Sensor Saturation for Hysteresis Reductionin GMR Magnetometers

Philip S. Mease and Robert R. KrchnavekElectrical & Computer Engineering

Rowan UniversityGlassboro, New Jersey 08028–1700

Email: mease@rowan.edu

Jacob T. Kephart and Peter FerraraNaval Surface Warfare Center

Carderock DivisionPhiladelphia, Pennsylvania 19112

Abstract—Giant Magnetoresistance (GMR) sensors offer sev-eral advantages over other technologies for the development ofgeneral-purpose magnetometers. They are applicable for bothAC and DC fields, are relatively sensitive, can be fabricatedto measure over a large range of field strengths, have a high-speed response and are low cost. One significant disadvantageof the GMR sensor is that the output is a function of thehistory of the magnetic fields that have been on the device,i.e., hysteresis. In this work, we demonstrate that hysteresiseffects can be virtually eliminated by saturating the GMR sensorprior to making a field measurement. This forces the sensorto follow the same path of output voltage versus applied fieldfor each measurement. Within the limits of our equipment,we cannot determine any hysteresis effects when using thissaturation technique and therefore measurement repeatability isdramatically improved.

I. INTRODUCTION

There are a variety of techniques available to measure thestrength of a magnetic field and instruments that make thismeasurement are known as magnetometers. The most commonmagnetometers include: Hall effect magnetometer; fluxgatemagnetometer; SQUID magnetometer; and, more recentlyGMR magnetometers [1]. A variety of additional magnetome-ters are available, but, in general, the measurement techniquesdo not readily lend themselves to field measurements and aretypically reserved for laboratory settings.

Different physical phenomena that are affected by magneticfields give rise to the techniques listed above. It is not thepurpose of this report to evaluate all of the available techniquesfor the measurements required, but it is sufficient to statethat some techniques are more sensitive than others, butusually at an increased level of measurement complexity. Forexample, a SQUID (superconducting quantum interferencedevice) magnetometer is one of the most sensitive magne-tometers. However, instruments based upon SQUIDs requirecooling down to liquid helium temperatures (4.2K) [2]. Thissignificantly complicates field measurements.

Giant Magnetoresistance (GMR) sensors provide a uniqueopportunity to create a field magnetometer with a desirableset of features. For example, GMR sensors detect a magneticfield directly and do not require a changing magnetic field foroperation. This makes them applicable for DC as well as ACmagnetic fields. GMR sensors can be fabricated to measure

over a large range of field strength and are the sensor of choicefor measuring the small fields associated with hard disk drives[3]. GMR sensors also respond quickly to magnetic fields andare applicable for varying fields up to 1 MHz.

The basis of measurement for a GMR manetometer is thatthe resistance of a GMR sensor drops when a magnetic fieldis applied to the sensor. If the GMR material is assembled ina balanced, resistive bridge configuration, significant outputvoltage can be measured due to a change in magnetic fieldstrength.

Unfortunately, GMR sensors exhibit significant hysteresis[4]. This means that the output of the device is a function ofthe history of the fields that have been present on the device.This is easily seen in Figure 1 below.

Fig. 1. The output of a GMR sensor is a function of the the history ofthe magnetic field. Note that the output voltage will be different for identicalmagnetic fields if the field was increasing from 0 or decreasing from thesaturated condition [4].

The hysteresis of the GMR sensor ultimately degrades theaccuracy of the measurement system. Traditional techniquesof hysteresis modeling add significant complexity to the mea-surement system in real world applications [5]. In this work,we demonstrate a method of reducing the effects of hysteresisby controlling the history of the fields on the sensor just priorto making a measurment.

978-1-4244-2787-1/09/$25.00 ©2009 IEEE

II. PROPOSED APPROACH

The proposed approach is to develop a circuit that will ener-gize a coil surrounding the GMR sensor that will momentarilysaturate the sensor. This will force the sensor to follow the pathfrom the positive saturated magnetic field back to zero – theupper curve in Figure 1. The field measurement is taken afterthe pulse current in the saturating coil has returned to zero.The design parameters for this approach include:

• minimizing the saturation on-time: measurements cannotbe taken in this region, therefore decreasing pulse-timecreates a larger measurement window

• eliminate ringing: allowing the GMR sensor to go neg-ative introduces additional hysteresis. Ringing deep intothe provided measurement window also causes erroneousmeasurements. Quick damping of this ringing is critical.

