A2.4

Giant magnetoresistive (GMR) Sensors for contactless position detection

W. Clemens, Siemens AG, Corporate Technology ZT MF 1, Post Box 3220, D-91050 Erlangen, Germany;
Tel: [phone], Fax: [phone], email: [e-mail]

E. Hufgard, Infineon Technologies AG i. Gr., GS MFH, P.O. Box 100944, D-93009 Regensburg,
Germany; Tel: [phone], Fax: [phone], email: [e-mail]

The market for magnetic sensors is dominated by field sensors, i.e. sensors that are sensitive the variation of
the magnetic fieldstrength acting on the sensor. The Infineon GMR Sensor is distinguished by its principle as
a pure angular sensor. Fig 1 schematically shows the internal set-up, that consists of a magnetically hard
and a magnetically soft layer. The electrical resistance of this stack varies almost sinusoidal with a period of
360° with the relative magnetic orientation of these two layers. We achieve a change in resistance,

∆

R/R of >4%.

AAF: pinned

magnetisation

Magnetisation

by external

magnetic Field

ϕ

M

Fe

Cu
Co

Co
Cu

Fe

Cu
Co

Cu

Fig.1:

Scheme of the sensorprinciple

The hard magnetic layer is an artificial antiferromagnet (AAF), with an orientation fixed relative to the part,
the soft magnetic layer is iron. The internal magnetisation of the iron layer, actually there are two iron layers,
rotates with the direction of an external magnetic field, as long as the fieldstrength of this field is within a
magnetic window between 5 .. 15 KA/m. It is the target to operate the sensor on a saturation mode, i.e. to
chose a fieldstrength that is suitable to rotate the orientation of the soft magnetic layer completely. As a
result of this operation mode, the comfortable alignment tolerances of the Infineon GMR Sensor are
achieved.
Overall, the active stack consists of 15 individual layers of a thickness of about 1nm each. To achieve an
electrical resistance of round about 1000

Ω

/ single resistor on a small die area, the resistors are shaped as

meanders, as can be seen on a photograph of the fullbridge chip GMR B6, which is published in our
Application note.

Presently 3 different types of dies are a available in 2 different packages, as listed below.

Type

Meander

Direction of internal

magnetisation

Package

S4, S6

Single sensor

1

0°

SOH, SMT(MW-6)

B6

1 full bridge / 2 antiparallel half
bridges

2 + 2

0°

180°

180°

0°

SMT(MW-6)

C6

2 crossed half bridges

2 + 2

0°

90°

180°

270°

SMT(MW-6)

GMR B6

V

↑

V

↓

GMR C6

V

→

V

↑

Fig.2: Types of GMR Sensors and set-up of the bridge types GMR B6, GMR C6;

the direction of internal magnetisation is indicated by the arrows.

The shape of the outputsignal of the types GMR B6 and GMR C6 over an angle of 360° is shown in fig 3.

a)

GMR B6

90°

180°

270°

360°

0°

V

↓

V

↑

−

V

↓

V

↑

Bridge Voltage

(I

× ∆

R)

0

1

-1

½

-½

b)

GMR C6

90°

180°

270°

360°

0°

V

↑

V

→

B

ridge V

o

lt

age (

I

×

∆

R)

0

½

-½

Fig.3: Bridgevoltages of full bridges B6

(V

↑

- V

↓

)

and half bridges B6

(V

↑

, V

↓

)

,. C6

(V

↑

, V

←

)

respectively

while

the external field is rotated 360°

While the type B6 offers a complete full bridge, the type C6 offers 2 halfbridges that are magnetically rotated
90° with respect to each other and which are to be completed to 2 full bridges by, for example 2 external
resistors. Of course, 2 GMR B6 rotated 90° on a PCB may replace GMR C6.
From fig 3 it is obvious, that the usage of a C6 allows a unique determination of the angle over 360°.
From the examples discussed below , it can be seen immediately which part is the best choice for the actual
application.

For every application that exceeds the status of a laboratory set-up, temperature compensation and signal
amplification circuitry is mandatory. An approved circuit, suitable for B6 is shown in fig.4.

a)

5 V

R

1

Sens 2

Sens 1

R

n

=

330

Ω

B = 3575 K

GMR bridge

S

3

S

1

S

2

S

4

330

Ω

N

T

C

b)

R

2

R

3

Sens 2

Sens 1

5 V

R

4

R

5

GMR bridge

R

1

S

3

S

1

S

2

S

4

68 k

Ω

3.9 k

Ω

98

Ω

3.3 k

Ω

10 k

Ω

OP:
LMC6494BEN

OP

OP

OP

R

1

R

2

R

3

R

3

R

2

T

1

D

1

5 V

2.5 V

Sens 2

Sens 1

OP

V

out

1 k

Ω

1 k

Ω

12 k

Ω

1 k

Ω

12 k

Ω

5 k

Ω

∆

U

Fig. 4: Temperaturecompensation (above: a

)

by means of a NTC- b

)

by means of a NIC circuit) and an

amplification circuit for a full bridge B6

In the case of a crossed bridge C6, the circuit for temperature compensation remains identical, the
amplification part has to be built twice, once for each half bridge. Measurements over a temperature range of
25° ... 150°C showed stability of the outputsignal of better than 1% for both, the NTC and the NIC circuit.
With the use of the NTC variant, the voltage drop over the bridge is higher than with the use of the NIC
circuit, at identical supply voltages.

