Basic Introduction to the use of Magnetoresistive Sensorseducypedia.karadimov.info › library › an37.pdf · of the sensors. Magnetic field sensor principle An AMR sensor is made

Basic Introduction to the use ofMagnetoresistive Sensors

Howard Mason, September 2003

This application note provides an introduction to the rudiments of anisotropic magnetoresistive(AMR) sensors for those users who may be unfamiliar with their characteristics and modes ofoperation and goes on to describe some applications, with guidelines to getting the best use outof the sensors.

Magnetic field sensor principle

An AMR sensor is made by depositing a verythin film of Permalloy. When a magnetic fieldis applied, the magnetic domains "swing"round and the electrical resistance changes byaround 2-3%.

The physical origin of the magnetoresistiveeffect in the transition metals lies in thedependence on the direction of themagnetization of the scattering of electrons. Inthe transition metals, the predominant carriersof current are the 4s electrons since they have ahigher mobility than the 3d electrons. Thescattering of electrons from the s to the d bandsis found to be highest when the electrons aretraveling parallel to the magnetization.

Lord Kelvin, formerly William Thompson,discovered the magnetoresistive effect. Hefirst observed this effect in ferromagneticmetals in 1856 when he noticed the slightchange in the electrical resistance of a piece ofiron when he placed it in a magnetic field. But ittook more than 100 years before thin filmtechnology could make it into a practicalsensor, which was when the Hunt element wasinvented in 1971.

Applying low magnetic fields to a Huntelement will lead only to small changes of themagnetization. A Hunt element is not sensitiveto small field strengths. In order to make theMR sensor sensitive for low magnetic fields,the MR transfer curve has to be modified. The

most common way to achieve this is bythe use of what are called "barber poles".The geometry of a Hunt element withbarber pole structure (a singleAMR-Resistor) is depicted in Figure 1.

AN 37 - 1 SEMICONDUCTORS

ISSUE 1 - SEPTEMBER 2003

Application Note 37

Figure 1. Barber pole construction showingdirection of magnetic fields

A magnetic field HY coupled into the softmagnetic sensor material will change theresistivity of the stripe, which is measured bypassing a sense current through the element.The linear behavior of the AMR-Resistor isachieved by the use of these barber polestructures. The stripes are covered withaluminum bars having an inclination of |45°| tothe stripe axis (for example -45° betweencurrent and HX-axis). Aluminum has aresistivity about 5 times lower than permalloy,so the barber poles cause a change of thedirection of current. Figure 2 shows theconstruction of the barber poles and how thecurrent direction is changed by them.

The characteristic of the AMR sensor after thebarber poles have been added is representedin Figure 3.

Predictable operation of the sensor is achievedby applying an auxiliary field HX. Thisstabilizing HX-field is usually generated by anexternal or internal permanent magnet.

This field defines the value and direction of themagnetization Ms. The range of HY for safesensor operation is determined by the strengthof the HX-field. In addition, the sensitivity of thesensor can be controlled by the strength of thisauxiliary field, making it very versatile.

In most applications, the single AMR-Resistor isnot suited as it does not provide a zero referenceas well as having an absolute resistance toleranceof ± 30% and a temperature coefficient of+0.3%/K. These disadvantages can be avoided byusing a Wheatstone bridge (AMR-Bridge).Furthermore the output signal of an AMR-Bridgeis twice as high as with the single AMR-Resistor.

The overall size is 1.3mm x 1.9mm. The stripesare arranged into a Wheatstone bridge wiredso that 2 of the 4 resistors increase inresistance when a field is applied and the other2 decrease in resistance. The bridge isbalanced by laser trimming. By comparing themid-point voltages, absolute resistortolerances are canceled and an extremelysensitive field detector can be made. Thecircuit of a AMR-Bridge with fourAMR-Resistors as used in the sensor chip isshown in Figure 4.

AN 37 - 2SEMICONDUCTORS


Figure 2. Barber pole construction

Figure 3. Characteristic of an AnisotropicMagnetoresistive Sensor

Figure 4. Arrangement of four AMR elementswith barber poles into a Wheatstone bridge,showing how resistance changes to give theoutput voltage

Let us assume that there is a field HX as shownin figure 4 above and no field HY to begin with.Bearing in mind that the resistance is lowestwhen the current is flowing parallel to themagnetic field, it can be seen that with no HYfield it starts with the current at 45° to the fieldin all four resistors and the bridge is balanced.