III. RESULTS

In this section, we discuss several important aspects of thecircuitry and show the results of magnetic field measurementswith and without the pulse-current saturating circuit in use.

A. Circuit Subsystems

The circuit can be divided up into 3 subsystems: PowerSection; GMR Sense; and GMR Saturation. For this paper, wewill ignore the Power Section and concentrate on the GMRSense and GMR Saturation subsystems.

1) GMR Sense: The GMR Sense subsystem consists of2 major components, the GMR sensor and instrumentationamplifier. In this circuit, we are using the AAH002-02 sensorfrom NVE Corporation [6]. The AAH series of sensors fromNVE Corporation use a 2 kΩ Wheatstone bridge. Because ofthe low internal impedance of the Wheatstone bridge, theoutput of the GMR sensor will go into an instrumentationamplifier. The high input impedance of the instrumentationamplifier will maximize the output voltage from the GMRsensor. However, the input impedance is not critical sinceit only needs to be high relative to the internal impedance(2 kΩ) of the GMR sensor. One leg of the Wheatstone bridgecontains the GMR material. The output of the sensor bridgemeasures the voltage difference between the two center pointsof each leg of the bridge. The sensor bridge is supplied witha regulated 5V supply. This sensor has a useable linear rangefrom 0.6 G to 3.0 G and the sensitivity has a minimum valueof 11.0 mV/V-G.

The output from the GMR sensor is connected to a high-precision instrumentation amplifier (AD620S) [7]. The pri-mary purpose of the instrumentation amp is to create adifferential measurement and decrease the load on the sensorbridge. The gain of the instrumentation amp is set by choosingthe value of R2. The relationship between the gain (G) andR2 is given by

R2 =49.4 kΩ

G− 1.

Although the AD620S has a gain range from 1-10,000, theinstrumentation amp will saturate at the supply voltage minus1.2 V. Since the supply to the instrumentation amp is set

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at 5 V, the maximum output voltage is limited to 3.8 V.Therefore, the gain should be limited to values that will notexceed 3.8 V. For this work, the gain was set to 2. Notethat increasing the gain will not improve the signal-to-noiseratio because the noise from the GMR sensor will also beamplified. However, increasing the gain may be necessaryfor the application-specific data acquisition instrumentation.Finally, if a voltage offset (Voffset) is desired, this can beaccomplished by applying an appropriate voltage at P5 of theAD620S. Then, the output from the instrumentation amplifieris given by

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Between the GMR sensor and the instrumentation amp is apassive filter network.

This network can be configured as either a low-pass or high-pass filter. This has the advantage of reducing amplification ofout-of-band signals. The cutoffs frequencies for the low-passand high-pass configurations are defined by

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Note, the cutoff frequencies should be the same for each inputto the instrumentation amp. By default the fiter is bypassed byjumping C8 and C9 and leaving R4 and R5 open. Low-passoperation is configured by swapping the positions of the Rsand Cs. The schematic is showing the configuration for high-pass operation. These simple first-order filters were intendedprimarily for testing. Depending on the environment in whichthe magnetometer will be used, more sophisticated filteringmay be required. In our testing, the filter was usually bypassed.

2) GMR Saturation: The third subsystem is the GMRSaturation subsystem (Figure 3). This subsystem is designedto create a large magnetic field around the GMR sensorwhich saturates the device. After saturating the GMR sensor,a magnetic field reading can be taken using the GMR Sensesubsystem. The purpose of saturating the GMR sensor is thatit forces the sensor to follow a particular hysteresis curve,therefore increasing the repeatability of the measurement.Unlike other hysteresis reduction techniques, memory is notan issue since the device is driven to the same limit for eachmeasurement.

