AN-1314: AMR Angle Sensors (Rev. 0)

AN-1314
APPLICATION NOTE
One Technology Way • P.O. Box 9106 • Norwood, MA 02062-9106, U.S.A. • Tel: 781.329.4700 • Fax: 781.461.3113 • www.analog.com
AMR Angle Sensors
by Robert Guyol
INTRODUCTION
Anisotropic magnetoresistive (AMR), thin film materials are
becoming increasingly important in today’s position sensing
technologies. Magnetoresistive (MR) position measurement
has many advantages over traditional technologies. Reliability,
accuracy, and overall robustness are the primary factors
contributing to the development of MR sensing technologies.
Low cost, small relative size, contactless operation, wide
temperature range, dust and light insensitivity, and operation
over a wide magnetic field range all lead to a robust sensor design.
The MR effect is the ability for a material to change its electrical
resistance with a change in direction or magnitude of an externally
applied magnetic field. The two different areas of operation for
AMR materials are high field and low field. This application note
discusses high field applications, where the applied external
magnetic field is much greater than the internal field of the sensor,
and the sensor is said to be operating in saturation. During this
mode of operation, the change in resistance only depends on
field direction and not on the applied field strength. Due to
the nature of AMR films, the resistance change of the material
is identical for opposing directions, meaning that the sensor
itself cannot distinguish a north magnetic pole from a south
magnetic pole. Therefore, the output information for a single
rotating dipole magnet repeats twice over an entire mechanical
revolution. This effect limits the measurement range to 180°. The
change in resistance can be modeled by the following equation:
R = R0 + ΔR0cos2(α)
where:
R is the sensor resistance.
R0 is the unexcited sensor resistance.
ΔR0 is the change in sensor resistance.
For AMR sensors, in general, ΔR0 is approximately 3% of the
overall resistance of the bridge. Due to this small change in
resistance, an instrumentation amplifier is needed to further
amplify the output signal to a useable value proportional to
the supply voltage.
Rev. 0 | Page 1 of 10
AN-1314
Application Note
TABLE OF CONTENTS
Introduction ...................................................................................... 1
Magnet Selection Considerations....................................................4
Revision History ............................................................................... 2
Magnet to Sensor Relationship ........................................................5
Magnet Configurations .................................................................... 3
Misalignment and Air Gap Measurements ....................................6
Linear ................................................................................................. 3
Diagnostics .........................................................................................7
Off Shaft ............................................................................................. 3
Error Sources......................................................................................9
End of Shaft ....................................................................................... 3
Calibration Procedure .......................................................................9
Bridge Configuration ....................................................................... 3
Layout Recommendations and Magnetic Interference ............. 10
AMR Sensor Element ....................................................................... 4
VTEMP Output .............................................................................. 10
Magnetic Angle vs. Mechanical Angle........................................... 4
REVISION HISTORY
10/14—Revision 0: Initial Version
Rev. 0 | Page 2 of 10
Application Note
AN-1314
MAGNET CONFIGURATIONS
END OF SHAFT
AMR technology can be used to detect both linear as well as
rotary position. There are many different types of magnetic
configurations used in conjunction with AMR angle sensors,
including linear, off shaft, and end of shaft magnet configurations.
The primary measurement configuration discussed in this
application note is a simple magnet configuration generally called
end of shaft. In end of shaft magnet configurations, a dipole magnet
that has been magnetized diametrically is located at the end of a
rotating shaft. The sensor is located underneath the rotating shaft
and magnet. In this mechanical setup the north and south poles
of the diametric magnet form a uniform field above the center of
the magnet. As the magnet and shaft rotates, so does the magnetic
field. The sensor is placed such that the uniform magnetic field
is located in the same plane as the sensing elements. Figure 3
shows an end of shaft magnet configuration.
LINEAR
12487-101
12487-001
For linear applications, the magnet must be located in the same
plane as the sensor, as shown in Figure 1. The red and blue sides
of the magnet show the orientation of the north and south poles.
