CN-0323: Magnetoresistive Angle Measurement PDF

Circuit Note
CN-0323
Devices Connected/Referenced
Circuits from the Lab™ reference circuits are engineered and
tested for quick and easy system integration to help solve today’s
analog, mixed-signal, and RF design challenges. For more
information and/or support, visit www.analog.com/CN0323.
AD7866
Dual Channel, 1MSPS, 12-bit,
Simultaneous Sampling SAR ADC
AD8227
Wide Supply Range, Rail-to-Rail,
Instrumentation Amplifier
AD8615
Low Offset, Low Noise, Precision Amplifier
Magnetoresistive Angle Measurement
EVALUATION AND DESIGN SUPPORT
The circuit provides all necessary signal conditioning including
instrumentation amplifiers, buffers, and a dual channel ADC
that efficiently process the AMR sensor low level bridge outputs.
Circuit Evaluation Boards
CN-0323 Circuit Evaluation Board (EVAL-CN0323-SDPZ)
System Demonstration Platform (EVAL-SDP-CB1Z)
Design and Integration Files
Schematics, Layout Files, Bill of Materials
The result is an industry leading angle measurement solution
suitable for machine tool speed control, crane angle control,
motor speed measurement, and other industrial or automotive
applications.
CIRCUIT FUNCTION AND BENEFITS
The circuit shown in Figure 1 provides a contactless, AMR
(anisotropic magnetoresistive) angle measurement solution
with 1° angular accuracy over a 180° range. The circuit is ideal
for applications where high speed, accurate, non-contact angle
measurements are critical.
5V
0.1µF
5V
5V
REF
0.1µF
10µF
2.96kΩ
10µF
2.5V
3.3V
5V
DVDD AVDD
REFSEL
RANGE
AD8615
VDRIVE
VA1
VCC
SCLK
+VO1
AA747
5V
AD7866
2.5V
DOUTA
–VO1
+VO2
–VO2
GND1
SDP
AD8227
REF
2.96kΩ
5V
CS
AD8227
AD8615
VB1
DGND AGND
DCAP A
470nF
DCAP B
470nF
VREF
5V
100nF
2.5V
11916-001
AD8615
Figure 1. Magnetoresistive Angle Detection System (Simplified Schematic: Decoupling and All Connections Not Shown)
Rev. 0
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CN-0323
Circuit Note
The Sensitec AA747 is an AMR-based angular sensor containing
two galvanically separated Wheatstone bridges at a relative angle of
45° to each other. The AA747 offers minimal offset voltage (±2
mV) and high signal amplitude (65 mV). A rotating magnetic
field stimulates the sensor, creating an output voltage of ±65 mV.
An AD8227 instrumentation amplifier amplifies the signal of
interest while rejecting the Wheatstone bridge common-mode
voltage of 2.5 V. The common-mode output voltage of the inamp is set to 2.5 V by driving the VREF pin to 2.5 V. A 2.96 kΩ
gain resistor sets the gain of 32. This creates an analog output
voltage of 0.2 V to 4.8 V for a bridge output of 2.5 V ±70 mV.
The circuit signal bandwidth is determined by the AD8227 that
has an approximate 100 kHz bandwidth for a gain of 32
A unity gain AD8615 op amp buffers the in-amp output voltage
and connects directly to the ADC. This buffer has a rail-to-rail
output stage that swings to within 200 mV of the supply rails.
The AD7866 is a dual channel 12-bit 1 MSPS SAR ADC. The
polarity of the RANGE pin determines the analog input range
and output coding. If this pin is tied to a logic high when the
chip select goes low, the analog input range of the next conversion
is 0 V to 2VREF (0 V to 5 V), leaving approximately 200 mV
headroom for the 0.2 V to 4.8 V input signal from the buffer
amplifier.
