ENGINEERING AND APPLICATION NOTES

Transcription

1 ENGINEERING AND APPLICATION NOTES GIANT MAGNETORESISTIVE (GMR) SENSORS Notes to Users: 1. For additional information, including product data sheets, new product releases, additional technical data, samples or technical assistance, please contact NVE: NVE Information line: (800) Specific Product or Company Information: (612) Customer Applications Engineering: (612) NVE Internet Home Page: 2. NVE reserves the right to make product changes and improvements from time to time. Technical data will be updated accordingly as soon as possible. 3. NVE reserves the right to discontinue any product without notice. 4. The information contained herein is believed to be accurate as of the date of printing, however NVE assumes no responsibility for its use. 5. If application questions or concerns exist, please contact NVE prior to use. Page 1

2 TABLE OF CONTENTS INTRODUCTION...4 PRODUCT OFFERINGS AND SPECIFICATIONS...4 GENERAL COMMENTS...4 COMPETITIVE TECHNOLOGIES...5 GIANT MAGNETORESISTIVE MATERIAL PHYSICS...5 GMR MAGNETIC FIELD SENSORS...6 GMR MAGNETIC FIELD GRADIENT SENSORS (GRADIOMETERS)...8 MAGNETIC REFERENCE INFORMATION...10 PERMANENT MAGNETS...10 MEASUREMENT SYSTEMS...10 CONVERSION FACTORS...11 SIGNAL CONDITIONING CIRCUITS...12 OPERATIONAL AMPLIFIER (OP AMP) BRIDGE PREAMPLIFIER...12 INSTRUMENTATION AMPLIFIER (IA) BRIDGE PREAMPLIFIER...16 THRESHOLD DETECTION CIRCUIT...18 NOISE IN NVE GIANT MAGNETORESISTIVE SENSORS...19 USE OF GMR MAGNETIC FIELD SENSORS...20 GENERAL CONSIDERATIONS...20 MAGNETIC BIASING...21 TYPICAL APPLICATIONS...24 MEASURING DISPLACEMENT...24 ANGULAR POSITION/SPEED MEASUREMENT...25 CURRENT MEASUREMENT...26 MAGNETIC MEDIA DETECTION...29 APPENDIX...32 APP 002 APP 003 CURRENCY DETECTION...32 GMR CURRENT SENSING...36 Page 2

3 LIST OF FIGURES FIGURE 1 - TYPICAL GMR MAGNETIC FIELD SENSOR LAYOUT...6 FIGURE 2 - GMR MAGNETIC FIELD SENSOR OUTPUT CHARACTERISTIC...7 FIGURE 3A - BASIC GRADIOMETER BRIDGE SENSOR LAYOUT...8 FIGURE 3B - GRADIOMETER BRIDGE SENSOR OUTPUT CHARACTERISTIC...9 FIGURE 4 - SINGLE OP AMP PREAMPLIFIER CIRCUIT...12 FIGURE 5 - TWO OP AMP PREAMPLIFIER CIRCUIT...14 FIGURE 6 - THREE OP AMP PREAMPLIFIER CIRCUIT...15 FIGURE 7 - INSTRUMENTATION AMPLIFIER PREAMPLIFIER CIRCUIT...16 FIGURE 8 - THRESHOLD DETECTION CIRCUIT...18 FIGURE 9 - AC MODULATION/ DEMODULATION BLOCK DIAGRAM...19 FIGURE 10- SENSITIVE MAGNETIC AXIS - AAXXX-02 SENSOR...20 FIGURE 11 - MAGNETIC BIASING CONFIGURATION...21 FIGURE 12 - BIASING UP THE CURVE...21 FIGURE 13 - FINDING MINIMUM RESISTANCE...23 FIGURE 14 - GRADIOMETER MAGNET POSITIONING...23 FIGURE(S) 15 A&B - CONFIGURATIONS FOR MEASURING LINEAR DISPLACEMENT...24 FIGURE 16 - ANGULAR POSITION SENSING...25 FIGURE 17 - SENSING MAGNETIC FIELD FROM A CURRENT-CARRYING WIRE...26 FIGURE 18 - SENSING MAGNETIC FIELD FROM A CURRENT CARRYING PCB TRACE...27 FIGURE 19 - CURRENT SENSOR WIRE POSITION COMPARISONS...28 FIGURE 20 - CURRENT SENSOR OUTPUT COMPARISONS...28 FIGURE 21 - EXTERNAL FLUX CONCENTRATOR EXAMPLE...30 FIGURE 22 - FERRITE BAR MAGNET...32 FIGURE 23 - FRONT...32 AND BACK BIASING...32 FIGURE 24 - BIASING MAGNET POSITIONING...33 FIGURE 25 - BIASING MAGNET POSITIONING...34 FIGURE 26 - GAP PERFORMANCE...34 FIGURE 27 - FUNCTION FOR 90 MIL TRACE...37 FIGURE 28 - FUNCTION FOR 60 MIL TRACE...38 FIGURE 29 - HYSTERESIS INITIALIZATION...39 FIGURE 30 - FUNCTION FOR 10 MIL TRACE...40 FIGURE 31 - FUNCTION FOR 7 X 10 MIL TRACE...41 FIGURE 32 - FUNCTION OF INTERFERENCE...43 Page 3