Applying an HY field pushes the resultant fieldin two of the resistors more into line with thecurrent (angle <45°), thus reducing theirresistances and the field in the other tworesistors more out of line (angle >45°), thisincreasing their resistances. This unbalancesthe bridge and gives an output. Applying an HYfield in the opposite direction (but keeping theHX field unchanged) unbalances the bridge inthe opposite direction. We can therefore detectboth amplitude and polarity of the HY field.

The basic resistance of the bridge R0 is 1.7k�and it will change by an amount dR which is:

RR

R021

∆sin sinα α−

where �R/R is set by the properties of thematerial and � is the angle between the currentflow and the resultant field as shown back inFigure 1. The resultant field is the vector sum ofHY (the applied field), HX (the auxiliary field)and H0 (the initial magnetization of thepermalloy).

Because the domains can have randomorientation without a field, there is an inherentmagnetism in the permalloy H0, but theresistance is not stable and predictable, so theauxiliary field HX should be provided to line upthe domains to be at 45° as explained to makethe barber poles work. This can be done using asmall external magnet or a coil, however Zetexoffer the sensors with built-in magnets for thispurpose. The suffix "M" after the part numbersignifies that a magnet is included. By aligningthe domains first, the change in resistance dueto the field at right angles HY will be much morelinear. Fields perpendicular to the chip face(HZ) do not affect the operation.

Safe Operating Area

It is necessary to explain in some detail the"Safe Area" of operation. The followingexplanation applied to the MR sensors whichdo not have internal magnets fitted. If anexcessive field is applied in the Y (measuring)direction with little field in the X direction, thedomains will line up with Y, almost regardlessof the magnitude of HY, and the change inresistance will not be proportional to the fieldwe wish to measure. Not only that, but they canget permanently magnetized in this directionand need "resetting" by a strong HX field.Figure 5 explains the area in which the sensorshould be operated.

An HX field of greater than 2.5kA/m willguarantee that the domains are always resetafter the HY field is removed and cannot be"flipped" into the Y direction and cause falsereadings.

The ZMY20M and ZMZ20M have internalmagnets fitted under the chips and theseprovide an adequate field to ensure that thedevices remain in a safe operating area. Thisalso means the user does not have to providean auxiliary field and the sensitivity is definedat manufacture.



Figure 5. Safe Operating Area

Sensitivity and Polarity

To use the devices effectively, some sensitivityand polarity issues should be understood. Forthe reason as explained above, a strong field inthe HX direction will try to keep the domainsaligned in X and, as the orientation of thedomains is the resultant of HX and HY, it nowneeds a larger HY field before the domains getpulled into the Y direction. Hence thesensitivity is reduced in the presence of astrong HX field, but the range of HY isincreased. The user can therefore choose themeasurable range by choosing HX. Bear inmind that, whether the 2 fields are weak orstrong, the domains are only really doing thesame thing by lining up with the resultantdirection of the two fields, so the maximumchange of resistance is still the same. Thismeans that the maximum output in volts pervolt is always limited at around 15mV/V andsensitivity will be reduced for large HX fields.This makes the device very versatile. Figure 6shows the graphs of sensitivity versus appliedfields in HX and HY.

It is clear from this that no HX field results in thehighest sensitivity, but the device will"saturate" with an output of 15mV/V when HY isonly 1kA/m. An HX field of 6kA/m makes thedevice capable of measuring HY up to 6kA/m,but now the output voltage will still only be15mV/V at this higher field. The device specgives a sensitivity which reflects the middlecurve above with an HX field of 3kA/m. As anexample of a sensor with internal magnet, theZMZ20M magnet provides a field of 2.5kA/m,so the sensitivity is slightly higher than thismiddle curve. The ZMY20M magnet onlyprovides 2.0kA/m, so the sensitivity is higherstill, but still not as high as the devices with nomagnet and therefore no intrinsic HX field.