An example of the AMR effect is shown in Figure 2. A current
(I), flowing through a conductor, is subject to an external
magnetic field (HY). The resistance of the conductor changes as
a function of the angle (Ø) between the magnetization vector
(M) and the current flow vector (I). The magnetization vector is
the net sum of the internal magnetic field (HX) and the applied
external magnetic field (HY).
The maximum resistance occurs when the magnetization vector
(M) is parallel to the current vector (I). The minimum resistance
occurs when the magnetization vector (M) is perpendicular to
the current vector (I).
Effective utilization of the AMR effect requires the conductor
itself to be a material insensitive to mechanical stress but sensitive
to magneto-restriction. For these reasons, permalloy (80%
nickel, 20% iron) is the most commonly used alloy in AMR
sensor manufacturing.
Permalloy Properties
There are two properties of permalloy strips that provide design
challenges when creating angular measurement systems.
First, permalloy has a narrow linear operating region (see Figure 3).
Only when the angle (Ø) between the magnetization vector (M)
and current flow vector (I) becomes larger, does the response
become linear. Unfortunately, shortly after the response becomes
linear, it will saturate.
Connecting the REFSEL pin low configures the ADC to use the
internal 2.5 V reference voltage. This voltage is available on the
VREF pin but requires a buffer before it can be used elsewhere in
the system. The DCAPA pin and DCAPB pin are decoupled with
470 nF capacitors to ensure proper operation of the ADC. The
reference voltage is buffered by the AD8615 and sets the
common-mode output voltage of the AD8227 in-amp.
The AD7866 simultaneously samples both channels of the
magnetoresistive sensor. The digital words are normally available
on DOUTA and DOUTB. Each data stream consists of one leading
zero followed by three status bits and then twelve bits of conversion
data. However, by holding the chip select low for an additional 16
clock cycles, both digital words are read from one channel, DOUTA.
An SPI interface allows access to both channels on one data line.
Magnetoresistive (MR) Theory
Magnetoresistivity is the ability of a material to change the value
of its resistance when subjected to an external magnetic field.
The most commonly used MR sensors are based on the
anisotropic magnetoresistive (AMR) effect.
M
Ø
HY
I
R
R0 + ΔR
R0
HY
–1.0 –0.5
0
0.5
1.0
H0
Figure 3. Permalloy Resistance vs. Magnetic Field
Secondly, permalloy is insensitive to polarity. The resistance of a
permalloy strip decreases whether the angle (Ø) between the
magnetization vector (M) and the current flow vector (I) is
positive or negative.
Barber Poles
A common method used to improve both the linearity and polar
insensitivity of the permalloy strip is to add aluminum stripes
angled at 45° to the strip axis called barber poles, as shown in
Figure 4. Any current flowing between barber poles takes the
shortest path—the perpendicular path, and the angle between the
current flow vector (I) and magnetization vector (M) shifts by 45°.
ALUMINUM STRIPES
HY
11916-002
I
M
Figure 2. Anisotropic Magnetoresistive Example
HX
HY = 0
11916-004
HX
11916-003
CIRCUIT DESCRIPTION
Figure 4. Barber Pole Effect in a Permalloy Strip
Rev. 0 | Page 2 of 6
Circuit Note
CN-0323
Figure 5 shows the result of adding Barber Poles to a permalloy
strip. The current flow vector shifts by 45°, but the magnetization
vector remains unchanged. Notice the linear behavior now
present in the middle of the graph.
–VO1
VCC1
GND1
+VO1
–VO2
VCC2
R
GND2
+VO2
11916-007
R0 + ΔR
Figure 7. AA747 Dual Wheatstone Bridge Configuration
HY
–1.0 –0.5
0
0.5
H0
1.0
11916-005
R0
Figure 5. Barber Pole Permalloy Resistance vs. Magnetic Field
Magnetic Field Strength
The AA747 magnetoresistive sensor requires a minimum
magnetic field strength of 25 kA/m to ensure the error
specification found in the data sheet. This stimulating magnetic
field must intersect the center of the sensor package.
