HMC1051/HMC1052/HMC1053
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.
NOISE CHARACTERISTICS
The noise density for the HMR105X series is around 50nV/sqrt Hz at the 1 Hz corner, and quickly drops below 10nV/sqrt
Hz at 5Hz and begins to fit the Johnson Noise value at just below 5nV/sqrt Hz beyond 50Hz. The 10Hz noise voltage
averages around 1.4 micro-volts with a 0.8 micro-volts standard deviation.
CROSS-AXIS EFFECT
Cross-Axis effect for the HMR105X series is typically specified at ±3% of full scale to 1 gauss. See application note
AN215 regarding this effect and methods for nulling.
OFFSET STRAP
The offset strap is a spiral of metalization that couples in the sensor element’s sensitive axis. In two-axis designs, the
strap is common to both bridges and must be multiplexed if each bridge requires a different strap current. In three-axis
designs, the A and B bridges are together with the C bridge sharing a common node for series driving all three bridges’
offset straps. Each offset strap measures nominally 15 ohms, and requires 10mA for each gauss of induced field. 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 STRAP
The set/reset strap is another spiral of metalization that couples to the sensor elements easy axis (perpendicular to the
sensitive axis on the sensor die). Like the offset strap, the set/reset strap runs through a pair of bridge elements to keep
the overall die size compact. Each set/reset strap has a nominal resistance of 3 to 6 ohms with a minimum required peak
current of 400mA 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
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 are likely to be completely erased by the duet
Iset
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
5 volts
pulses, several pulses together become close in
performance to a set/reset pulse circuit. Figure 1 shows a
quick and dirty manual pulse circuit for uni-polar application
of pulses to the set/reset strap.
Figure 1
Set Pulse Circuit
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