If using the device with no HX field to obtainmaximum sensitivity, the domains may have"flipped" round the opposite way if the sensorhas previously been subjected to a large fieldand the measurement will be incorrect. Toovercome this, the sensor should be "flipped"into the correct HX direction using a pulse ofcurrent in a coil of 2.5kA/m, which is thenswitched off just before the sensit ivemeasurement is made. This is necessary forcompassing applications for example. Afterthe coil is switched off, the field should belimited to 0.5kA/m to avoid flipping the sensor.It must be said that compassing applications ofAMR devices require some very sophisticatedelectronics.

Although the internal magnet on the ZMY20Mand ZMZ20M guarantees the sensor can not beflipped, there is still a maximum allowable fluxof 40kA/m with the devices with internalmagnets. Above this, the internal magnet willbe damaged and the sensor will no longer workcorrectly.



Figure 6. Sensitivity of Output voltage toHY field for different values of HX field.

Temperature Coefficients

If the user wishes to measure fields accuratelyover a wide temperature range, bear in mindthat the sensitivity in mV/V itself has atemperature coefficient of minus 0.3%/K. Thebridge resistors themselves have atemperature coefficient of resistance of plus0.3%/K. There are two ways to cancel out thetemperature effects , ei ther viatemperature-dependent circuitry in theexternal amplifier or by using constant currentdrive to the bridge.

Using constant current drive, the +0.3%/Kcoefficient of the bridge resistance results inthe bridge voltage rising at +0.3%/K whichcancels out the -0.3%/K decrease in thesensitivity. This is because the output is inmV/V, so a 0.3% greater bridge voltage resultsin 0.3% more mV for the same field. Oneproblem here is that the absolute bridgeresistance has a tolerance of ±30%, so thecurrent must be set up for the particular bridge,otherwise the output voltage would beexcessively low for low resistance bridges andvice versa. Figure 7 shows a way of using theZetex ZMR500 to force constant currentthrough a bridge to obtain reasonablytemperature independent sensitivity to the HYfield. In this case it is actually used with theZMC10 10 amp current sensor, so it will obtaina temperature independent sensitivity tocurrent being measured.

It can be preferable to supply the bridge with aregulated voltage and design an amplifier witha temperature coefficient of gain of +0.3%/K.This way the bridge resistance tolerance doesnot affect the circuit and it is not necessary toset up the current for each bridge.

Another important parameter is thetemperature coefficient of the sensor offsetTCOFF. This effect is caused by smalldifferences in the temperature behavior of thefour sensor bridge resistors. In practice, itleads to a drift in the output voltage which cannot easily be separated from the output signalcaused by the magnetic field HY which is beingmeasured. In applications using DC-signalcoupling the TCOFF will limit the measurementaccuracy. If the offset voltage is nulled out byexternal adjustment at one temperaturechanges of temperature will result in the offsetchanging, limiting accuracy.



Figure 7. Constant Current Drive to ZMC10Current Measuring Bridge

Frequency Response

It is useful to know the two effects which limitthe frequency response of MR sensors. One isthe "magnetic inertia" of the permalloy, whichmeans that the magnetic domains take a finitetime to align themselves with the externalfield.

In addit ion, eddy current reduces thepermeability of magnetic materials such asNi81Fe19 at higher frequencies. This meansthat, when the sensitivity is plotted inmV/V/kA/m versus frequency, it rolls off at1MHz and the devices can not be used abovethis.This limiting frequency applies to the AMRsensor itself and therefore affects the wholerange of devices.

However, in the case of the current measuringdevices (ZMC range), there is anotherlimitation. The conductor carrying the currentis subject to the "skin effect" which means thatthe field is not proportional to the currentabove 100kHz. The AMR chip measuring thisfield will therefore give erroneous readingsand so the ZMC series can not be used above100kHz.

Figure 8 shows the current path being alteredand a formula to explain this effect.

If the field sensors are used to measure thecurrent in a conductor (as explained later inthis article) the skin effect will have to becalculated for that particular conductor, unlessthe frequency is low, for example whenmeasuring mains currents.