4 INTRODUCTION PRODUCT OFFERINGS AND SPECIFICATIONS Presently NVE offers standard and custom magnetic field sensors based on the Giant Magnetoresistive (GMR) effect. These sensors come in two categories; GMR Magnetic Field Sensors (without conditioning electronics) and integrated sensors employing onchip signal conditioning. We are continually adding to our standard product offering while providing engineering services to create custom GMR sensors for specific customer requirements. This publication does not contain specifications for all NVE GMR sensors. Applicable technical data sheet(s) are available per request.. Please consult NVE for further information. GENERAL COMMENTS The NVE GMR sensors are designed to measure or sense magnetic field strength over a wide range of fields. GMR sensors directly detect magnetic field rather than the rate of change in magnetic field. Therefore, they are useful as DC field sensors. NVE s GMR sensors are sensitive to small changes in magnetic fields. This allows for accurate measurement of position or displacement in linear or rotational systems. The extremely small size of the sensing element enhances the position sensitivity, especially in applications incorporating small magnets and large field gradients. Magnetic fields produced by current carrying conductors make our devices useable as current sensors or detectors. Listed below, are applications for GMR magnetic sensors: Current sensing Linear or rotary motion detection Linear or rotary position sensing Wheel speed sensing Ignition timing Throttle position sensing Media detection (inks, currency, etc.) Page 4

5 COMPETITIVE TECHNOLOGIES GMR sensors have greater output than conventional anisotropic magnetoresistive (AMR) sensors or Hall effect sensors and are able to operate at fields well above the range of AMR sensors. In addition, high fields will not flip GMR sensors or reverse their output as is possible with AMR sensors. The output of GMR sensors is frequency insensitive. The GMR Sensor produces an output even with a constant field. This sets them apart from inductive (variable reluctance) field sensors, which respond only to changes in magnetic field. High resistivity GMR material enables the fabrication of sensors with high resistance. Sensors with 5 kω resistance is standard. Special low power devices can be manufactured with 30 kω or higher resistance. Sensors can also be fabricated with built-in offset at zero field that provide for a zero crossing in output at a specified field value. GIANT MAGNETORESISTIVE MATERIAL PHYSICS The giant magnetoresistive phenomenon is a recently discovered (1988) effect found in metallic thin films consisting of magnetic layers a few nanometers thick separated by equally thin nonmagnetic layers. Researchers observed, a large decrease in the resistance with a magnetic field to the films. The cause of this effect is the spin dependence of electron scattering and the spin polarization of conduction electrons in ferromagnetic metals. With layers of the proper thickness, adjacent magnetic layers couple antiferromagnetically to each other with the magnetic moments of each magnetic layer aligned antiparallel to the adjacent magnetic layers. Conduction electrons, spin polarized in one magnetic layer, are likely to be scattered as they reach the interface to an adjacent magnetic layer with antiparallel conduction electron spins. Frequent scattering results in high resistivity. If an external field overcomes the antiferromagnetic coupling and achieves parallel alignment of moments in adjacent ferromagnetic layers, the spin dependent scattering of conduction electrons decreases and resistivity decreases. The size of this decrease in resistivity can be 10% to 20% and higher in GMR materials with multiple nonmagnetic layers. Page 5

