The highly compact Hall effect current sensor ICs without concentrators that are now available make it possible to enhance performance of small-current differentiation by simple layout steps and shielding 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 passes adjacent to the Hall element (see figure 1).
The lack of a concentrator has the advantage of nearly eliminating magnetic hysteresis as a source of error in the IC. 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 magnetic flux lines are perpendicular to the plane of the Hall element. If greater protection against external fields is needed, a surface mount magnetic alloy shield covering the device package is recommended.
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 the Hall element from the perpendicular flux passing through the PCB from below.
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.
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This particular magnetic engine puzzle has been attracting a lot of attention lately from people who say they had similar ideas. They measure this field directly, without the use of a magnetic concentrator, which is a common feature in other magnetic devices (for example, in the Allegro® MicroSystems CA and CB packages, used for the ACS75x families of current sensor ICs). However, this also leaves the ACS71x devices less shielded from external magnetic fields that could distort the current measurement.
The Hall element only responds to the flux component that is perpendicular to its surface, and is only susceptible to magnetic fields in this direction.
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 package around the primary current path.
You can't fix the problem by making the shield stationary, for then it won't "follow" the rotating magnet. In applications where large magnetic fields may be present, care must be taken in the alignment and spacing of the Hall element relative to those fields. As shown in figure 2, although the path of the primary current, IP, is in the same plane as the Hall element, the magnetic flux vectors generated by current passing through the U-loop are perpendicular to the Hall element plane.
With this alignment, the magnetic flux would circulate in the plane of the Hall element rather than through it, and have little effect on the output of the Hall IC.

Although this is a consideration for optimizing designs, the effect of the neighboring trace is small and much less than the signal generated by the current being measured within the device.
The minimal influence of magnetic effects from an adjacent trace diminish rapidly as the distance, D, is increased. This design provides protection against perpendicular magnetic flux lines impinging from above an SOIC8-packaged Allegro current sensor IC. A simple ferrous alloy shield deflects virtually all perpendicular flux lines, effectively shielding the Hall element. Two types of ferrous alloys were used, silicon steel (SiFe), and HyMu alloy, which is a generic class of alloys that have a high magnetic permeability level, ┬Á.

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