This orientation direction is interchangeable because AMR sensors
do not distinguish between a north and south pole. To achieve
the most linear response from the AMR sensor, the center of the
sensor and the center of the magnet must be located half of the
magnet distance away from each other.
Figure 3. End of Shaft Magnet Configuration
Figure 1. Linear Magnet Configuration
OFF SHAFT
One magnet configuration for rotary measurement uses a pole
ring. Figure 2 shows an idealized pole ring. The colored regions
depict the outer magnetic field orientation as seen by the AMR
sensor. Similar to linear measurement, the ring must be placed
in the plane of the sensor at half of the distance of the pole length to
achieve a linear response. The sensor response for this type of
magnet configuration repeats as many times as there are poles
over a full mechanical revolution. For the ring in Figure 2, there
are 5 north poles and 5 south poles for a total of 10 magnetic
poles seen by the sensor. The AMR sensor output for the ring
shown repeats 10 times for each revolution and, therefore, gives
36° of absolute information.
The end of shaft magnet configuration lends itself well to brushless
dc motor position and control. In the case of the ADA4571, or
any 180° angle sensor, the brushless dc motor used must be an
even pole pair motor, because odd pole pair motors require full
360° position information.
Most closed-loop, brushless dc motor controls use hall sensors
to provide rotor position feedback to determine the correct
position for commuting the coils. These sensors range in accuracy,
but are generally on the order of 5° to 10° accurate. To create a
smoother and more efficient motor response and to reduce
torque ripple, more accurate rotor angle information is needed.
Analog Devices, Inc., AMR sensors provide mechanical accuracies
on the order of ±0.1° typical, ±0.5° maximum. This level of
accuracy can also be achieved through a more conventional
means of incremental encoders. However, startup, stall errors,
and environmental effects are much more of a problem with
incremental encoders. Analog Devices AMR sensors provide
absolute position information on startup or stall condition
regardless of motor position. This absolute position information
allows much better torque control, a smoother motor startup, as
well as more efficient motor start and stall performance.
12487-102
BRIDGE CONFIGURATION
Figure 2. Off Shaft Magnet Configuration
Analog Devices AMR sensors are manufactured in a wheatstone
bridge configuration, which allows both a wider output voltage
swing as well as rejecting large dc offsets compared to a single
resistive element. When a single bridge output is measured
differentially over the revolution of a single dipole magnet, it is
only possible to obtain 90° of usable range. The output waveform
for a single bridge element over the 360° mechanical revolution
Rev. 0 | Page 3 of 10
AN-1314
Application Note
of a single dipole magnet is shown in Figure 4. Note that for each
voltage output level, there are four possible mechanical positions.
5
AMPLITUDE (V)
4
3
AMR SENSOR ELEMENT
The layout of the AMR sensing element determines the end
performance of the device. Analog Devices uses AMR sensing
technology from Sensitec GmbH, a proven industry leading MR
sensor manufacturer. The Sensitec AMR sensor used in Analog
Devices products leverages PERFECTWAVE® technology.
PERFECTWAVE sensors use curved sensor elements to reduce
higher order harmonics and to improve accuracy.
2
1
0
0
90
180
270
360
MECHANICAL ANGLE (Degrees)
12487-002
MAGNETIC ANGLE vs. MECHANICAL ANGLE
Figure 4. Single Bridge Output
By placing two sensing elements on the same die rotated 45° from
one another; the sensor can be used over a full 180° measurement
range. Figure 5 shows a simplified circuit of two bridges.
α=0
α
Two different angle scales must be understood for AMR
technology: magnetic angle and mechanical angle. Due to the
nature of AMR technology, the ADA4571 is a 180° mechanical
sensor for a single dipole magnet. Because the outputs of two
AMR bridges rotated at 45° from each other are sinusoidal with
a relative phase shift of 90°, the absolute angle over 180° can be
obtained by performing an arctangent2 calculation.
