Description

Anisotropic Magnetoresistive (AMR) thin film materials are gaining increasing significance in contemporary position sensing technologies. Magnetoresistive (MR) position measurement offers numerous advantages compared to traditional methods, with reliability, accuracy, and overall robustness being key factors driving the advancement of MR sensing technologies. Features such as low cost, compact size, contactless operation, wide temperature range, dust and light insensitivity, and operation across a broad magnetic field spectrum collectively contribute to a resilient sensor design.

The MR effect, which involves a material altering its electrical resistance in response to changes in the direction or magnitude of an externally applied magnetic field, forms the foundation of AMR materials. These materials operate in two distinct areas: high field and low field. This application note focuses on high field applications, where the externally applied magnetic field far exceeds the internal field of the sensor, leading to saturation. In saturation mode, the change in resistance is solely dependent on the field direction, irrespective of the applied field strength.

Owing to the characteristics of AMR films, the resistance change for opposing directions is identical, making it challenging for the sensor to differentiate between a north and south magnetic pole. Consequently, the output information for a single rotating dipole magnet repeats twice over a complete mechanical revolution, limiting the measurement range to 180°. The change in resistance is mathematically modeled by the equation:

where:
R is the sensor resistance.
R₀ is the unexcited sensor resistance.
ΔR₀ is the change in sensor resistance.

For AMR sensors, in general, ΔR₀ is approximately 3% of the overall resistance of the bridge. Due to this modest change in resistance, an instrumentation amplifier becomes essential to further amplify the output signal to a usable value proportional to the supply voltage.