Figure 8. The Skin Effect in conductors

Stress, Anisotropy and Hysteresis

The user should be aware of three effects if theMR sensor is used for very accuratemeasurements, especially if these are aroundits zero field point.

Package Stress

Firstly, stress applied to the package of an AMRsensor can cause bending of the permalloystructure. This causes magnetostriction effectswhich can cause the bridge to becomeunbalanced even when the HY field is zero.Figure 9 shows how mounting a package on aPCB badly will induce stress and zero offsets:

The package stress may be reduced by the useof a small carrier if the MR sensor is beingmounted on a very large PCB where stress maybe expected.



Figure 9. Package stressed by PCB distorting AMR sense element

Anisotropy caused by Asymmetrical Electrical Loads on Bridge

The bridge will need an amplifier to amplify thesmall signals and this will present a finiteelectrical load to the bridge. For large fieldoperation, such as proximity detector giving arail to rail output voltage, it does not matter ifthe two outputs of the bridge are loadedasymmetrical ly. However for accuratemeasurement of fields the amplifier shouldpresent a symmetrical load. The two examplesare shown in Figure 10 for the 5 amp currentsensor ZMC05. Version 1 is recommended andversion 2 is not recommended.

The asymmetrical current load for the sensorwould increase the anisotropy effect of themagnetostrictive sensor and lead to offsets,making it impossible to measure very low fieldstrengths accurately.



Figure 10. Symmetrical and Asymmetrical Loading of Bridge

Hysteresis

Another characteristic of the AMR sensorswhich should be noted is that of hysteresis.The accuracy of the magnetoresistive sensorsduring low field measurements is affected byhysteresis. The magnetizat ion of thepermalloy strips is not always completelyhomogenous, and an effect called "pinning"can occur, where some magnetic domains inthe permalloy can swing other domains intoline with them, causing hysteresis.

If the output must be measured accuratelyaround the zero field point, it is possible todetermine the amount of hysteresis and cancelit out electronically, in a manner similar toOp-Amp offset voltage cancellation.



Applications

Field Sensors

It is worth describing the various types of fieldsensor. These all use the same AMR chip, but itis available either in a SOT223S package, withthe prefix "ZMY" or in a 4 pin E-line packagewith the prefix "ZMZ". The number "20"signifies the bridge resistance (this used to be2k�, but is now 1.7k�). Both these devices areavailable with an internal magnet to providethe auxiliary field, in which case they have thesuffix "M" or without the magnet, in which casethey have no suffix. Hence the ZMZ20M is a1.7k� bridge in a 4 pin E-line package with aninternal magnet.

Some simple applications will be described,firstly a proximity switch using the ZMZ20M,the circuit of which is shown in Figure 11.

The output from the bridge is amplified usingan LM339 and used to light the LED oralternatively the output can be brought out todrive other external circuits. It is advisable toinclude some positive feedback on circuits toprovide hysteresis, otherwise if the metalobject is on the threshold of the circuit, theoutput could jitter or oscillate.



Figure 11. Proximity Detector using ZMZ20M plus LM339.

Another circuit uses two Zetex ZR431regulators, one of which regulates the voltageon the top of the bridge to 5 volts and the otheracts as an amplifier The power consumptioncan be reduced and operation from a 5 volt railis possible if the ZR431L regulator is usedinstead, as this will regulate the voltage on thetop of the bridge to 2.48 volts, which means thebridge will only draw 1.5mA. The circuit usingtwo ZR431s is shown in Figure 12.

A typical application for this proximity detector(nowadays called "Proxies") could be thedetection of a rotating wheel, e.g. for enginetiming, shown in Figure 13.



Figure 12. Proximity Detector using ZMZ20M

ZMX50MT8: target presentation, engineering sample

Figure 13. Proximity Detector

Current Measurement is possible using theField Sensors, by placing them close to aconductor and suitable calibrating them interms of bridge output versus conductorcurrent. Two obvious advantages are that thegalvanic isolation can be made as high asdesired and there is no upper limit to thecurrent to be measured, as the sensor can bepositioned as near or far away as is needed.Currents of hundreds of amps in conductors athundreds of volts can be measured. Figure 14shows some suggestions for arranging theZMY20M near to conductors.

