6 Preliminary Document. Subject to change. www.magneticsensors.com
BASIC DEVICE OPERATION
The Honeywell HMC1043L magnetoresistive sensors are Wheatstone bridges to measure magnetic fields. With power
supply applied to the bridges, the sensors convert any incident magnetic field in the sensitive axis directions to a
differential voltage outputs. In addition to the bridge circuits, each sensor has two on-chip magnetically coupled straps; the
offset strap and the set/reset strap. These straps are Honeywell patented features for incident field adjustment and
magnetic domain alignment; and eliminate the need for external coils positioned around the sensors.
The magnetoresistive sensors are made of a nickel-iron (Permalloy) thin-film deposited on a silicon wafer and patterned
as a resistive strip element. In the presence of a magnetic field, a change in the bridge resistive elements causes a
corresponding change in voltage across the bridge outputs.
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.
The offset straps allow for 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 in the sensor element’s sensitive axis. The straps will easily
handle currents to buck or boost fields through the ±6 gauss 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, or ground one connection node. Do not tie both strap connections together to
avoid shorted turn magnetic circuits.
Set/Reset Straps
The set/reset strap is another spiral of metallization that couples to the sensor elements easy axis (perpendicular to the
sensitive axis on the sensor die. 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
magnetic domains 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 magnetic domains 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 will be completely erased by the duet of pulses. For
simpler circuits with less critical requirements for noise and accuracy, a single polarity pulse circuit may be employed (all
sets or all resets). With these uni-polar pulses, several pulses together become close in performance to a set/reset pulse
circuit. Figure 1 shows an H-Bridge Set/Reset circuit that will generate both set and reset pulses to the set/reset strap.
Additional information and examples on set/reset pulse circuits can be found in our application notes AN201 and AN213.