The maximum peak signal amplitude of the AA747 is 70 mV
(14 mV/VCC on a 5 V supply). The sensor offset voltage is
±10 mV (±2 mV/VCC on a 5 V supply) giving a useable 2.5 V
±0.70 mV output signal. A rotating magnetic field produces the
sin (2ø) and cos (2ø) outputs seen in Figure 8. Both signals are
periodic over a 180° range, making the detection of full 360°
measurements impossible without additional circuitry and
components.
2.57
2.55
OUTPUT VOLTAGE (V)
When selecting a magnet, consider the air gap between the
sensor and the magnet as shown in Figure 6. If the magnet is
not in close proximity to the sensor (the distance, d, is very
large), a larger magnetic field strength may be necessary to
ensure the magnetic field strength at the sensor location and
maintain the minimum error specification.
2.53
2.51
COS (2Ø)
SIN (2Ø)
2.49
2.47
2.43
0
90
ANGLE (°)
135
180
Figure 8. Magnetoresistive Sensor Output Voltage
MAGNET
Channel Sensitivity
11916-006
d
SENSOR
45
11916-008
2.45
Figure 6. Magnet Orientation and Air Gap for Rotating Shaft Angle
Measurement.
Sensor Basics
The standard AMR sensor consists of two Wheatstone bridges,
with one bridge at a relative angle of 45° with respect to the
other. Permalloy strips comprise each element of both bridges
and have nominal resistance values of 3.2 kΩ.
The sensor has a nominal sensitivity of 2.35 mV/° for each
channel. This means each degree of change between the
magnetization vector and the sensor orientation produces an
output voltage change of 2.35 mV. The sensitivity is not constant
with respect to the angle. The areas of decreased sensitivity are
the portions of each output where the slope of the line
approaches zero.
Referring to Figure 8, channel one (the blue line) loses sensitivity as
the magnetization vector angle nears 45° or 135°. Similarly,
channel two (the red line) loses sensitivity around 0° and 90°.
Fortunately, when one channel has reduced sensitivity, the other
channel is in a region of high sensitivity.
The software takes advantage of this, measuring the angle based
on whichever sensor is most accurate at the time. If channel one
is approaching 45°, channel two is used to calculate the angle
and maintain the system accuracy.
Rev. 0 | Page 3 of 6
CN-0323
Circuit Note
The angular velocity of the magnetic field is an important
component in understanding the bandwidth of the circuit. The
ADC converts one sample every microsecond. In order to
achieve a resolution of 1°, the magnetic field can only move one
degree in one microsecond (2.778 kHz); otherwise, the ADC
cannot sample fast enough to keep up with the magnetic field.
This sets the maximum useable angular velocity of the magnetic
field at 2.778 kHz for a 1 MSPS ADC.
Test Results
The EVAL-CN0323-SDPZ PCB is tested by mounting a magnet
to the end face of a rod capable of spinning freely through 360°.
The EVAL-CN0323-SDPZ PCB sits in position with the face of
the AA747 AMR sensor (U5) parallel to the face of the magnet.
The EVAL-CN0189-SDPZ tilt sensor PCB attaches to the second
end of the rod. Thus, as the rod spins, both CN-0189 and the
magnet also spin through 360°. A functional diagram of the
setup is shown in Figure 9, and a photo of the setup is shown in
Figure 10.
position while the magnet is rotated to produce a 0° angle
reading in the CN-0323 evaluation software.
It is important to align the center of the magnet with the center
of the face of the IC. Any misalignment shifts the magnetic field
with respect to the sensor and cause errors in the final angle
calculation of the CN-0323 evaluation software.
Data is collected by turning the center rod and comparing the
two evaluation software display readings. Figure 11 shows the
output angle error recorded through a ±90° range. The error is
±0.4°over the entire range. The LabVIEW evaluation software
can be seen in Figure 12.
