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APPLICATION INFORMATIONMETHODS AND VALIDATION OF STRAY MAGNETIC FIELDMITIGATION IN MAGNETIC SPEED SENSOR ICS
By Braeden Benedix, Cedric Gillet, and Zachary Richards
Allegro MicroSystems

INTRODUCTION
While tremendous progress has been made in recent years on the technological underpinnings of magnetic sensor ICs, the environments in which these devices are used have been rapidly changing in ways that confound their usage. The increasing densityof high-power electronics inside automotive and industrial applications has led to the rise of the phenomena of stray fields—thepresence of extraneous magnetic fields that interfere with the successful tracking of a target object via its magnetic profile.
Allegro MicroSystems has been designing magnetic sensor ICs for more than thirty years and is a leader in the supply of magnetic sensor ICs for the automotive industry. The increase of electrification in vehicles and the proliferation of external magnetic
perturbations in applications has required Allegro to adapt its designs to mitigate stray field effects in order to continue to offer
accurate speed sensor ICs.
Stray fields are a particular challenge for magnetic sensor ICs since they are a disturbance of the very effect that is intended to besensed. One way to consider this predicament is to imagine trying to read a scrolling news ticker while there are nearby lights
flashing on and off. Whether or not one is able to read the words as they appear depends on the size and clarity of the text, as
well as the intensity and orientation of the distracting light sources. In the same way, magnetic sensor IC performance during
stray field events depends on the quality of the desired signal as well as the intensity and orientation of stray field sources.
This document provides an overview of the phenomena of stray magnetic fields, its various sources, solutions developed tominimize its effects, and product proposals tolerant to stray field perturbations.

SOURCES OF STRAY FIELDS
For most applications, stray field perturbations are induced by an electrical device, actuator, coil, or a high-current cable mounted
near the sensor IC. In some instances, the source of the magnetic perturbation can also be external to the vehicle, as in the caseof tram or train rails, or has been observed on some bridges where the residual effect of the construction and test process is apermanent DC magnetic field over the bridge.
The above-mentioned perturbations will generate a magnetic stimulus in addition to the magnetic signal to be sensed, acting
like an added DC offset to the signal or an added AC fluctuation. The impact on the speed sensor IC could be output inaccuracy,
additional output edges, or even latch-up in a worst-case scenario.
N

N

S
S

Current
in WireLoopof WireSolenoid

Bar Magnet

Figure 1: Magnetic Field Sources

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APPLICATION INFORMATIONIn engine management, cam and crank speed sensor ICs provide the information necessary for an ECU to determine engine
position and the timing of fuel injection for a given cylinder. The engine environment is becoming increasingly condensed, andthe sensor ICs must provide accurate output while sometimes exposed to magnetic perturbation from an actuator mounted next
to it. Well-known examples are fuel injectors using a coil to move the injection nozzle, engine starter generating some magnetic
field pulses at start-up, and the operation of coil-driven engine valves. Actuators are not the only source of perturbation—a simple
cable next to the device will generate a magnetic field. The level of perturbation will depend on the distance from the sensor IC
to the cable and on the level of current—in hybrid vehicles, a peak current level of 300 to 500 A peak flowing through a cable
is common and may generate significant magnetic perturbation. The Biot-Savart Law as shown in Equation 1, can be used to
calculate the magnetic field produced by this scenario (illustrated in Figure 4).
Equation 1:

B=

μ0I
2πr

Here, μ0 is the permeability of the free space, I is the current flowing in the conductor, r is the distance from the conductor to thesensor IC, and a long wire (relative to r) is assumed. Assuming 500 A flowing in a conductor placed 10 cm away from a sensing
element yields Equation 2 below.
Equation 2:B=(4π × 10-7 H⁄m)(500 A)(2π)(0.01 m)= 10 GThus, the stray field seen by a sensing element under these conditions would be 10 G.
Transmission speed sensor ICs typically encounter relatively little stray field interference due to their location. In general, being set
back from the engine block, they are distanced from major sources of stray fields like starter coils, and because they are inserted
into the transmission, they can benefit from some amount of shielding from the case. There are still some stray field sources toconsider, such as shift solenoids, but they are typically minor and very manageable.
Finally, wheel speed sensor ICs see numerous sources of stray field interference including railroad tracks, heating coils embedded in garage floors, powerlines, and (again) high current wires throughout the vehicle.

