HMC1001/1002/1021/1022
These resistive elements are aligned together to have a common sensitive axis (indicated by arrows on the pinouts) that
will provide positive voltage change with magnetic fields increasing in the sensitive direction. Because the output only is in
proportion to the one-dimensional axis (the principle of anisotropy) and its magnitude, additional sensor bridges placed at
orthogonal directions permit accurate measurement of arbitrary field direction. The combination of sensor bridges in two
and three orthogonal axis permit applications such as compassing and magnetometry. See Figure 1 for a representation
of the magneto-resistive elements.
Permalloy Thin Film
Out-
Vb
Gnd
Easy Axis
Out+
Sensitive
Axis
Figure 1 – Magneto-Resistive Wheatstone Bridge Elements
The offset strap allows for several modes of operation when a direct current is driven through it. These modes are: 1)
Subtraction (bucking) of an unwanted external magnetic field, 2) null-ing of the bridge offset voltage, 3) Closed loop field
cancellation, and 4) Auto-calibration of bridge gain.
The set/reset strap can be pulsed with high currents for the following benefits: 1) Enable the sensor to perform high
sensitivity measurements, 2) Flip the polarity of the bridge output voltage, and 3) Periodically used to improve linearity,
lower cross-axis effects, and temperature effects.
Offset Straps
The offset strap is a spiral of metallization that couples to the sensor element’s sensitive axis. The offset strap has some
modest resistance and requires a moderate current flow for each gauss of induced field. The straps will easily handle
currents to buck or boost fields through the linear measurement range, but designers should note the extreme thermal
heating on the die when doing so.
With most applications, the offset strap is not utilized and can be ignored. Designers can leave one or both strap
connections (Off- and Off+) open circuited.
Set/Reset Straps
The set/reset strap is another spiral of metallization that couples to the sensor element’s easy axis (perpendicular to the
sensitive axis on the sensor die). Each set/reset strap has a low resistance with a short but high required peak current for
reset or set pulses. With rare exception, the set/reset strap must be used to periodically condition the magnetic domains
of the magneto-resistive elements for best and reliable performance. A set pulse is defined as a positive pulse current
entering the S/R+ strap connection. The successful result would be the sensor aligned in a forward easy-axis direction so
that the sensor bridge’s polarity is a positive slope with positive fields on the sensitive axis result in positive voltages
across the bridge output connections.
A reset pulse is defined as a negative pulse current entering the S/R+ strap connection. The successful result would be
the sens or aligned in a reverse easy-axis direction so that sensor bridge’s polarity is a negative slope with positive fields
on the sensitive axis result in negative voltages across the bridge output connections.
Typically a reset pulse is sent first, followed by a set pulse a few milliseconds later. By shoving the magnetic domains in
completely opposite directions, any prior magnetic disturbances are likely to be completely erased by the duet of pulses.
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