V
arctan 2 SIN
 VCOS
α=
2
DIRECTION OF
MAGNETIC FIELD




The information gathered from the arctangent2 calculation repeats
twice over 360° for a single dipole magnet and more for a multipole
pair magnet. Figure 7 shows an example output waveform after
this arctangent2 calculation for a single dipole magnet.
360
Figure 5. Simplified Circuit Diagram for Double Wheatstone Bridge Sensor
An AMR sensor has an identical output whether looking towards
a magnetic north or south pole. Due to this effect, only a 45°
relative rotation is needed to create a 90° phase shift between
the two sinusoidal outputs when monitoring each channel
differentially from the respective wheatstone bridge. Figure 6
shows the two outputs from both AMR bridges over an entire
mechanical revolution in the dipole magnet configuration.
270
180
90
0
5
0
90
180
270
MECHANICAL ANGLE (Degrees)
Figure 7. Magnetic vs. Mechanical Angle
When the arctangent2 calculation is performed, a linear angle
response is generated. Neither the absolute voltage nor the absolute
field strength is important in the calculation of the magnetic angle,
which makes the sensors insensitive to magnetic and amplitude
shifts and drifts, compared to competing angle sensor technologies.
3
2
MAGNET SELECTION CONSIDERATIONS
1
0
90
180
270
MECHANICAL ANGLE (Degrees)
Figure 6. Two Bridge Output
360
12487-004
AMPLITUDE (V)
4
0
360
12487-006
CALCULATED ANGLE (Degrees)
12487-003
MAGNETIC
MECHANICAL
When working with AMR sensors, it is important to mate the
sensor with an appropriate magnet to obtain optimal
performance. Due to their direction dependency on the field,
the magnet used must be magnetized diametrically instead of
Rev. 0 | Page 4 of 10
Application Note
AN-1314
12487-007
axially. Such a magnet is shown in Figure 8. The blue and red
areas of the magnet indicate the north and south poles. The
magnetic field lines travel from the north pole of a magnet to a
south pole. On top of the magnet, where the AMR sensor must
be located for an end of shaft magnet configuration, as shown
in Figure 3, the field lines are uniform in the plane of the sensor.
The AMR sensing element used in the ADA4571 has a minimum
operating magnetic field of 25 kA/m. Operation at a lower field
strength is possible, but results in reduced accuracy. Higher field
strengths increase accuracy and do not damage the device. Due
to the field direction measurement of AMR sensors, as opposed
to flux measurement, a larger temperature coefficient of magnetic
field strength can be tolerated while still achieving the error
specified by the device. However, magnets selection must ensure
that field strength degradation is accounted for at operating
temperature extremes. The degradation can be calculated from
the nominal field strength and the temperature coefficient.
MAGNET TO SENSOR RELATIONSHIP
Figure 8. Dipole Magnet Pole Orientation
Generally, rare earth magnets are mated with AMR sensors due
to their high magnetic energy to weight ratio. However, lower cost
ferrite magnets can be used as long as the minimum saturation
field strength requirements of the sensor are met. However, for
high performance and high temperature applications, an increase
in performance comes with a rare earth magnet due to the higher
field strength seen by the AMR sensor. A higher field strength also
helps to reduce the influence of stray fields on the sensor accuracy.
The two most common types of magnetic material for permanent
rare earth magnets are neodymium (NdFeB) and samarium-cobalt
(SmCo). Table 1 shows a comparison of the two different magnetic
materials, showing the main advantages of each material. There are
many different grades of these two magnetic materials; therefore,
this is a high level comparison. For specific characteristics, each
grade of material must be examined separately. The grades of the
magnetic materials indicate the energy product of the material
and are expressed in megaGauss Oersteds (MGOe). This value is
taken by the maximum of the BH curve for that magnetic material.
As a general rule, a material with double the MGOe has double
the pull strength for a same size magnet.
Table 1. Comparison of NdFeB and SmCo Magnetic Materials
Parameter
Price
Field Strength
Maximum
Temperature
Temperature
Coefficient
Corrosion
Protection
the devices. The Sensitec AMR sensors used in Analog Devices
products have a curved structure that reduces the fourth harmonic
present in many other sensors, which allows lower field strength
operation to achieve similar performance.