0.5
0.4
0.3
0.2
ERROR (°)
System Bandwidth, Magnetic Field Rotation
EVAL-SDP-CB1Z
0.1
0
–0.1
–0.2
USB
–0.3
–0.4
EVAL-SDP-CB1Z
USB
–0.5
ANGLE (°)
+6V
11916-011
MAGNET
90
82
74
66
58
50
42
34
26
18
10
2
–6
–14
–22
–30
–38
–46
–54
–62
–70
–78
–86
MR SENSOR
Figure 11. Magnetic Field Angle Error
+6V
EVAL-CN0189-SDPZ
11916-009
CN-CN0323-SDPZ
11916-010
11916-012
Figure 9. Data Collection Test Setup
Figure 10. Photo of Bench Test Setup
The CN-0189 tilt measurement system provides a reference
angle to test CN-0323. The center rod turns until the CN-0189
evaluation software reads 0°. The center rod is held in this
Figure 12. Screenshot of the CN-0323 evaluation software
The calibration tab determines the maximum and minimum
voltage (VMAX and VMIN) output of each Wheatstone bridge.
Knowing these values allows a more precise mapping of voltage
to digital code. The user has two methods of determining the
VMAX and VMIN values.
Rev. 0 | Page 4 of 6
Circuit Note
CN-0323
The first method is manually entering the values. The second
method is to spin the magnet through a 180° range while the
software automatically identifies the values. This method can be
faster than manually identifying the values but can also
introduce errors if the magnetic field is rotating too quickly.
PCB
PCB Layout Considerations
In any circuit where accuracy is crucial, it is important to consider
the power supply and ground return layout on the board. The
PCB should isolate the digital and analog sections as much as
possible. The PCB for this system was constructed in a 4-layer
stack up with large area ground plane layers and power plane
polygons. See the MT-031 Tutorial for more discussion on
layout and grounding, and the MT-101 Tutorial for information
on decoupling techniques.
D=
SENSOR
1
POLE LENGTH
2
11916-014
D
P
P = POLE LENGTH
Figure 14. Linear Position Measurement Magnet, PCB, and Sensor
Decouple the power supply to all ICs with 1 µF and 0.1 µF
capacitors to properly suppress noise and reduce ripple. Place
the capacitors as close to the device as possible. Ceramic
capacitors are advised for all high frequency decoupling.
The AA745 comes in a horizontal package that mounts flush
against the edge of the PCB. This allows optimization of the
distance between the magnet and sensor, the ideal distance being
one-half the pole length of the magnet.
Power supply lines should have as large a trace width as possible to
provide low impedance paths and reduce glitch effects on the supply
line. Shield clocks and other fast switching digital signals from
other parts of the board by digital ground. Figure 13 is a photo
of the PCB.
As the sensor moves parallel to the magnet it detects the magnetic
field which rotates 180° for every pole length travelled. The pole
length of the magnet (P) and the angular accuracy of the sensor
(ΔØ = 0.05°) determine the theoretical accuracy (Δx).
A complete design support package for this circuit note is at
www.analog.com/CN0323-DesignSupport.
This provides an absolute measurement system for only one pole
length. If the magnet has more than one pole, counting the number
of poles passed provides a more accurate reading. Additional
electronics are required to implement this functionality, and
traditionally a second magnet with different pole length provides a
reference point for an additional sensor.
Δx = P × ΔØ/180°
CIRCUIT EVALUATION AND TEST
This circuit uses the EVAL-SDP-CB1Z System Demonstration
Platform (SDP) evaluation board and the EVAL-CN0323-SDPZ
circuit board. The two boards have 120-pin mating connectors,
allowing for the quick setup and evaluation of the performance
of the circuit.
The EVAL-CN0323-SDPZ contains the circuit to be evaluated, as
described in this note. The EVAL-SDP-CB1Z is used with the
CN-0323 evaluation software to capture the data from the
EVAL-CN0323-SDPZ evaluation board.
