LINEAR MAGNETIC FIELD SENSORS
NOISE CHARACTERISTICS
The noise density curve for a typical MR sensor is shown then it can be compensated for by applying an equal and
in Figure 3. The 1/f slope has a corner frequency near 10 opposite field using the OFFSET strap. Another use for the
Hz and flattens out to 3.8 nV/√Hz. This is approximately OFFSET strap would be to drive a current through the strap
equivalent to the Johnson noise (or white noise) for an that will exactly cancel out the field being measured. This is
850Ω resistor—the typical bridge resistance. To relate the calledaclosedloopconfigurationwherethecurrentfeedback
noise density voltage in Figure 3 to the magnetic fields, use signal is a direct measure of the applied field.
the following expressions:
Thefieldoffsetstrap(OFFSET+andOFFSET-)willgenerate
For Vsupply=5V and Sensitivity=3.2mV/V/gauss,
Bridge output response = 16 mV/gauss
a magnetic field in the same direction as the applied field
being measured. This strap provides a 1 Oersted (Oe) field
per50mAofcurrentthroughitinHMC1001/2and1Oe/5mA
in HMC1021/2. (Note: 1 gauss=1 Oersted in air). For
example, if 25 mA were driven from the OFFSET+ pin to the
OFFSET- pin in HMC1001/2, a field of 0.5 gauss would be
added to any ambient field being measured. Also, a current
or
16 nV/µgauss
The noise density at 1Hz ≈ 30nV/√Hz
and corresponds to 1.8 µgauss/√Hz
For the noise components, use the following expressions: of -25 mA would subtract 0.5 gauss from the ambient field.
The OFFSET strap looks like as a nominal resistance
1/f noise(0.1-10Hz) = 30 * √(ln(10/.1)) nV
64 nV (rms)
between the OFFSET+ and OFFSET- pins.
4 µgauss (rms)
27 µgauss (p-p)
The OFFSET strap can be used as a feedback element in
a closed loop circuit. Using the OFFSET strap in a current
feedback loop can produce desirable results for measuring
magnetic fields. To do this, connect the output of the bridge
amplifier to a current source that drives the OFFSET strap.
Using high gain and negative feedback in the loop, this will
drive the MR bridge output to zero, (OUT+) = (OUT-). This
method gives extremely good linearity and temperature
characteristics. The idea here is to always operate the MR
bridge in the balanced resistance mode. That is, no matter
what magnetic field is being measured, the current through
the OFFSET strap will cancel it out. The bridge always
“sees” a zero field condition. The resultant current used to
cancel the applied field is a direct measure of that field
strength and can be translated into the field value.
white noise (BW=1KHz) = 3.8 * √BW nV
120 nV (rms)
50 µgauss (p-p)
1000
(1001/1002)
100
1 0
1
The OFFSET strap can also be used to auto-calibrate the
MR bridge while in the application during normal operation.
This is useful for occasionally checking the bridge gain for
that axis or to make adjustments over a large temperature
swing. This can be done during power-up or anytime during
normal operation. The concept is simple; take two point
along a line and determine the slope of that line—the gain.
When the bridge is measuring a steady applied magnetic
field the output will remain constant. Record the reading for
the steady field and call it H1. Now apply a known current
through the OFFSET strap and record that reading as H2.
The current through the OFFSET strap will cause a change
in the field the MR sensor measures—call that delta applied
field (∆Ha). The MR sensor gain is then computed as:
0.1
1
1 0
Frequency (Hz)
100
1000
Figure 3—Typical Noise Density Curve
WHAT IS OFFSET STRAP?
Any ambient magnetic field can be canceled by driving a
defined current through the OFFSET strap. This is useful
for eliminating the effects of stray hard iron distortion of the
earth’s magnetic field. For example, reducing the effects of
a car body on the earth’s magnetic field in an automotive
compass application. If the MR sensor has a fixed position
within the automobile, the effect of the car on the earth’s
magnetic field can be approximated as a shift, or offset, in
this field. If this shift in the earth's field can be determined,
MRgain = (H2-H1) / ∆Ha
TherearemanyotherusesfortheOFFSETstrapthanthose
described here. The key point is that ambient field and the
OFFSET field simply add to one another and are measured
by the MR sensor as a single field.
7
HONEYWELL [ Honeywell ]