NdFeB
Medium
High
80°C to 180°C
SmCo
High
Medium to high
160°C to 300°C
−0.08 %/K to −0.13 %/K
−0.03 %/K to −0.04 %/K
Nickel (typical)
Not needed
In the case of AMR technology, a higher strength magnet always
performs better than a lower strength magnet.
Increasing the field strength seen by the AMR sensor element
improves the performance of the device. Higher order harmonics
are present in all AMR sensors due to the physical limitations of
Mechanical alignment is critical for maximizing the performance
of an AMR sensor. There are several key parameters to keep in
mind when designing the physical system. The x-y alignment
tolerances between the magnet and the sensor must be sufficiently
controlled so that the field direction seen by the sensor is in the
desired direction. The physical misalignment of the center of
the sensor to the center of the magnet contributes an error to
the whole system that is dependent on the size and uniformity
of the magnetic field around the sensor location. Within the
ADA4571 8-lead SOIC package, the center of the magnetic sensor
is located between the top edge of Pin 2 and Pin 7 in the center of
the package. During packaging, the position accuracies are within
a precision of ±50 µm in each direction of this nominal position.
See the ADA4571 data sheet for specific alignment drawings.
The end of shaft system being controlled must have its magnetic
center of axis in line with the center of the magnetic sensor.
Air gap, or z direction, is also important to the performance
of an AMR sensor. While not as critical in absolute alignment as
the x-y relative position, the air gap must be understood to
maximize the performance of the sensor. To achieve the specified
performance of the AMR sensor, the magnetic stimulus must be
designed to provide at least the minimum required field strength
of the sensor. The required field strength for the ADA4571 is
25 kA/m. One way to increase the magnetic field strength seen
by the sensor is to decrease the working air gap. However, it is
important to note that decreasing the distance to a magnet does
not always increase the performance of the device. Near the surface
of magnets, the magnetic field produced becomes non-uniform.
Air gap insensitive operation is an important feature of AMR
technology. As long as the sensor is fully saturated by the exciting
magnetic field, the angle information gathered from the sensor
does not change with magnetic field strength. This tolerance
means that small z axis movement due to vibration, stress, or
lifetime mechanical drift has little impact on angular accuracy.
The amount of movement tolerance depends on the magnet
material and geometries, but can range from a few millimeters
to a centimeter or more.
Rev. 0 | Page 5 of 10
AN-1314
Application Note
MISALIGNMENT AND AIR GAP MEASUREMENTS
Measurement results showing varying magnet size, strength,
air gap, and misalignment are shown in the following sections.
Additional magnets have been tested using the methods outlined.
Contact Analog Devices for more information on magnet
selection for specific applications.
field strengths. There are also different NdFeB and SmCo
magnetic materials with various energy grades available.
In Figure 10 and Figure 11, the color scale indicates angular
error in degrees. The minimum angular error for these plots,
located in the center of the plot when the magnet is perfectly
aligned with the sensor, is 0.07°.
1.0
Measurement Setup
Each magnet was mounted onto a slotless, brushless dc motor
and spun at a constant rotational velocity of 3000 rpm.
Y
2.0
0.6
Y ALIGNMENT (mm)
The motor is mounted on a movable platform with two linear
actuators, one for the x direction and one for the y direction
with respect to the sensor. Figure 9 shows the defined direction
for x and y movement.
2.5
0.8
45
0.4
1.5
0.2
0
1.0
–0.2
–0.4
0.5
–0.6
–1.0
1.0
8
PD
VCOS 2
7
VDD
GND 3
6
GND
VSIN 4
5
VTEMP
0.6
0.4
0.2
0
–0.2
–0.4
–0.6
–0.8
–1.0
X ALIGNMENT (mm)
X
Figure 10. SmCo (32MGOe) with 1 mm Air Gap
12487-009
GC
0
0.8
1.0
2.5
0.8
Figure 9. Defined Alignment Directions Relative to the ADA4571 Package
The two linear actuators are moved in 50 μm increments to cover
an entire square of 2 mm × 2 mm, or 1 mm in each direction
away from the center of the AMR sensor. The maximum radial
misalignment tested by this method is 1.4 mm as located at the
corners of the sweep.