Note that in both version 1 and version 2 thefield is strongly curved near the sensor, so if itmoves laterally, the HY component of the fieldwill change appreciably. The devices are notsensitive to fields vertical to the chip (we maycall this the HZ field) so the sensitivity wouldreduce as the device moves to the side as thesame resultant field from the conductor wouldhave a smaller HY component and a larger HZcomponent.



Figure 14. ZMY20M used for Current Measurement in Circular Conductors

This does not matter as long as the ZMY20M isheld rigidly in a fixed position relative to theconductor.

If the conductor is flat, it will give a parallelmagnetic field and slight lateral movements ofthe ZMY20M will have less effect on the valueof HY and therefore the sensitivity. This isshown in Figure 15.

If the expense of an iron or ferrite core can bejustified, very linear current measurement ispossible with this arrangement shown inFigure 16.



Figure 15. ZMY20M used for Current Measurement in Flat Conductors

Figure 16. ZMY20M used for accurate proportional currentmeasurement

ZMC Series of Current Sensors

The next sensors to be described are the ZMCcurrent measuring devices. These contain thesame MR sensor chip, but the leadframe isshaped into a conductor running beneath thechip, so that magnetic fields produced by thecurrent in the conductor can be measured bythe sensor. The current conductor is isolatedfrom the leadframe by a glass insert capable ofwithstanding 200 volts on the ZMC05 and aceramic insert capable of withstanding 2000volts on the ZMC10 and ZMC20. The 2000 volttypes are ideal for measuring high-sidecurrents, e.g. in mains circuits. The ZMC05 is inan SM8 package and measures currents up to 5amps. The ZMC10 is in a modified DIL14 stylepackage and measures up to 10 amps. TheZMC20 is also in a modified DIL14 stylepackage and measures up to 20 amps.

Figure 17 shows how the ZMC05 can be usedfor measuring the current in a motor withcomplete galvanic isolation between themotor circuit and the measuring electronics.



Figure 17. ZMC05 measuring motor current

ZMC10 Current Measurement using Burr Brown INA125

Another application circuit is shown here inFigure 18 which uses the ZMC10 in conjunctionwith a Burr Brown INA125 instrumentationamplifier. The bridge is fed by constant currentset to about 6mA. As explained earlier under"Temperature Coefficients" this enables thenegative tempco of sensitivity to be canceledout by the positive tempco of the bridgeresistance, hence bridge operating voltage.

An evaluation board for the ZMC10 and ZMC20is available which has 2 devices and so canmeasure and compare 2 currents. It hascomparators and 6 LEDs which give anindication of various current measurementsbeing performed.

There are also analog test points to look at thevarious voltages, enabling the user tounderstand the operation of MR sensors ascurrent measuring devices. The sensorconstant current can be set by links, enablingthe user to understand how the operatingcondit ions affect the accuracy of themeasurements.



Figure 18. ZMC10 used with Burr Brown INA125 Instrumentation Amplifier

ZMT31 Rotational Measurement

Although the ZMY20M and ZMZ20M can beused for the measurement of shaft rotation oraccurate positioning of a shaft at a single point(e.g. top dead center) they are sensing thepresence or absence of teeth on a wheel. Amagnet could be mounted on the shaft, but thissensor would only give a one-dimensionalmeasurement of the field. This would besymmetrical as the shaft moved either side of acenterline through the sensor and would notgive full positional information.

The ZMT31 contains 2 bridges at 45° to eachother in an SM8 package. If a magnet isplaced above the device and rotated, thesensors will give two sinusoidal outputsversus magnet angle, phase shifted by 45° asshown in Figure 19.

Please note that there is NO internal magnet asthis would interfere with the measurement ofthe external field. This means that, asexplained in the section on "Sensitivity andPolarity" earlier, the bridge will go out ofbalance in the same way when it sees either anorth or south pole. This is the reason it gives acomplete cycle for 180° rotation of the magnetas can be seen in Figure 20 above.

An important point to consider is that thisdevice is designed to give a sinusoidalvariation of resistance varying with the ANGLEof the field, not its intensity. The field should bevery strong (>50,000 Amps/meter) to ensurethat ALL the magnetic domains line up with it. Ifa low field strength is used, there would besome domains not lined up, as with the linearfield sensors, and the output voltage would notbe exactly proportional to the sine of the angle.