The first component in this subsystem is a buffer/gate-driverIC (U2 – TC4424). The input to the driver is a rectangularpulse that determines the turn-on time and duration for thesaturating pulse. The buffer provides a high impedance loadfor the input signal as well as sufficient drive for the MOSFET(Q1 – IRLZ44NS). Components R7, D2, and R10 allow forfast turn-on of Q1, but can be set to limit the rate of turn-off. When turning on Q1, diode D2 is forward biased andprovides a low impedance path for driving the gate of Q1.The RC time constant for turn-on is given by the product ofR11 and the gate-source capacitance of Q1, Cg. Since Cg isfixed for a given MOSFET, R11 becomes the primary methodof controlling the turn-on time constant. Of greater concernis the time constant for turn-off. When turning off Q1, diodeD2 becomes reverse biased and current must flow throughR7 and R10 in parallel. These resistors, in conjunction withthe large gate-source capacitance of the MOSFET, Cg, andR11, create an RC time constant limiting the speed of turn-off. The RC time constant for turn-off is given by the productof Cg and the equivalent resistance of R11 in series with theparallel combination of R7 and R10. If Q1 is turned off tooquickly, the current flowing through the saturating inductor,L1, would create a high voltage which could damage Q1, causesignificant circuit ringing into the critical measurement region,and may reverse the magnetic field on the GMR sensor. Allof these are undesirable effects. Whether specific values forR7 and R10 are needed will depend on the desired switchingfrequency as well as the drive current through the inductor. Ininitial testing, both R7 and R10 were bypassed.

The output side of the MOSFET driver utilizes two snubberswith transient voltage suppression (TVS) clamping. The coil-parallel RCD (resistor-capacitor-diode) snubber is tuned toremove much of the high frequency ringing, which was testedto be in the MHz range for the particular 3 µH inductor thatwas used. Determining values for this snubber requires knowl-edge of the circuit switching frequency, inductance used tosaturate the GMR sensor, and the peak current used to saturatethe GMR sensor. Values for R6 and C10 are determined asfollows.

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Fig. 3. A schematic drawing of the GMR Saturation portion of the circuitry.

where Vclamp is the desired clamping voltage, L is theinductance, Ip is the peak current, and f is the switchingfrequency and

C10 =Vclamp

VrippleR6f

where Vripple is the ripple voltage superimposed on the clampvoltage and is chosen at 5% [8, 9].

The MOSFET-parallel RC snubber (R9 in parallel with Cdsof Q1) is used for lower band ringing. The TVS (D4) limits theintial spikes to a reasonable level while lowering the amountof work required by the other damping networks. Due to thenature of this circuit, there is constant leakage through thedamping network and active devices when the MOSFET, Q1,is off. The majority of the leakage is through D3 and R9.However, since this current is constant and relatively small,leakage current should have no effect on the measured sensorsignal so long as the components and inductor temperatureshave stabilized and the data acquisition instrument is zeroed.

B. Magnetic Field Measurements

The goal of this sensor system is to increase the repeatabilityof magnetic field measurements by reducing the hysteresis ofthe sensor. This is accomplished by saturating the sensor priorto taking a measurement, waiting the small deadband (whilethe circuit damps ringing), and measuring the output. To testthe circuit, we set up a pair of 200-turn, 21cm Helmholtz coilsand an independent magnetic field measuring unit (LakeShore475 DSP Gaussmeter) to confirm field inside the Helmholtzpair. As expected, inserting the idle circuit into the magneticfield produced by the Helmholtz coils did not change thereadings on the gaussmeter.

We then used the GMR sensor to measure the magneticfield produced by the Helmholtz coils while we systematicallyvaried the field created by the Helmholtz coils by adjusting theHelmholtz coil current. We confirmed GMR sensor readingswith the gaussmeter. The sensor we used had a linear rangefrom 0.6 - 3.0 G. In this set of measurements, we beganwith 0 amps in the Helmholtz coils, made a measurement,then increased the current, made another measurement, andso on. We did this until the field strength was 4 G, which iswell above the saturation point of the GMR sensor. At thispoint, we continued making measurements while decreasingthe current in the Helmholtz coils and therefore the magneticfield. This data is shown in Figure 4. The arrows indicatewhich measurements were taken while increasing the current(right-pointing arrow) and which were taken while decreasingthe current (left-pointing arrow). The hysteresis of the GMRsensor is clearly visible in this figure.