Note that all results were digitally filtered and upsampled to create a
smoother looking plot. Absolute error values remain the same.
Field Strength Study
Table 2. Magnet Dimensions for Comparison 1
Parameter
Diameter
Thickness
NdFeB (35 MGOe)
6 mm
3 mm
SmCo (32 MGOe)
6 mm
3 mm
Y ALIGNMENT (mm)
0.6
The z direction, or air gap from sensor to magnet, is fixed
throughout a measurement sweep. The indicated air gap for
each measurement is defined as the distance from the magnet
to the top of the package. The AMR sensor is located 0.38 mm
nominally, with a tolerance of ±0.025 mm, from the top of the
package. To find the distance from the AMR sensor die within
the ADA4571 package to the magnet, add this distance to the
air gap measurements.
2.0
0.4
1.5
0.2
0
1.0
–0.2
–0.4
0.5
–0.6
–0.8
–1.0
1.0
0
0.8
0.6
0.4
0.2
0
–0.2
–0.4
–0.6
–0.8
–1.0
X ALIGNMENT (mm)
12487-011
1
12487-010
–0.8
Figure 11. NdFeB (35 MGOe) with 1 mm Air Gap
Due to the increase in field strength from the NdFeB magnet, this
magnet maintains higher performance over a larger displacement
distance from the magnet position in comparison to the SmCo
magnet. The effective field strength at the sensor element for these
two magnets at a 1 mm air gap is approximately 60 kA/m for the
NdFeB magnet and 50 kA/m for the SmCo magnet. A larger,
lower energy grade magnet is considered in the following section.
Large, Lower Energy Grade Magnet
Table 3. Magnet Dimensions for Comparison 2
To study the effect of different field strengths on misalignment,
two magnets were chosen. Both magnets are 6 mm in diameter
and 3 mm thick. One magnet is NdFeB with an energy grade of
35 MGOe, and the other magnet is SmCo with an energy grade
of 32 MGOe. Other reasons to choose a SmCo magnet over
NdFeB include a higher temperature grade for SmCo, as well as
a lower temperature coefficient of the magnetic material. These
effects are more critical for high temperature applications. The
magnets chosen are examples to show the effect of different
Parameter
Diameter
Thickness
SmCo (24 MGOe)
10 mm
5 mm
The SmCO magnet described in Table 3 was tested at 2 mm, 4 mm,
and 6 mm air gaps, as shown in Table 4, which lists the minimum
and maximum reported errors (the center and edges of the plot).
Rev. 0 | Page 6 of 10
Application Note
AN-1314
Table 4. Minimum and Maximum Error for Comparison 2
DIAGNOSTICS
Parameter
Minimum Error
Maximum Error
(1.4 mm Misalignment)
Approximate Field Strength (kA/m)
Several post processing diagnostics can be helpful for monitoring
the ADA4571 to ensure proper system operation and/or to monitor
performance. In an end of shaft or off shaft configuration, the
magnetic field strength must be uniform throughout an entire
mechanical revolution. The magnitude of this field must exceed
25 kA/m to fully saturate the sensor to overcome its internal
magnetization. With this condition met, the output amplitude
of both the sine and cosine channels must be synchronous with
a 90° phase difference. As a result of this output synchronization,
the radius is constant at a constant temperature. The radius can
be calculated using the following formula:
2 mm
0.0774
0.6118
4 mm
0.1002
0.7522
6 mm
0.1477
0.7074
60
35
20
In Figure 12 to Figure 14, the color scale indicates angular error
in degrees. The maximum angular error for these plots is 0.8°.