6 GMR MAGNETIC FIELD SENSORS The NVE standard line of magnetic field sensors use a unique configuration employing a Wheatstone bridge of resistors and various forms of flux shields and concentrators. Using magnetic materials for shielding eliminates the need for a bias field with GMR sensors. NVE has developed a process, that plates a thick layer of magnetic material on the sensor substrate. This layer forms a shield over the GMR resistors underneath, essentially conducting any applied magnetic field away from the shielded resistors. The configuration allows two resistors (opposite legs of the bridge) to be exposed to the magnetic field. The other two resistors are located under the plated magnetic material, effectively shielding them from the external applied magnetic field. When the external field is applied, the exposed resistors decrease in electrical resistance while the other resistor pair remain unchanged, causing a signal output at the bridge terminals. The plating process developed by NVE for use in GMR sensor applications has another benefit: it allows flux concentrators to be deposited on the substrate. These flux concentrators increase the sensitivity of the raw GMR material by a factor of 2 to 100. The flux concentration factor is roughly equivalent to the length of one shield divided by the length of the gap. This allows use of GMR materials that saturate at higher fields. For example, to sense a field from 0 to 50 Oersteds, NVE deposits a GMR sensor that saturates at a nominal 300 Oersteds and flux concentrators with a magnification factor of (6) six. Figure 1 shows the basic layout of the device. Axis of sensitivity Ceramic substrate (1300 µm x 400 µm) Flux concentrator (2) Metal interconnects Bonding pads (4) (100µm x 100µm) Magnetically active resistor pair Magnetically shielded resistor (2) FIGURE 1 - TYPICAL GMR MAGNETIC FIELD SENSOR LAYOUT Page 6

7 Phone (952) (800) Fax (95612) The magnetic characteristic of a shielded bridge device is shown in Figure 2. This characteristic was taken from an actual production device, with 1 ma supplied to the bridge power terminals (5k Ω bridge). Field(Oers Output Voltage Applied Field (Oersteds) FIGURE 2 - GMR MAGNETIC FIELD SENSOR OUTPUT CHARACTERISTIC This signal output can be coupled directly into a linear amplifier or a comparator to generate a high level electrical signal proportional to the strength of the magnetic field seen by the sensor Page 7

8 GMR MAGNETIC FIELD GRADIENT SENSORS (GRADIOMETERS) The NVE gradiometer is a GMR magnetic field sensor used to detect field gradients between Wheatstone bridge configured resistors. This device is unshielded (i.e., it does not employ resistor shields) therefore all four (4) legs of the Wheatstone bridge are active (they respond to changes in field level). Gradiometers can be used to detect either magnetic or ferrous targets. To detect gradient changes caused by the proximity to a moving ferrous target, a biasing magnet is required. Refer to the Magnetic Biasing section on page 20 for gradiometer biasing guidelines. The output of the gradiometer differs from that of a standard GMR Magnetic Field Sensor. The gradiometer s output can be bipolar versus unipolar and can be shaped by the use of magnetic biasing and the application of external flux shaping devices (flux guides). Figure 3A shows the basic layout of the design. Figure 3B shows the output characteristic from a gradiometer biased for gear tooth detection. Axis of sensitivity Ceramic substrate (1250 µm x 650 µm) Active resistor pairs Metal interconnects Bonding pads (4) (100µm x 100µm) Unused resistor pairs (different resistor spacing may be incorporated based on application) FIGURE 3A - BASIC GRADIOMETER BRIDGE SENSOR LAYOUT Page 8

