LINEAR MAGNETIC FIELD SENSORS
WHAT IS
SET/RESET
STRAP?
Most low field magnetic sensors will be affected by large
magnetic disturbing fields (>4 - 20 gauss) that may lead to
output signal degradation. In order to reduce this effect, and
maximize the signal output, a magnetic switching technique
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
state for measuring magnetic fields. This is done by pulsing
a large current through the S/R strap. The Set/Reset (S/R)
strap looks like a resistance between the SR+ and SR- 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.
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
domains in one direction (Figure 4B). This will ensure a high
sensitivity and repeatable reading. A negative pulse (Reset)
will rotate the magnetic domain orientation in the opposite
direction (Figure 4C), and change the polarity of the sensor
outputs. The state of these magnetic domains can retain for
years as long as there is no magnetic disturbing field
present.
Permalloy (NiFe) Resistor
Random
Domain
Orientations
0.2µF
S/R+
5,6
7,8
longer, to conserve power. The only requirement is that
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).
• The bridge output, Vout, can be expressed as: Vout =
[Vout(set) - Vout(reset)]/2. This technique cancels out
offset and temperature effects introduced by the electron-
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
which approach will be best for a given application. A simple
set/reset circuit is shown in Figure 5.
6-9V
3
25K
IRF7105
4
RESET
SET
RESET
Signal should be in
RESET state when idle
Easy Axis
Magnetization
Fig.4A
0.1µF
2
Signal input
5V
Manual Switch
After a
Set Pulse
S/R-
1
Fig.4B
Magnetization
Figure 5—Single-Axis Set/Reset Pulse Circuit (1001)
After a
Reset Pulse
Fig.4C
Figure 4—
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
µgauss
in HMC1001/2, then a 3 amp pulse (min) is adequate.
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.
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.
The on-chip S/R should be pulsed with a current to realign,
or “flip”, the magnetic domains in the sensor. This pulse can
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
8
HONEYWELL [ HONEYWELL SOLID STATE ELECTRONICS CENTER ]