The increase in minimum error in the center of these plots, and
as shown in Table 4 is a result of a much lower field strength at
the AMR sensor, especially at 6 mm. This magnet was designed
to provide 25 kA/m at 3 mm.
0.8
V RAD  (V SIN 
0.7
0.6
When the radius, VRAD, is monitored by an external processor or
electronic control unit (ECU), any significant deviation from
the nominal radius points to a fault in the system. Real time
mechanical failure and misalignment, as well as magnetic field
degradation, can be monitored by this radius calculation.
0.3
–0.4
0.2
–0.6
0.6
0.4
0.2
0
–0.2
–0.4
–0.6
–0.8
–1.0
X ALIGNMENT (mm)
Figure 12. SmCo (24MGOe) with 2 mm Air Gap
1.0
0.8
0.8
0.7
0.6
0.4
0
0.3
–0.4
0.2
–0.6
–1.0
1.0
0
0.8
0.6
0.4
0.2
0
–0.2
–0.4
–0.6
–0.8
–1.0
X ALIGNMENT (mm)
12487-013
0.1
–0.8
VSIN MAGNITUDE (%VDD)
Y ALIGNMENT (mm)
0.5
–0.2
Figure 13. SmCo (24MGOe) with 4 mm Air Gap
1.0
0.8
0.8
0
5
10
0
VCOS MAGNITUDE (%VDD)
0.4
–0.2
GC ON
Figure 15. Bounded Radius with GC On
0.3
–0.4
0.2
–0.6
0.1
–0.8
0
0.8
0.6
0.4
0.2
0
–0.2
–0.4
–0.6
–0.8
–1.0
X ALIGNMENT (mm)
VCOS
Figure 14. SmCo (24MGOe) with 6 mm Air Gap
Rev. 0 | Page 7 of 10
VCOS
30
25
20
0
0.5
VRAD
45
40
35
0.6
12487-017
Y ALIGNMENT (mm)
60
55
50
15
10
5
0.4
–1.0
1.0
75
70
65
0.7
0.6
0.2
+150°C
+125°C
+25°C
–40°C
95
90
85
80
0.4
0.2
VSIN
100
0.6
VSIN
0
0.8
65
70
75
80
–1.0
1.0
12487-012
0.1
–0.8
Depending on the gain control pin (GC) configuration of the
ADA4571, the allowable output radius is bounded by the following
values. This range is represented by the shaded region shown in
Figure 15 and Figure 16. Typical VRAD values for −40°C, +25°C,
+125°C, and +150°C are also indicated. Minimum and maximum
values are included in the ADA4571 data sheet. Monitoring the
temperature at the device can further tighten up the allowable
range.
12487-015
0.4
0
–0.2
85
90
95
100
0.5
50
55
60
Y ALIGNMENT (mm)
0.6
0.4
0.2
V DD 2
V
)  (VCOS  DD ) 2
2
2
30
35
40
45
0.8
15
20
25
1.0
Application Note
Figure 18 shows the FFT of a well aligned sensor. Figure 19 shows
the FFT of a sensor that has been misaligned 1 mm away from
the magnetic center. Figure 20 shows the FFT of an unsaturated,
high air gap sensor with a stimulus of 10 kA/m.
VSIN
100
95
90
85
80
75
70
65
60
55
50
45
40
35
30
25
20
15
10
5
0
+150°C
+125°C
+25°C
–40°C
Note that the measurement noise floor was increased in both
the misaligned and unsaturated sensor plot. In the case of the
unsaturated sensor, higher order harmonics are much more
prevalent in the system. These harmonics are the main
contributors to a decrease in sensor accuracy.
VSIN
VRAD
VCOS
VCOS
100000
10000
MAGNITUDE
1000
12487-016
0
5
10
15
20
25
30
35
40
45
50
55
60
65
70
75
80
85
90
95
100
VCOS MAGNITUDE (%VDD)
GC OFF
100
10
Figure 16. Bounded Radius with GC Off
1
While the amplitude of the sine and cosine signals are largely
independent of the magnetic field strength, an unsaturated
sensor begins to show degradation in output amplitude
compared to a saturated sensor.