9 Ferrous gear tooth FIGURE 3B - GRADIOMETER BRIDGE SENSOR OUTPUT CHARACTERISTIC Page 9

10 MAGNETIC REFERENCE INFORMATION PERMANENT MAGNETS The Magnetic Materials Producers Association (MMPA) publishes two reference booklets that will provide the reader with valuable reference information relative to basic magnetic theory, permanent magnet materials and their practical application. They are: MMPA Standard no Standard Specifications for Permanent Magnet Materials MMPA PMG-88 Permanent Magnet Guidelines These booklets can be obtained from the MMPA: Magnetic Materials Producers Association 8 South Michigan Ave. Suite 1000 Chicago, IL (312) (312) (fax) MEASUREMENT SYSTEMS Unit Symbol cgs System SI System English System Length L centimeter (cm) meter (m) inch (in) Flux φ maxwell weber (Wb) maxwell Flux density B gauss (G) tesla (T) lines/in 2 Magnetizing force H oersted (Oe) ampere turns/m (At/m) ampere turns/in (At/in) Magnetomotive force F gilbert (Gb) ampere turn (At) ampere turn (At) Permeability in air µ 0 1 4π x Page 10

11 CONVERSION FACTORS To Convert Into Multiply by µwb maxwell 10 2 A/cm Oe A/m Oe x 10-2 At Gb G Oe 1 (when µ o =1) G T 10-4 G mt 10-1 G nt 10 5 G Wb/cm G Wb/in x 10-8 G Wb/m Gb At ka/m Oe x 10 1 maxwell Wb 10-8 maxwell µwb 10-2 mt G 10 maxwell volt second 10-8 nt G 10-5 nt gamma (γ) 1 Oe A/cm x 10-1 Oe A/m x 10 1 Oe ka/m x 10-2 T G 10 4 T Wb/m 2 1 volt second maxwell 10 8 volt second Wb 1 Wb maxwell 10 8 Wb/cm 2 G 10 8 Wb/m 2 G 10 4 Page 11

12 SIGNAL CONDITIONING CIRCUITS A number of methods exist for pre-amplification of an NVE GMR bridge sensor output. This section shows some representative circuits and compares the relative advantages and disadvantages of some common configurations. The circuits shown were designed for low power and 5V operation. Low noise or high performance applications should be designed with lower noise, higher performance components. OPERATIONAL AMPLIFIER (OP AMP) BRIDGE PREAMPLIFIER Single Op Amp Bridge Amplifier Figure 4 shows a simple circuit for amplifying an NVE AAxxx-02 GMR Magnetic Field Sensor s bridge output using a single 5V supply. The advantages of this configuration is its simplicity, small number of components, and inherent low cost. +5V C1 0.1uF Vref +5V C2 0.1uF R4 R V+ 8 V+ U1 AAxxx-02 Bridge Sensor 5 OUT+ 1 OUT- R1 2 - U2 LMC7101A/NS 4 V- 6 Vout 4 V- R2 FIGURE 4 - SINGLE OP AMP PREAMPLIFIER CIRCUIT Page 12

13 The equation for the amplified voltage is V out = ( out ) ( out ) V R R + R R4 + V + Vref Assuming R R1 + R2 R3 R3 1 +R 2 =R 3 +R 4 >>5K This type of amplifier has two significant limitations in that; 1) the feedback resistors load the output of the NVE bridge sense resistors and 2) the circuit has a poor common mode rejection (CMRR) if the resistor ratios are not ideally matched. Users of this circuit should be aware of the deficiencies and ensure that the feedback resistors are large compared to the bridge resistor values and that the bridge supply is stable and free from noise and ripple. Any pickup on the bridge leads should be minimized through proper layout and or shielding. Page 13

14 Two Op Amp Bridge Amplifier The two op amp circuit of Figure 5 reduces the loading of the preamplifier on the NVE bridge outputs but still has a CMRR that is dependent on the ratio of resistor matching. The AC CMRR is also poor in that any delay of the common mode signal through op amp U2 provides a mismatch in the signals being delivered to op amp U3 for cancellation. Its main advantage is simplicity and low cost. +5V C1 0.1uF +5V C3 0.1uF V+ 8 V+ U1 AAxxx-02 Bridge Sensor 4 V- 5 OUT+ 1 OUT- +5V V+ U2 LMC7101A/NS 2-4 V- C2 0.1uF 6 R1 2 - U3 LMC7101A/NS 4 V- R2 6 Vout Vref R4 R3 RG FIGURE 5 - TWO OP AMP PREAMPLIFIER CIRCUIT The equation for DC gain of the two op amp circuit (assuming infinite input impedance of the op amps) is: R2 2R2 Vout = Vref + VIN 1+ + R1 RG for R2 R4 = R1 and V R3 IN =(V out+ ) - (V out- ) Page 14