TARGET IMPACT
Allegro speed sensor ICs provide speed (and/or direction) information for a rotating target by means of digital output transitions
representing target profile (speed only output) or output pulses with defined width depending on direction of rotation (speed
and direction). The target will be one of two types: a ring magnet or ferromagnetic target.
A ring magnet target is a magnetic trigger wheel generally made of plasto-magnetic material which is magnetized in such a waythat a period is made of adjacent north and south magnetic poles. The target can have as many of these pole-pairs (periods) as
magnetization and required dimension permit, though a range of 30 to 90 pole-pairs are used in targets across applications.
Since such targets are made of material which cannot concentrate external magnetic fields, they will not influence speed sensor IC
performance over stray field. That is, the stray field performance of the speed sensor IC can be tested with or without the target.
Ferromagnetic targets are used in a majority of applications and require the magnetic sensor IC to be back-biased by a magnet
to detect rotation. These targets will influence speed sensor IC stray field performance because they concentrate magnetic
lines—including those of stray magnetic fields—toward the magnetic sensor elements. For such cases, it is therefore recommended to perform stray field tests with the application target.
To illustrate the impact of ferromagnetic targets on an external magnetic field, the field strength can be measured with and withouta ferromagnetic target in a Helmholtz coil. When a ferromagnetic target is placed in a Helmholtz coil, it acts as a concentratorof the flux lines and significantly increases the level of magnetic perturbation at the sensor position as illustrated in Figure 2. For
example, applying a field of 1000 A/m RMS will generate 36 G peak-to-peak of homogenous AC perturbation in the flux density at the sensor IC location (at the center of the coil) when no ferromagnetic material (target) is installed. If a target is placed in
the coil, the flux density at the sensor IC location can increase up to 103 G peak-to-peak at an air gap of 2.5 mm. The air gap isdefined as the distance from active face of the sensor IC to the target tooth.

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APPLICATION INFORMATION

Magne�c Field Measured with Calibrated Linear Hall Device
200

Magne�c Field (G)Without Target
With Target Tooth 2.5 mm

180
160
Almost 3 �mes larger than the reference ﬁeld !

140

Magne�c ﬁeld generated bycoil with target.
Distance from target tosensor: 2.5 mm

120
100
80

Magne�c ﬁeld generated bycoil without target

60
40
36 G
Reference
20
0

0

200

400

600

800

1000

1200

Reference Point for 1000 A/m

1400

1600

1800

Coil Current (mA)

Figure 2: Impact of Ferromagnetic Target in a Helmholtz Coil

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APPLICATION INFORMATIONTEST METHODOLOGIESThe first major specification on the testing of stray fields was the US DOD’s MIL-STD-461 in 1967, and this document is the originof the radiating loop coil as it is used to this day. At present, the primary specification defining stray magnetic field testing is
the ISO 11452-8 standard, “Immunity to Magnetic Fields”; however, most automotive manufacturers have created their own
offshoot specifications, some even with variant waveforms. A composite of these various specifications is included in Figure 3.
104

Stray Field Requirement (A/m RMS)

103

102

101

100

10–1
100

101

102

103

104

105

106

Frequency (Hz)Figure 3: Stray Field Requirements

To test magnetic sensor ICs to these various specifications, three established test methodologies are commonly used: the single
current wire, Helmholtz coil, and radiating loop tests.
The single current wire test specification is defined by the customer to be as close as possible to the application configuration. It
consists of positioning a wire (generally a straight wire) at a distance, r, to the sensor IC under test (and its corresponding rotatingtarget) in a specified orientation. A specified current, I, is then forced through the wire as illustrated in Figure 4. The current isgenerally applied continuously (DC) but can also consist of an applied current step.

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APPLICATION INFORMATIONI
AppliedCurrent

Magne�c ﬁeld
generated bycurrent
Current conductor
Target toothRota�ng targetr

Device under test

Air Gap
Figure 4: Single Current Wire Test

The Helmholtz coil test consists of two identical circular coils symmetrically placed and separated by a distance equal to theradius of the coils as illustrated in Figure 5. The radius is generally specified by ISO 11452-8 to be a minimum of 150 mm, which
has the advantage to generate a relatively large homogenous magnetic field zone around its center and permits for a full system
to be tested. Often defined as the “reference” setup in ISO11452-8 specification, the ISO standard also refers to the radiating
loop coil as an “optional” setup.

