LINEAR MAGNETIC FIELD SENSORS
WHAT IS SET/RESET STRAP?
longer, to conserve power. The only requirement is that
Most low field magnetic sensors will be affected by large
magnetic disturbing fields (>4 - 20 gauss) that may lead to
outputsignaldegradation.Inordertoreducethiseffect,and
maximizethesignaloutput,amagneticswitchingtechnique
can be applied to the MR bridge that eliminates the effect
of past magnetic history. The purpose of the Set/Reset
(S/R) strap is to restore the MR sensor to its high sensitivity
stateformeasuringmagneticfields. Thisisdonebypulsing
a large current through the S/R strap. The Set/Reset (S/R)
straplookslikearesistancebetweentheSR+andSR-pins.
This strap differs from the OFFSET strap in that it is
magnetically coupled to the MR sensor in the cross-axis, or
insensitive, direction. Once the sensor is set (or reset), low
noise and high sensitivity field measurement can occur. In
the discussion that follows, the term “set” refers to either a
set or reset current.
each pulse only drive in one direction. That is, if a +3.5 amp
pulse is used to “set” the sensor, the pulse decay should not
drop below zero current. Any undershoot of the current
pulse will tend to “un-set” the sensor and the sensitivity will
not be optimum.
Using the S/R strap, many effects can be eliminated or
reduced that include: temperature drift, non-linearity errors,
cross-axis effects, and loss of signal output due to the
presence of a high magnetic fields. This can be accom-
plished by the following process:
• A current pulse, Iset, can be driven from the S/R+ to the
S/R- pins to perform a “SET” condition. The bridge output
can then be measured and stored as Vout(set).
• Another pulse of equal and opposite current should be
driven through the S/R pins to perform a "RESET" condi-
tion. The bridge output can then be measured and stored
as Vout(reset).
When MR sensors exposed to a magnetic disturbing field,
the sensor elements are broken up into ramdonly oriented
magnetic domains (Figure 4A) that leads to sensitivity
degrading. A current pulse (set) with a peak current above
minimum current in spec through the Set/Reset strap will
generate a strong magnetic field that realigns the magnetic
domainsinonedirection(Figure4B).Thiswillensureahigh
sensitivityandrepeatablereading.Anegativepulse(Reset)
will rotate the magnetic domain orientation in the opposite
direction (Figure 4C), and change the polarity of the sensor
outputs.Thestateofthesemagneticdomainscanretainfor
years as long as there is no magnetic disturbing field
• The bridge output, Vout, can be expressed as: Vout
=
[Vout(set) - Vout(reset)]/2. This technique cancels out
offsetandtemperatureeffectsintroducedbytheelectron-
ics as well as the bridge temperature drift.
There are many ways to design the set/reset pulsing circuit,
though, budgets and ultimate field resolution will determine
whichapproachwillbebestforagivenapplication. Asimple
present.
Permalloy (NiFe) Resistor
set/reset circuit is shown in Figure 5.
6-9V
Random
Domain
3
25K
RESET
SET
RESET
Orientations
4
2
Signal should be in
RESET state when idle
0.2µF
Fig.4A
Magnetization
Easy Axis
S/R+
S/R-
0.1µF
5,6
7,8
Signal input
After a
5V
1
Set Pulse
Manual Switch
Fig.4B
Figure 5—Single-Axis Set/Reset Pulse Circuit (1001)
Magnetization
After a
The magnitude of the set/reset current pulse depends on
the magnetic noise sensitivity of the system. If the minimum
detectable field for a given application is roughly 500
µgaussinHMC1001/2,thena3amppulse(min)isadequate.
If the minimum detectable field is less than 100 µgauss,
then a 4 amp pulse (min) is required. The circuit that
generates the S/R pulse should be located close to the MR
sensor and have good power and ground connections.
Reset Pulse
Fig.4C
Figure 4—
The on-chip S/R should be pulsed with a current to realign,
or“flip”, themagneticdomainsinthesensor. Thispulsecan
be as short as two microsecond and on average consumes
less than 1 mA dc when pulsing continuously. The duty
cycle can be selected for a 2 µsec pulse every 50 msec, or
The set/reset straps on the Honeywell magnetic sensors
are labeled S/R+ and S/R-. There is no polarity implied
since this is simply a metal strap resistance.
8
HONEYWELL [ Honeywell ]