15 Three Op Amp Bridge Amplifier The three op amp circuit of Figure 6 is the most robust version of an op amp implementation. In this circuit the CMRR still depends on the resistor ratios of the differential amplifier (U4) but is not dependent on the resistors R1, RG, and R2. Therefore to minimize common mode errors the gain of the first stage should be made large compared to the gain of the second stage. The minimum gain of the second stage is dependent on amplifiers U2 and U3 output voltage range and Op Amp U4 s common mode input range which for the LMC7101 is rail to rail, allowing a gain of one (1) in the second stage. +5V C2 0.1uF +5V C1 0.1uF V+ U2 LMC7101A/NS 2-4 V- 6 R4 Vref R6 +5V C4 0.1uF 8 V+ U1 AAxxx-02 Bridge Sensor 4 V- 5 OUT+ 1 OUT- RG R2 +5V C3 0.1uF V+ U3 LMC7101A/NS V- R3 7 V+ 3 + U4 LMC7101A/NS 2-4 V- R5 6 Vout R1 FIGURE 6 - THREE OP AMP PREAMPLIFIER CIRCUIT The DC transfer function of the circuit is: Vout V 1 2R 1 R = ref + + RG R 4 3 V IN for R 1 =R 2, R 3 =R 5, R 4 =R 6 and V IN =(V out+ ) - (V out- ) The symmetrical nature of this configuration also allows for cancellation of common mode errors in amplifiers U2 and U3 if the error's track. Page 15

16 INSTRUMENTATION AMPLIFIER (IA) BRIDGE PREAMPLIFIER The advent of low cost, high performance IA s such as Analog Devices AD620 and the Burr Brown s INA118, have greatly simplified the design of bridge preamplifiers while adding significant advantages in noise, size, and performance over op amp implementations. Figure 7 shows the design of a bridge amplifier circuit using a INA118 (the AD620 is pin for pin compatible with the INA118). +5V C1 0.1uF +5V C2 0.1uF Use for AC coupling 8 V+ U1 AAxxx-02 Bridge Sensor 4 V- 5 OUT+ 1 OUT- C3 C3 R3 R3 RG 2 IN- 3 IN+ 1 RG1 8 RG2 5 REF 7 U2 INA118/BB 4 6 Vout Vref The gain of this circuit is: FIGURE 7 - INSTRUMENTATION AMPLIFIER PREAMPLIFIER CIRCUIT 50K VOUT = 1+ VIN + Vref with V RG IN =(V out+ ) - (V out- ) and the frequency 3dB point is 1 given by f = πr C Integrated circuit instrumentation amplifiers utilize circuit techniques where resistor matching is not as critical to the CMRR as active device matching. Active device matching can be easily controlled on integrated circuits allowing for greatly improved CMRR of instrumentation amplifiers over op amp implementations. Also the gain bandwidth product and CMRR are functions of gain of the instrumentation amplifier that Page 16

17 allows for wider bandwidth and improved CMRR at higher gains than is normally achievable with op amp implementations. Page 17

18 THRESHOLD DETECTION CIRCUIT Figure 8 shows the implementation of a low power threshold detection circuit that utilizes the AAxxx-02 GMR Magnetic Field Sensor and Burr Brown s INA118 instrumentation amplifier. Comparator hysteresis has been added around the LM311 comparator to minimize random triggering of the circuit on potential noise sources and pickup. The gain of the instrumentation amplifier is the same as before. The hysteresis of the comparator is approximately: R1 Vh 2.5 R + R + R R2 Neglecting the finite output swing of the comparator. R1 + R2 +5V C1 0.1uF +5V C2 0.1uF 8 V+ U1 AAxxx-02 Bridge Sensor 4 V- 5 OUT+ 1 OUT- RG 2 IN- 3 IN+ 1 RG1 8 RG2 5 REF 7 U2 INA118/BB 4 6 Vref +5V R2 C3 0.1uF +2.5V R V+ U3 IM311 7 R3 Vout 3-4 V- 1 G FIGURE 8 - THRESHOLD DETECTION CIRCUIT Page 18

