Giant magneto-resistive sensing makes better current and magnetic field sensors

Allegro has added magneto-resistive sensors to its Hall-based range of current and magnetic field devices, for high-sensitivity high-bandwidth applications.

Allegro-ACS70331-GMR-sensor

The first product is a 5A 1MHz isolated current sensor.

Based on GMR – the giant magneto-resistive effect, the technology has been branded ‘xMR’ and combines magneto-resistive sensor elements and BiCMOS circuits monolithically.

“The launch is the culmination of more than five years of research and development efforts, a sizeable investment that supports Allegro’s mission to create a portfolio of magnetic sensor ICs within the automotive and industrial markets,” said the firm. “Most other solutions are not monolithic. The commitment to develop this platform has been augmented with a substantial investment in precision thin film deposition tools and equipment within an advanced BCD wafer manufacturing facility, where high-volume manufacturing is under way.”

For automotive use, the technology can be thermally stable to greater than 150°C, even in the presence of large magnetic fields.

“Hall-effect technology meets the needs of many applications in Allegro’s target markets, but there are certain applications, particularly in the areas of ADAS and high efficiency vehicles, where xMR technology offers more elegant solutions relative to Hall technology,” said v-p business development Michael Doogue. “In these applications, xMR ICs provide more accurate measurement data, enable more favourable mechanical placement of a sensor IC within a mechanical system, or create smaller form factor solutions.”

According to the firm, it is already supplying an xMR ring-magnet speed sensor IC, along with signal processing algorithms, to a Tier 1 automotive supplier for wheel speed sensors. “GMR technology enables low jitter rotational speed measurements that are not possible with Hall effect ICs,” it said. The ring speed sensor is discussed in this white paper, which also includes further GMR background reading.

It is also developing monolithic back-biased GMR sensors for engine management and transmission gear speed sensing applications, “where the same low jitter measurements and large air gap performance are essential for advanced power-train system operation”.

However, the first broadly available xMR chip is the ACS70331 5A 1MHz current sensor, capable of resolving currents below 1mA – and the first in a line of GMR current sensors.

Allegro-ACS70331-GMR-sensor-packageWithin this, the built-in isolated current conductor has a series resistance of only 1mΩ, and the QFN package version will sit on a 3 x 3mm footprint – there is also an SOIC8 version.

The GMR elements inside the ACS70331 operate differently than the Hall-effect sensors.

“The main advantage of GMR is that it is much more sensitive than the Hall-effect, making it ideal for measuring small currents. This is what enables the ACS70331 to have over 25 times lower input-referred noise than Allegro’s lowest noise Hall-effect based current sensors,” said the firm.

It is this noise level that is leading the firm to claims 25x sensitivity compared with Hall devices.

Noise in the device is 6mArms at 1MHz – the maximum bandwidth. Sensing response time is <550ns.

A subtle difference in GMR sensing is that new mechanical arrangements are needed because the applied field needs to be parallel to the surface of the sensor instead of perpendicular to the sensor plane as is required with planar Hall sensors.

Allegro-ACS70331-GMR-sensor-graphGMR elements are essentially resistors which change resistance with applied field. A typical representative response curve for one of the four GMR elements in the ACS70331 is shown in the graph.

The equation of each magneto-resistor is: R(B) = 1,000(1 – 0.04sin(tan-1(B/100))).

The basic 1,000Ω increases and decreases with field, and saturation limits usefulness at high fields – the linear limits of the ACS70331 sensing elements are around ±50G. Overload capability is limited by the current loop which can withstand up to +/-10A.

Allegro-ACS70331-GMR-sensorTo keep the magnetic plane in the same plane as the resistors, the die sits above the primary current path (see package diagram), electrically isolated from it for fields up to 100V.

To enable differential measurement of the sensed current while rejecting external (stray) fields, GMR elements 1 and 2 (see circuit digram) sense field in the +X direction for positive current current flow, and GMR elements 3 and 4 sense field in the –X direction for positive current flow.

Allegro-ACS70331-GMR-sensor-blockThe four GMR elements are arranged in a Wheatstone bridge configuration, such that the output of the bridge is proportional to the differential field sensed by the four elements, rejecting common fields.

“Theoretically, the bridge configuration will perfectly cancel out all external common-mode fields that could interfere with the sensor; however, the performance is limited by non-idealities, such as mis-match,” said Allegro.

In practice, the device is most sensitive to external fields along the x-axis, where 20G will cause +/-4.2% sensitivity error and an offset of +/-76mA, but completely insensitive to fields along the z-axis, and somewhere in between for other alignments.

Power for the chip comes from a single 3.3-4.5V rail, which does not need to be regulated due to the high power supply rejection ration, and this part works over -40 to 85°C.

There are four current range options: 0-2.5A (800mV/A), +/-2.5A (400mV/A), 0-5A (400mV/A) and +/-5A (200mV/A), in both SOIC and QFN packages.


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