Figure 17 shows a radius plot of a saturated sensor, excited with
40 kA/m, and an unsaturated sensor, excited with 10 kA/m.
5.0
0
100
200
300
400
500
600
700
800
900
1000
900
1000
900
1000
FREQUENCY
Figure 18. FFT of Well Aligned Sensor
100000
10000
1000
MAGNITUDE
The following radius and fast Fourier transform (FFT) plots,
Figure 17 to Figure 20, were taken with the previously discussed
SmCo magnet with a diameter of 10 mm and a thickness of
5 mm. The sensor was biased at a 5 V supply and held at room
temperature.
0.1
12487-019
VSIN MAGNITUDE (%VDD)
AN-1314
100
10
4.5
1
0.1
3.5
0
100
200
300
500
600
700
800
FREQUENCY
3.0
VSIN
400
12487-020
4.0
Figure 19. FFT of Misaligned Sensor
2.5
100000
2.0
10000
1.5
1000
0
0
0.5
1.0
1.5
2.0
2.5
3.0
VCOS
3.5
4.0
4.5
5.0
12487-018
UNSATURATED
SATURATED
100
10
Figure 17. Radius of Unsaturated and Saturated Sensors
1
An FFT of the sine and cosine output signals can be a powerful
tool for examining the performance of the sensor and in overall
system troubleshooting.
0.1
0
100
200
300
400
500
600
700
800
FREQUENCY
Figure 20. FFT of Unsaturated Sensor
Rev. 0 | Page 8 of 10
12487-021
0.5
MAGNITUDE
1.0
Application Note
AN-1314
ERROR SOURCES
To minimize the angular error of the sensor, it is important to
understand the different error contributions and how each can
be calibrated.
Offset Error
Error due to the offset of the sensor is the largest error contribution
to the system. However, after proper calibration, as described in
the Calibration Procedure section, the offset error can be reduced
to near zero.
the overall waveform provides an accurate representation of
the offset. Averaging the past sampled values over multiple full
mechanical revolutions also provides an accurate offset value.
The offset of each channel is different and must be stored
separately.
When the offset from each channel is collected, the controller
must subtract the offset from its respective channel before
performing an arctangent2 calculation to gather the angle
information from the device.
Amplitude Synchronization Error
ECU/MICRO
COS SINE
VOS VOS
COS
OUTPUT
Phase Error
As a result of the production layout of the two AMR wheatstone
bridges on a single die, the inherent phase error between the
sine and cosine channels is negligible. However, a phase error is
introduced if the outputs are not sampled synchronously, that
is, by a muxed analog-to-digital converter (ADC). The error
due to asynchronous sampling is greater at higher magnetic
field rotation speeds due to a larger phase lag between samples.
It is recommended to use two separate ADCs or a muxed ADC
with two simultaneous track-and-hold amplifiers to sample the
sine and cosine outputs synchronously to avoid error due to
phase lag. The degree of sampling phase error maps directly
onto the degree of calculated electrical error.
CALIBRATION PROCEDURE
To achieve the best performance from the ADA4571, a single
calibration procedure is required. Fix the mechanical tolerances
and align the mechanical setup as closely as possible following
the recommendations for magnet to sensor alignment and air
gap distance. Once the system is set, the offset and offset drift of
the sensor are the primary sources of angular error. There are two
different types of calibration that can be performed: dynamic
calibration or a single point calibration. Dynamic calibration
results in a lower angular error than a single point calibration
but requires further real time processing.
