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參數(shù)資料
| 型號(hào)： | HMC1021Z |
| 英文描述： | Magnetoresistive Sensor |
| 中文描述： | 磁阻傳感器 |
| 文件頁數(shù)： | 7/15頁 |
| 文件大?。?/td> | 190K |
| 代理商： | HMC1021Z |
HMC10XX FAMILY
7
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,
then it can be compensated for by applying an equal and
opposite field using the OFFSET strap. Another use for the
OFFSET strap would be to drive a current through the strap
that will exactly cancel out the field being measured. This
is called a closed loop configuration where the current
feedback signal is a direct measure of the applied field.
The field offset strap (OFFSET+ and OFFSET-) will generate
a magnetic field in the same direction as the applied field
being measured. This strap provides a 1 Oersted (Oe) field
per 50 mA of current through it. (Note: 1 gauss=1 Oersted
in air). That is, if 25 mA were driven from the OFFSET+ pin
to the OFFSET- pin, a field of 0.5 gauss would be added to
any ambient field being measured. Also, a current of -25
mA would subtract 0.5 gauss from the ambient field. The
OFFSET strap looks like as a nominal 2.5 ohm resistance
between the OFFSET+ and OFFSET- pins.
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.
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 field the MR sensor measures—call that the delta applied
field (
Ha). The MR sensor gain is then computed as:
MRgain = (H2-H1) /
Ha
There are many other uses for the OFFSET strap than
those 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.
NOISE CHARACTERISTICS
The noise density curve for a typical MR sensor is shown
in Figure 3. The 1/f slope has a corner frequency near 10
Hz and flattens out to 3.8 nV/
√
Hz. This is approximately
equivalent to the Johnson noise (or white noise) for an
850
resistor—the typical bridge resistance. To relate the
noise density voltage in Figure 3 to the magnetic fields, use
the following expressions:
For Vsupply=5V and Sensitivity=3.2mV/V/gauss,
Bridge output response =
or
16 mV/gauss
16 nV/
μ
gauss
The noise density at 1Hz
≈
30nV/
√
Hz
and corresponds to
1.8
μ
gauss/
√
Hz
For the noise components, use the following expressions:
1/f noise(0.1-10Hz) =
30 *
√
(ln(10/.1)) nV
64 nV (rms)
4
μ
gauss (rms)
27
μ
gauss (p-p)
white noise (BW=1KHz) =
3.8 *
√
BW nV
120 nV (rms)
50
μ
gauss (p-p)
1
10
100
1000
0.1
1
10
100
1000
Frequency (Hz)
N
√
H
Figure 3—Typical Noise Density Curve
WHAT IS OFFSET STRAP
(1001/1002)
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