Figure 5: Helmholtz Coil Generating Large Homogeneous Field Area

Finally, the radiating loop coil test uses a single 12 cm diameter, 20-turn coil which is calibrated to a test point 5 cm from its
center. In contrast with the Helmholtz coil, the radiating loop is more susceptible to misalignment because it does not have alarge homogenous field. However, for the same reason it is less impacted by nearby metal objects, like ferromagnetic targets.

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APPLICATION INFORMATIONMITIGATION METHODOLOGIESThere are several methods manufacturers can employ to mitigate the stray field interference that will be seen by a sensor IC,
including wire routing, shielding, and IC and field source orientation.
Proactive wire routing can greatly diminish the scale of stray field interference by orders of magnitude. By maximizing the physicaldistance between any high-current conductors and sensor ICs sensitive to magnetic interference, manufacturers can leverage
the inverse nature of the Biot-Savart law as explored in Equation 1 and Equation 2.
Disregarding complex active techniques, magnetic shielding can be broken down to two methods, with the major difference
between them being the frequency of the external field. For alternating field in general, a conductive surface—such as sheet
metal—can reflect radiation through eddy currents which respond with an opposing field. The conductor thickness necessary
for this shielding depends on the material properties and the frequency of the signal due to the skin effect; higher frequencies
have a smaller skin depth and thus require less material as described in Equation 3.
Equation 3:
Here, δ is the skin depth, ρ is the resistivity of the material, μr is the relative permeability of the material, and μ0 is the permeabilityof free space. The other method of shielding, for low-frequency or constant magnetic fields, also involves the use of a metal
surface, but the primary factor is the magnetic permeability of the material rather than conductivity. With shielding constructedfrom such metals, the magnetic field is redirected through rather than reflected by the surface. These shielding methods areillustrated in Figure 6.

Magnetostatic Shielding
Flux-Shunting Mechanism

Shielding by Eddy Currents
Induced Currents Mechanism

Figure 6: Flux Shunting and Eddy Currents

The orientation of the interference field source relative to the magnetic sensor IC is also an important consideration. Hall-effectbased speed sensor ICs are sensitive to fields perpendicular to the die (into the branded face of the IC) whereas GMR-based speed
sensor ICs are sensitive to fields in the plane of the die (parallel to the branded face of the IC). Where possible, sources of stray fieldinterference can be oriented such that its incidence angle as seen by the speed sensor IC reduces or even eliminates interference.
The same example of 500 A flowing into a single-current wire explored in Equation 2 can be used to demonstrate the impactof sensor orientation. In an experiment, the Allegro single Hall cam sensor ATS16301PSL was positioned at 10 cm from thewire, but at two different sensing positions along the target rotation axis—at 0° and 90°, as illustrated in Figure 7. This experiment highlighted the implied magnetic offset was doubled when the sensitive axis of the device was aligned with the external
perturbation, and a small offset likely generated by the target concentrating magnetic flux lines remained observable when theperturbation was applied to the non-sensitive orientation.

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APPLICATION INFORMATIONI

Current ConductorrAppliedCurrent
Device Under Test

Simulated Target signal
without DC current (I) applied
Simulated Target signal with
DC current (I) appliedr

Device Under Test

Air Gap

Orienta�on #1 – 500 A

Single Hall Sensor – Air Gap = 3.0 mm~10 G of Oﬀset

Orienta�on #2 – 500 A
Single Hall Sensor – Air Gap = 3.0 mm~5 G of Oﬀset

Simulated Target
signal with DC
current (I) applied
Simulated Target
signal without
DC current (I)appliedFigure 7: Impact of External Field Orientation to Single Hall Cam Sensor (ATS16301PSL)