A dynamic calibration can only be performed in 360° continuous
or free running applications. In this mode, offset can be
continuously monitored to calibrate the ADA4571 and to
null the error contribution due to both offset and offset drift
over the lifetime and temperature range of the system. There
are several ways to gather offset information from the sensor
output. Collecting the maximum and minimum values from
–
ArcTangent
(CORDIC)
PHASE/SPEED
CORRECTION
CALCULATED
ANGLE
–
12487-022
SINE
OUTPUT
Figure 21. Dynamic Calibration Operating Flow Chart
A single point calibration can be performed in either a free running
or static application in which the angle measurement does not
move through an entire mechanical revolution. To perform a single
point calibration for a 360° range, an even number of electrical
revolutions must be captured before accurate offset information
is extracted. For a 180° movement application, only one electrical
revolution can be captured before accurate offset information is
extracted. Store the relevant offset information for each output
channel in the controller for offset compensation.
Regardless of the method to capture the offset, it is recommended
that at least two full mechanical revolutions be used for offset
calculations. This offset value is then subtracted from the signal
output before recovering angle information. While a single
point initial calibration can help to reduce the angular error due
to offset, perform a dynamic calibration when possible to minimize
the sensor error. This dynamic calibration helps to counter the
temperature dependent offset drift inherent to AMR sensors.
AVERAGE PAST
SINE VALUES
AVERAGE PAST
COS VALUES
COS SINE
VOS VOS
SINE
OUTPUT
COS
OUTPUT
Rev. 0 | Page 9 of 10
ECU/MICRO
–
ArcTangent
(CORDIC)
PHASE/SPEED
CORRECTION
CALCULATED
ANGLE
–
Figure 22. Single Point Calibration Operating Flow Chart
12487-023
For the ADA4571, careful layout of both the sine and cosine
channels has been taken for both the sensing element and signal
conditioning circuit to ensure good matching performance. As
a result, the error due to amplitude synchronization on the
ADA4571 is negligible, and correction for amplitude mismatch
error is not needed.
AN-1314
Application Note
LAYOUT RECOMMENDATIONS AND MAGNETIC
INTERFERENCE
VTEMP OUTPUT
Due to the nature of magnetic sensing applications, the materials
used near the sensor must be nonferrous or nonmagnetic in nature.
High current carrying ac and dc wires or traces also must not be
placed near the AMR sensor. Due to Lenz’s law, any high current
carrying wires or traces can create magnetic interference that
distorts the magnetic field direction being sensed and introduce
extra error into the system. Magnetic fields deteriorate by a cubic
term as the distance is increased from the magnet. Due to this
cubic nature, any extra space that can be made between high
current carrying wires and the sensor greatly decreases the
amount of stray fields near the sensor.
If high currents must be placed near the sensor, there are several
ways to help reduce the interference issue. Magnetic shielding
around the sensor with a magnetically conductive material,
such as steel, can help isolate the magnetic sensor and stimulus
from the external environment. A higher strength magnet also
helps to minimize the impact of an interference field.
The ADA4571 has an on-chip, coarse temperature sensor that
can be used for diagnostic purposes. If temperature measurement
is required, the readout must have an initial calibration at a
known temperature. Temperature information can be calculated
from the VTEMP pin readout by the following equation:
TVTEMP

 VTEMP
   VCAL

VDD  –  
VDD  – TCAL × TCO 

=
TC VTEMP
where:
TVTEMP is the calculated temperature (°C) from the VTEMP
output voltage.
VTEMP is the VTEMP output voltage during operation.
VDD is the supply voltage.
VCAL is the VTEMP output voltage during calibration at a
controlled temperature.
TCAL is the controlled temperature during calibration.
TCO is the temperature coefficient of the internal circuit
TCVTEMP is the linear temperature coefficient for the VTEMP
readout.
To increase accuracy, it is recommended that VDD is consistent
between initial known temperature calibration and operation.
TCVTEMP is the linear temperature coefficient for the VTEMP
readout. While TCVTEMP varies with supply voltage used,
TC = 3.173 mV/V/C results in a typical TVTEMP accuracy of ±5°C.
Whether in use or not, a 22 nF capacitor to ground must be in
place for electromagnetic interference (EMI) purposes.
©2014 Analog Devices, Inc. All rights reserved. Trademarks and
registered trademarks are the property of their respective owners.
AN12487-0-10/14(0)
Rev. 0 | Page 10 of 10
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