Fig.5: Flowchart angular position evaluation using GMR C6

( S1, S2: signal 1,2; lL, uL: lower Limit, upper Limit; I.P. intermediate Position)

Start

Initialize

Catch the bridge voltages

S2>uL

S1<lL

S2<lL

I. P..

N

N

S1>uL

N

N

Evaluate

the angle

Evaluate

the angle

Evaluate

the angle

Evaluate

the angle

Evaluate

the angle

Display
1°... 89°

Display

91°...179°

Display

181°...269°

Display

271°... 359°

Display 0,

90,180,270°

Let’s come to the first practical application, the evaluation of a 360° angle, a case that may occur quite
frequently , for example in contactless rotary knobs for radios, household equipment, computers etc.. The
ideal part is the GMR C6, the signal temperaturecompensated and amplified as shown above. Now following
the flowchart above, first identify the correct quadrant, choose the signal with the steeper slope and evaluate
the position.
Using an AD converter with 8 bit resolution you get 256*0.7 values / quadrant, overall a total of about
700 values/ 360°, equivalent to a resolution of roughly 0.5°. For dataprocessing Infineon offers an evaluation
board (Starter kit) with ports and flash memory etc., on the basis of a C505 microcontroller. Of course, the
programme is available without Starter kit also. The starter kit is shown in photograph 1.

Photograph 1: Starter kit “contactless rotary switch with GMR”

Additionally we offer a small PCB with two MW6 footprints and some space for additional sockets for
development engineers, remember the GMRs are SMD parts!
A case of frequent practical interest is the evaluation of angles in the range <130°, for example for seat
memory, pedal position sensing, valveangle position etc.. In this case, the GMR B6 is the sensor of choice,
for two reasons: First the signal is twice that of the C6 and second, there is only 1 signal to be evaluated.
For these small angles the hysteresis is significantly smaller than the value shown in the datasheet. For
example the measurement over an angle of 30°, as shown in fig. 6, hysteresis is <0.1°, for 90° this value
is < 0.2°. This feature opens new applications for GMR angle sensors, such as throttle position-, pedal-
position- or valvepositionsensors.

Fig. 6: Amplified signal (12-fold) and error of a 30° rotation

2,0

2,2

2,4

2,6

2,8

3,0

-20

-15

-10

-5

0

5

10

15

20

Angle of rotation [°]

-1,0

-0,5

0,0

0,5

1,0

Signal

Error

What are the arguments for the use of a GMR –Sensor in this case?
Well, the size of the part – SOT 143, the low powerconsumption, the part can be used in a pulsed mode or
can be driven by lower voltages than 5 V and still offers a reasonable signal and last but not least, the high
assembly tolerances, that are possible without a loss of accuracy, a fact that allows simple i.e. cheap
mechanical solutions. A typical tolerance plot is shown in Fig. 7 for the case of a SmCo magnet with the
dimensions 20*10*5 mm

3

.

-15

-10

-5

0

5

10

15

0

5

10

15

20

25

30

35

Axialer Abstand (mm)

Lateral

er A

b

stand (m

m

)

100%

>75%

>50%

S

N

Fig.7: Relative strength of a GMR–Sensorsingal as a function of the relative position of magnet and sensor.

In combination with polewheels, GMR –Sensors are ideally suited to build noncontacting incremental
potentiometers with direction sensing with very low effort. Depending on the magnetic pitch length, either
GMR B6 or GMR C6 may be used. The basic set-up and the outputsignal are shown in fig. 8.
For a pitchlength <0.5 mm we recommend the use of the GMR B6, while for higher poledistances the
GMR C6 is the better choice. Obviously, the principle shown here for tonewheels, can be easily transferred
to a linear arrangement of magnets.

0

0

Winkel

V

↑

V

↓

N

N

S

R

R

OP

OP

V

↑

V

↓

Fig. 8: Outputsignal and evaluationcircuit for an application as incremental angular or length detection.

Of course, this short presentation cannot cover the complete spectrum of potential applications for GMR-
Sensors but I hope I made you curious and triggered your creativity to discover this type of sensor as a
versatile tool to solve a lot of positionsensing problems. More examples can be found in the web at:

http://www.infineon.com/products/sensors

/383.htm

There you will find, among other things, our application notes in German and English that are updated
continuously. Definitely planned are the topics listed below:
1. Torquesensing with the aid of torsion bars;
2. Small angle evaluation for throttleposition, pedalposition and
3. incremental potentiometers;
Just one hint, there is a printed version of our actual application note in English available