There are several device-side methods which can also be used to mitigate any stray field interference, including magnet designin packages with integrated magnets (back-biased devices), differential sensing architectures, and the fine tuning of certain
algorithm parameters.
Magnet design in back-biased devices can be made to improve the ratio of external magnetic perturbation over sensing signal
peak-to-peak, somewhat like a magnetic signal-to-noise ratio. The simplest way is to design the largest magnet possible—so long as
magnetic field strength stays within device input range, of course—leading to a larger sensing signal and smaller external magnetic
field impact. Some other magnet designs require less magnetic material but more technical shapes, like “donut” or “sandwich”
magnets. The main advantage of these technical designs is to maintain a high sensing magnetic signal range while minimizing thecommon mode (baseline) field at the sensing location—this allows for both higher stray field rejection and minimization of offset
drift over temperature.
Differential sensing is a simple if powerful method to mitigate stray field interference. To the extent that the wavelength of any
interference is sufficiently large relative to the spacing of sensor elements internal to the IC, the interference will present as acommon mode signal and thus will be mathematically eliminated in a differential sensing architecture.
There are several algorithmic adjustments which can be made to mitigate stray field interference. The most common of theseused in Allegro speed sensor ICs is dynamic signal tracking. The benefit of this feature is to eliminate any switch point error due
to offset. For example, the DC magnetic offset generated by the single current wire in Figure 4 would have no effect in mostAllegro speed sensor ICs, because the peak tracker would eliminate this offset. A second feature of Allegro speed sensor ICs
minimizing impact of external magnetic perturbation is the switch-point calculation method using a percentage of the signal peakto-peak to define switch point. Any impact on sensitivity is attenuated or even eliminated. Determination of the threshold level forswitch point and the peak-to-peak hysteresis is a subtle way to mitigate AC magnetic perturbation effects. It has been observedthat the amplitude of stray field interference as seen by the sensor IC is larger under certain target conditions than others—when
sensing a tooth as opposed to a valley, for example. Thus, the threshold position can be adjusted in the direction of the sensing region that experiences the lowest stray field interference amplitude, while the peak-to-peak hysteresis can be adjusted tomarginally exceed the highest stray field interference amplitude. These adjustments can greatly increase performance even in
the presence of significant interference.
The sensing element technology also plays a role. The same single current wire scenario previously explored (500 A flowing in asingle current wire and sensor IC positioned 10 cm away) was simulated to compare a single Hall cam sensor IC, the ATS16301PSL,
to Allegro’s newest GMR cam sensor, the ATS16351PSM. Since the sensing signals were different, it was more appropriate tocompare the level of offset to the signal amplitude in percentage of the peak-to-peak as displayed in Figure 8.

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APPLICATION INFORMATIONI

Current conductorrr

AppliedCurrent
Device under test

Device under test

500 A - ATS16301PSL
Single Hall Sensor

500 A - ATS16351PSM
GMR Cam Sensor

Figure 8: Comparison of Hall and GMR Cam Sensor ICs Over a Single Current Wire Magnetic Perturbation

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APPLICATION INFORMATIONCONCLUSION
As discussed throughout this document, the sources of stray magnetic fields are numerous, multiplying, and are increasingly
proximate to magnetic sensor ICs across applications. Allegro MicroSystems continues to adapt to these evolving challenges
by both implementing the mitigation methodologies discussed here and pursuing active research on new solutions. Rigorous
testing using the three methods discussed here establishes the efficacy of these efforts and ensures device performance to any
relevant specifications—the ISO 11452-8 standard or customer specific.
Allegro’s comprehensive magnetic speed sensor IC portfolio includes both GMR and Hall technologies, both back-biased andnon-back-biased, with many output protocol options—a solution for any application need. Some of the available stray-field-tested
Hall and GMR options for various applications are included in Table 1 below.
Table 1: Released Allegro Speed Sensor ICs
DeviceTechnology

Application Area

ATS16351

Back-Biased GMR

Cam

ATS16951

Back-Biased GMR

Crank

A1696

Hall

Crank

ATS696Back-Biased Hall

Crank

A19520Hall

Transmission

ATS19520

Back-Biased Hall

Transmission

ATS19580

Back-Biased GMR

Transmission

A19420Hall

Transmission

ATS19420

Back-Biased Hall

Transmission

A19350

GMR

Wheel Speed

A19200Hall

Wheel Speed

A19250

GMR

Wheel Speed

ATS19200

Back-Biased Hall

Wheel Speed

A17501Hall

xEV Speed

A17502Hall

xEV Speed

It is important to note that many of these devices can also be used in various industrial applications. Contact an Allegro sales
representative for more information.

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Revision History
NumberDate

Description–

March 25, 2022

Initial release

Copyright 2022, 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 notguarantee 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 meetsall design requirements.
Copies of this document are considered uncontrolled documents.

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