Revision History
NumberDate

Description

Responsibility–

November 2, 2020

Initial release

E. Burdette

1

October 25, 2023

Fixed hyperlink (page 1)

R. Couture

Copyright 2023, Allegro MicroSystems.

The information contained in this document does not constitute any representation, warranty, assurance, guaranty,
or inducement by Allegro to the customer with respect to the subject matter of this document. The information
being provided does not guarantee that a process based on this information will be reliable, or that Allegro has
explored all of the possible failure modes. It is the customer’s responsibility to do sufficient qualification testing of
the final product to ensure that it is reliable and meets all design requirements.
Copies of this document are considered uncontrolled documents.

955 PERIMETER ROAD • MANCHESTER, NH 03103 • USA+1-603-626-2300 • FAX: +1-603-641-5336 • ALLEGROMICRO.COM