Equipment Needed
11916-013
The following equipment is needed:
Figure 13. Photo of the EVAL-CN0323-SDPZ PCB
COMMON VARIATIONS
Two changes are required to create a linear position measurement
system. First, replace the AA747 AMR sensor with the AA745.
This sensor specifically senses linear movement and has identical
electrical characteristics as the AA747. Second, replace the magnet
with a multi-pole bar magnet consisting of a series of alternating
north and south poles as shown in Figure 14.
• PC with a USB port and Windows® XP or Windows Vista®
(32-bit), or Windows® 7 (32-bit)
• EVAL-CN0323-SDPZ evaluation board
• EVAL-SDP-CB1Z evaluation board
• 6 V power supply or wall wart
• CN-0323 evaluation software
• Neodymium magnet with a minimum magnetic field
strength of 25kA/m at the package of the sensor.
Rev. 0 | Page 5 of 6
CN-0323
Circuit Note
Getting Started
Test
Load the evaluation software by placing the CN-0323
evaluation software CD into the PC. Using My Computer,
locate the drive that contains the evaluation software CD and
open the Readme file. Follow the instructions contained in the
Readme file for installing and using the evaluation software.
Apply power to the DC barrel jack, connector J4. Launch the
CN-0323 evaluation software and connect the USB cable from
the PC to the mini-USB connector on the EVAL-SDP-CB1Z.
Functional Block Diagram
Figure 15 shows the functional block diagram of the test setup.
6V
POWER
SUPPLY
J4
LEARN MORE
EVAL-SDP-CB1Z
120-PIN
CONNECTOR
CN-0323 Design Support Package:
http://www.analog.com/CN0323-DesignSupport
11916-015
J1
Information regarding the EVAL-SDP-CB1Z can be found in
the SDP User Guide.
Information and details regarding test setup and calibration, and
how to use the evaluation software for data capture can be found in
the CN-0323 Software User Guide at: www.analog.com/CN0323UserGuide .
PC
EVAL-CN0323-SDPZ
Once USB communications are established, the EVAL-SDP-CB1Z
can now be used to send, receive, and capture serial data from
the EVAL-CN0323-SDPZ.
MT-031 Tutorial, Grounding Data Converters and Solving the
Mystery of “AGND” and “DGND”, Analog Devices.
Figure 15. Test Setup Block Diagram
Setup
MT-101 Tutorial, Decoupling Techniques, Analog Devices.
Connect the 120-pin connector on the EVAL-CN0323-SDPZ to
the connector on the EVAL-SDP-CB1Z. Use nylon hardware to
firmly secure the two boards, using the holes provided at the
ends of the 120-pin connectors.
AN-688 Application Note, Phase and Frequency Response of
iMEMS Accelerometers and Gyros, Analog Devices
With power to the supply off, connect a 6.0 V DC barrel jack to
connector J4. Connect the USB cable supplied with the EVALSDP-CB1Z to the USB port on the PC. Note: Do not connect
the USB cable to the mini-USB connector on the SDP board at
this time.
Place the neodymium magnet directly on top of the IC or in
some fixture designed to spin the magnet, which minimizes the
distance between the IC and magnet itself.
It is important to keep other sources of magnetic fields away
from the IC as any stray magnetic field can cause errors in the
output voltage of the sensor.
AA700 Application Note, AMR Freepitch Sensors for Angle and
Length Measurement, Sensitec
Data Sheets and Evaluation Boards
CN-0323 Circuit Evaluation Board (EVAL-CN0323-SDPZ)
System Demonstration Platform (EVAL-SDP-CB1Z)
AD7866 Data Sheet
AD8227 Data Sheet
AD8615 Data Sheet
REVISION HISTORY
10/13—Rev. 0: Initial Version
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CN11916-0-10/13(0)
Rev. 0 | Page 6 of 6
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