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Application Notes
Hysteresis in GMR Sensors
All magnets and magnetic materials (iron, nickel, etc.) have magnetic hysteresis. Hysteresis refers to
the history of the magnetic field applied to the material and how it affects the material properties and
performance. NVE’s GMR sensors are made of magnetic materials, so they are subject to hysteresis
effects.
Hysteresis can make GMR sensors easier or harder to use, depending on the application. In nearly all
digital applications, hysteresis is desirable because it prevents “jitter” at the sensor operate point. With
sensor elements that do not have hysteresis, electrical hysteresis is normally built into the signal
processing electronics.
In a linear application, hysteresis can be problematic, but this depends on the application. For example,
if a GMR sensor is used as a current sensor, and it is detecting the magnetic field from a repetitive
current such as a sinusoidal waveform, then the recent magnetic history at sensor will always be the
same. This will result in repeatable output from the sensor at each current level. In this case, hysteresis
is not an issue. Another important point is that time is not a factor, only the magnitude of the fields that
the sensor is exposed to. For example, if the frequency of the current’s sinusoidal waveform changes,
or if the current stops at a given level for some period of time, and then restarts in the same sinusoidal
pattern, there will be no hysteresis effects at the sensor.
On the other hand, if the magnetic field to be detected is not repetitive in magnitude, but more random
in nature, then an error can result in the sensor output reading. The size of this error will depend on the
amount of hysteresis in the sensor element and the difference in the polarity and magnitudes of the
applied fields that the sensor was recently exposed to. The error will take the form of a voltage offset
change in the sensor element.
Because the error is essentially an offset change, it can be eliminated in cases where the signal is high
in frequency by AC coupling the output of the sensor to an amplifier. This is a common solution in
applications such as currency detection where a very small signal that can be random in nature must be
detected from a moving object. AC coupling and a high gain amplifier are employed to see this small
signal with GMR sensors.
If DC coupling the sensor to an amplifier or output stage is required, and the magnetic field will not be
repeatable, then the hysteresis in the sensor element must be taken into account as a potential error in
the reading. For NVE’s AA-Series sensors, this potential error can be as high as 4% if the sensor is
exposed to one polarity of magnetic field (unipolar mode of operation), and as high as 20% if the
sensor is exposed to a bipolar field. However, even in these cases the large output signal of the GMR
sensor elements can provide advantages over other technologies.
The polarity of the magnetic field that is applied to the sensor has a strong effect on the amount of
hysteresis. Unipolar operation is when the applied magnetic field at the sensor is always in the same
polarity, or direction. Bipolar operation is when the magnetic field at the sensor changes direction.
Hysteresis is much more exaggerated in the sensor element when a bipolar magnetic field is applied to
the sensor.
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www.nve.com phone: 952-829-9217 fax: 952-829-9189
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