Figure 19. Outputs of the ZMT31 versusangle of magnet

The output voltages of both Wheatstonebridges versus angle (a) of the magnetic fielddirection is as follows:

Bridge 1 output voltage 1 = sin 2( +45°)

Bridge 2 output voltage 2 = sin 2( )

sin 2( +45°) = - sin -2( +45°) = - sin 2(45°- ) = -sin (90°-2 ) = - cos 2( )

output voltage 2 / output voltage 1 = sin 2( ) / -cos 2( ) = - tan 2( )

A circuit for processing the outputs of theZMT31 is shown in Figure 20.



Figure 20. ZMT31 Applications Circuit

This device is ideal for designing a contactless,completely smooth potentiometer as well asfor shaft encoders, machine tool control andsensing of actuating levers in automotiveapplications.

Figure 21 shows a pictorial representation theconstruction of a contactless potentiometer.

ZMX40M Dual Sensor

The ZMX40M contains two sensor chips,mounted parallel to each other in an SM8package. In addition an internal magnet isincluded to bias the sensors into the linearregion. The number "40" is because there are 2bridges inside, each bridge is still 1.7k�.

The spacing between the chips is exactly 3mm.If a magnet travels horizontally above thesensor, each chip will give an output which willpeak as the magnet passes above it and the twopeaks will be spatially separated by 3mm.

When the 2 peaks are the same amplitude, themagnet must be mid-way between the twochips. This sensor can be used to measure theposition of, for example, a wheel tooth veryaccurately for automotive and machine-toolapplications. With calibration to allow for thetolerances on the bridge outputs being slightlydifferent, the ZMX40M has been used inmachine tool applications to resolve distancesdown to 30µm. By comparing the two outputsand adding some hysteresis, a large-geometrymagnetic tape reader can be made.

Conclusion

The Zetex range of Linear FieldMagnetoresistive Sensors have applications inlinear position sensing for process control,used for counting devices, door interlocks,proximity detectors, engine position andspeed measurement, the sensing of a rotatingimpeller inside a pipe for fluid metering,vehicle sensing and traffic counting andmeasurement of current in a conductor withcomplete galvanic isolation up to very highvoltages.

The Angle Sensor applications include pedaland gear lever sensing, speed and anglesensing of shafts in machine tools or enginesand the construction of a contactless, highresolution potentiometer.

The Current Sensor range can be used inautomotive, motor control, power tools, audioamplifier overload protection, traffic lamp bulbsensing and high intensity discharge lampballast circuits.



Figure 21. Rotating magnet above ZMT31



AN 37 - 20

Application Note 37ISSUE 1 - SEPTEMBER 2003

Europe

Zetex plcFields New RoadChaddertonOldham, OL9 8NPUnited KingdomTelephone (44) 161 622 4444Fax: (44) 161 622 [email protected]

Zetex GmbHStreitfeldstraße 19D-81673 München

GermanyTelefon: (49) 89 45 49 49 0Fax: (49) 89 45 49 49 [email protected]

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Zetex Inc700 Veterans Memorial HwyHauppauge, NY 11788

USATelephone: (1) 631 360 2222Fax: (1) 631 360 [email protected]

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Zetex (Asia) Ltd3701-04 Metroplaza Tower 1Hing Fong RoadKwai FongHong KongTelephone: (852) 26100 611Fax: (852) 24250 [email protected]

These offices are supported by agents and distributors in major countries world-wide.This publication is issued to provide outline information only which (unless agreed by the Company in writing) may not be used,applied or reproduced for any purpose or form part of any order or contract or be regarded as a representation relating to the productsor services concerned. The Company reserves the right to alter without notice the specification, design, price or conditions of supply ofany product or service.For the latest product information, log on to www.zetex.com

© Zetex plc 2003

SEMICONDUCTORS

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Basic Introduction to the use of Magnetoresistive Sensorseducypedia.karadimov.info › library › an37.pdf · of the sensors. Magnetic field sensor principle An AMR sensor is made