Application Information
Managing External Magnetic Field Interference
When Using ACS71x Current Sensor ICs
by Richard Dickinson and William Bentley
ABSTRACTINTRODUCTION

The highly compact Hall-effect current sensor
ICs without concentrators that are now available
make it possible to enhance performance of smallcurrent differentiation by simple layout steps andshielding the device from extraneous magnetic
flux along its primary sensing axis.

The ACS71x families of Hall-effect-based electrical current sensor ICs measure current by sensing the magnetic field it generates as it passesadjacent to the Hall element (see figure 1). They
measure this field directly, without the use of amagnetic concentrator, which is a common featurein other magnetic devices (for example, in theAllegro MicroSystems CA and CB
packages, used for the ACS75x families ofcurrent sensor ICs).

U-Loop

8IP
1IP

Figure 1. Current Path in an ACS71x device. Current passes
in either direction through the U-loop and around the Hallelement (X). The U-loop is mounted underneath the die inthe SOIC8 package.

AN26030, Rev. 3
MCO-0001258

The lack of a concentrator has the advantage ofnearly eliminating magnetic hysteresis as a sourceof error in the IC. However, this also leaves theACS71x devices less shielded from external
magnetic fields that could distort the current measurement. In applications where large magnetic
fields may be present, care must be taken in thealignment and spacing of the Hall element relative
to those fields. Shielding the device may also be
desirable in some circumstances.
ALIGNMENT
Flux lines form circles around a conductor inplanes that are orthogonal to the direction of current flow through a conductor. The Hall element
only responds to the flux component that is per-May 11, 2022

pendicular to its surface, and is only susceptible
to magnetic fields in this direction. As shown infigure 2, although the path of the primary current,
IP , is in the same plane as the Hall element, themagnetic flux vectors generated by current passing through the U-loop are perpendicular to theHall element plane. Those that intersect the Hallelement induce a voltage across it, which is thenamplified and used to generate the output voltage.
Magnetic Flux Induced by Current FlowHall ElementIP

High-current conductors in the vicinity of thedevice should be, if possible, oriented perpendicular to the plane on the board on which thedevice package is mounted. This is shown infigure 3. With this alignment, the magnetic flux
would circulate in the plane of the Hall element
rather than through it, and have little effect on theoutput of the Hall IC.
SPACING AND LAYOUT GUIDELINES
When laying out a PCB where the ACS71x device
will be mounted, it is good practice to keep asmuch space as possible between the device andother traces carrying significant amounts of current. Figure 4 shows the measured effect of anadjacent current trace on the same side of the PCB
as the device. Although this is a consideration foroptimizing designs, the effect of the neighborMagnetic Flux Induced
by Current Flow

Figure 2. U-Loop and Hall Element. Current flowing throughthe U-loop generates flux lines in planes orthogonal to its
path. Flux perpendicular to the plane of the Hall element cangenerate Hall voltage.

DACS71x

Adjacent Current-Carrying Trace

Magnetic Flux Induced
by Current Flow
Response (mV/A)

Hall Element

0.70.6
0.5
0.4
0.3
0.2
0.1
00Figure 3. Adjacent Conduction Perpendicular to Hall Element
Plane. Flux lines generated perpendicular to current flow areparallel to Hall plane and do not generate Hall voltage.

246D (mm)81012Figure 4. Adjacent Conduction Traces Carrying 50 A, Effecton an ACS71x device. The minimal influence of magnetic effects from an adjacent trace diminish rapidly as the distance,
D, is increased.

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955 Perimeter RoadManchester, NH 03103-3353 U.S.A.
www.allegromicro.com

2ing trace is small and much less than the signal
generated by the current being measured within
the device.
SHIELDING
Figure 4 shows the effect of the magnetic flux
generated by an adjacent current-carrying trace on
the Hall element when the current path is oriented
in the same plane as the Hall element, and so its

Figure 5. Simple Shield. This design provides protection
against perpendicular magnetic flux lines impinging fromabove an SOIC8-packaged Allegro current sensor IC. If
necessary, a second shield can be placed on the opposite
side of the PCB.

magnetic flux lines are perpendicular to the planeof the Hall element.
If greater protection against external fields isneeded, a surface mount magnetic alloy shieldcovering the device package is recommended. A
shield such as the one depicted in figure 5 shunts
external magnetic flux away from the SOIC8
package, and results in no significant effect on
the magnetic field generated inside the packagearound the primary current path. Typical magnetic
field results are depicted in figure 6.
The shield can be fastened to the PCB by epoxy,and is it not mandatory that it have a conductive
path to ground. Because the Hall element is not
susceptible to flux lines parallel to its plane, it is
not necessary to enclose the sides of the shield. Infact, leaving the shield open in the vicinity of theIC leads is preferred for creepage and clearance
considerations.

Figure 6. Magnetic Field Map of Shield. A simple ferrous alloy shield deflects virtually all
perpendicular flux lines, effectively shielding the Hall element.

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Manchester, NH 03103-3353 U.S.A.
www.allegromicro.com

3

In cases where a high level of shielding is required, a second shield of a similar type can be attached to the underside of the PCB, protecting theHall element from the perpendicular flux passing
through the PCB from below.
EXPERIMENTAL RESULTS
An experiment was performed with ±240 G applied perpendicularly to the Hall element of an
8-pin device package, through the use of an air
core field source. The attenuation levels achievedby shields constructed of various ferrous alloys
and material thicknesses were recorded. The
results are provided below. Two types of ferrous
alloys were used, silicon steel (SiFe), and HyMu
alloy, which is a generic class of alloys that have ahigh magnetic permeability level, µ. It should be

noted that in most applications the Hall element
will not be exposed to field levels as great as 60
G. For example, up to 60 G of field would passthrough the Hall transducer only if the ACS71x
were placed within 6 mm of an adjacent busbar
carrying 500 A.
SUMMARY
The alignment, spacing, and shielding techniques
described in this note may be used as increasingly
aggressive steps to mitigate the effects of external
magnetic fields when applying ACS71x current
sensor ICs.
If you need further assistance in dealing with theeffects of external fields, please contact your local
Allegro sale office to be put in touch with one ofour applications engineers.

Test ShieldABC

Configuration

Single-sided
11-mil thick
3% SiFe

Single-sided
20-mil thick
HyMu

Double-sided (C-core)
20-mil thick
HyMu

Attenuation-2.4 dB-4.5 dB-13 dB

2.9

No shielding

2.8

shield A

Vout (V)

2.7

shield B
shield C

2.6
2.5
2.4
2.3
2.2
2.1
-60

-50

-40-30-20-1001020
Applied Field (gauss)

30

405060Allegro MicroSystems
955 Perimeter RoadManchester, NH 03103-3353 U.S.A.
www.allegromicro.com

4Revision History
NumberDate

3May 11, 2022

Description
Updated document branding and minor editorial updates

Copyright 2022, Allegro MicroSystems.
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www.allegromicro.com

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