We then repeated the measurements, following the sameprocedure of making a measurement (increasing the currentin the Helmholtz coils, making another measurement, and soon, until the GMR sensor was saturated, and then continuedby decreasing current in the Helmholtz coils while makingmeasurements until we reached 0 current) with one majordifference: the GMR Saturation subsystem was utilized to

Fig. 4. A graph of the GMR sensor output as a function of applied magneticfield. The current creating the applied magnetic field was ramped up and thendown. The hysteresis of the GMR sensor is easily observed.

saturate the GMR sensor prior to making each measurement.Note, the current in the Helmholtz coils remained constantduring the saturation phase of the measurement. By saturatingthe GMR sensor prior to each measurement, the measurementis pushed to the same side of the hysteresis curve, regardlessof whether the current in the Helmholtz coils is increasingor decreasing. This data is shown in Figure 5. Again, thearrows indicate the set of measurements corresponding toincreasing/decreasing current flow in the Helmholtz coils. Notethat the hysteresis is eliminated. Within the accuracy andresolution of our instrumentation, we were unable to quantify adifference between field measurements made for increasing ordecreasing current flow in the Helmholtz coils. In other words,saturating the GMR sensor prior to making a measurementgives the sensor the same history for each measurement andtherefore is independent of the history of the field strengthbeing measured, i.e., the Helmholtz coils in this experiment.

As a final example of circuit functionality, we examined theresponse at critical circuit points on an oscilloscope. Figure 6shows three signals of the circuit in operation during the GMRsaturation phase. The top signal is the input to the MOSFET.The next signal is the output voltage from the GMR Senseportion of the circuit board. As expected, this voltage saturatesunder the presence of the large magnetic field. The bottomsignal is the current flowing through the coil used to saturatethe GMR sensor. In this example, the peak current reachesapproximately 2 A. The horizontal time step is 200 µs. Notethe very small overshoot in current as the MOSFET turns off.

IV. CONCLUSION

The circuit developed and fabricated is successful at re-moving hysteresis from the GMR sensor. Within the limits ofour instrumentation, randomly varied magnetic fields producean output equal to the down-going curve of the GMR output

Fig. 5. A graph of the GMR sensor output as a function of applied magneticfield. The GMR sensor was saturated prior to making each measurement. Thiseliminates the hysteresis seen in Figure 4.

Fig. 6. An oscilloscope screen capture showing 3 signals on the circuit. Thetop signal represents the input voltage to Q1 and the bottom signal is thecurrent through the coil. The saturating current reached approximately 2 A.

vs magnetic field curve. The circuit has been designed withadjustable gain, input filtering, transient voltage suppressionto minimize ringing, low-leakage through device choice, andhigh-speed operation through appropriate drive circuitry. Mostof these features are designed so that the user can tailor thecircuit operation for the specific application.

REFERENCES

[1] R. Schneider and C. Smith, “Low Magnetic Field Sensing with GMRSensors – Part I: The Theory of Solid-State Magnetic Sensing,”http://www.sensorsmag.com/sensors/article/articleDetail.jsp?id=325285,last checked 10/1/09, 1999.

[2] D. Drunga), C. Amanna, J. Beyera, A. Kirstea, M. Petersa, F. Ruedea,Th. Schuriga, C. Hinnrichsb, and H.-J. Barthelmessb, “High-Performancedc SQUID Sensors and Electronics,” IEEE/CSC & ESAS EuropeanSuperconductivity News Forum, , No. 1, July 2007.

[3] C. Smith and R. W. Schneider, “Low-Field Magnetic Sensing with GMRSensors,” in Proceedings of Sensors EXPO-Baltimore, May, 1999.

[4] Application Notes for GMR Sensors, NVE Corporation, www.nve.com.[5] Z. Sari and A. Ivanyi, “Statistical approach of hysteresis,” Physica B 372,

pp 4548, 2006.[6] Data sheet, AA and AB-Series Analog Sensors, NVE Corporation,

www.nve.com.[7] Application Note for AD620S, Analog Devices, Inc. www.analog.com.[8] Application Note AN848, Maxim Integrated Products, Inc., www.maxim-

ic.com/an848.[9] Application Note AN4138, Fairchild Semiconductor, Inc.,

www.fairchildsemi.com.