HDPM01
6.1. Compass measurement
THEORY:
The anisotropic magnetoresistive (AMR) sensors are special resistors made of permalloy thin film
deposited on a silicon wafer. During manufacturing, a strong magnetic field is applied to the film to
orient its magnetic domains in the same direction, establishing a magnetization vector. Subsequently,
an external magnetic field applied perpendicularly to the sides of the film causes the magnetization to
rotate and change angle. This in turn causes the film’s resistance to vary. The HOPERF AMR sensor is
included in a Wheatstone bridge, so that the change in resistance is detected as a change in differential
voltage and the strength of the applied magnetic field may be inferred.
However, the influence of a strong magnetic field (more than 5.5 gausses) along the magnetization axis
could upset, or flip, the polarity of the film, thus changing the sensor characteristics. A strong restoring
magnetic field must be applied momentarily to restore, or set, the sensor characteristics. The HOPERF
magnetic sensor has an on-chip magnetically coupled strap: a SET/RESET strap pulsed with a high
current, to provide the restoring magnetic field.
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