19 NOISE IN NVE GIANT MAGNETORESISTIVE SENSORS The 1/f noise characteristic of NVE GMR sensors is approximately an order of magnitude higher than noise for thin film resistors. The actual cause of the 1/f noise characteristics is presently not completely understood and is under investigation. The noise has been shown to follow the usual characteristics of being proportional to the square of the current density. NVE data on 1/f noise is limited at the time of writing this application note but the present data implies that NVE s AAxxx-xx sensors should be thermal noise limited at a frequency of 10 khz. For use in low field applications, the noise of the NVE GMR sensors limits the minimum signal detected. For measuring low fields it is recommended that an AC modulation/demodulation scheme be implemented. Figure 9 shows a block diagram of an AC modulation/demodulation circuit. The phase shifter block is required to account for parasitic phase shift around the loop. PHASE SHIFTER SINE WAVE GENERATOR 8 V+ IN2 U1 AAXXX-02 BRIDGE SENSOR 4 V- 5 OUT+ 1 OUT- RG + RG1 RG2 - INST AMP IN1 DEMODULATOR OUT Rf Cf Vout FIGURE 9 - AC MODULATION/ DEMODULATION BLOCK DIAGRAM Page 19

20 USE OF GMR MAGNETIC FIELD SENSORS GENERAL CONSIDERATIONS All of NVE s GMR Magnetic Field sensors have a primary axis of sensitivity. Figure 10 shows an AAxxx-02 series GMR Magnetic Field Sensor with a cut away view of the die orientation (not to scale) within an SO8. Flux Concentrators Axis of Sensitivity FIGURE 10- SENSITIVE MAGNETIC AXIS - AAXXX-02 SENSOR The flux concentrators on the sensor die gather the magnetic flux along the axis shown and focus it at the GMR bridge resistors in the center of the die. The sensor will have the largest output signal when the magnetic field of interest is parallel to the flux concentrator axis. For this reason, care should be taken when positioning the sensor to optimize performance. Although sensor position tolerance may not be critical in gross field measurement, small positional variation can introduce undesirable output signal error for certain application and accuracy requirements. Page 20

21 MAGNETIC BIASING General Comments In many applications, GMR Magnetic Field Sensors make use of biasing magnetic fields. Biasing magnetic fields provide either a magnetic field to sense (where one is not present) or create a pseudo zero field. Back biasing a sensor consists of applying a magnetic field through the sensor package without influencing the sensor. The purpose is to create a magnetic field that the device can sense for those applications where a magnetic field is not present, i.e., ferrous material detection. Figure 11 shows a permanent magnet use for this purpose. The magnet adjusted in the X direction, as shown, achieves the maximum field in the Z direction and minimum field in the sensitive X direction. N Z S X FIGURE 11 - MAGNETIC BIASING CONFIGURATION Another means of biasing a GMR Magnetic Field Sensor is to provide a constant magnetic field in the sensitive direction. The result is a sensor biased part way up its output curve shown in Figure Output Voltage Possible bias point Applied Field (Oersteds) FIGURE 12 - BIASING UP THE CURVE Page 21

22 This biasing technique creates a bipolar output with a DC offset. Another typical purpose for this kind of biasing is to bias the sensor away from the zero field area where, as seen in Figure 12, the hysteresis increases. Since gradiometers respond to the flux gradient rather than the field itself, setting the bias by positioning the magnetic is not as straightforward as it is with GMR Magnetic Field Sensors. One possible method is, as described below, bias the gradiometer to the center of its range. It is important not to bias a gradiometer to so high a field that the GMR resistors will saturate and the bridge comes back into balance. The range of the gradiometer, P/N AB001-xx, is ±250 Oe. The resistance of the bridge (and of the individual resistors) is 2.5 kω. Procedure 1. Without any magnets near the gradiometer, measure the resistance between V + and Out A (or any two adjacent corners). The value should be about 1875 Ω or ¾ of the bridge resistance. (One (1) resistor in parallel with three (3) resistors in series.) 2. Move a magnet towards the sensor as shown in Figure 13 and find the minimum resistance across the same two pins. The value should be 10 to 12% lower or about 1700 Ω. This value represents the saturated resistance of the resistors in this configuration. 3. Now place the magnet in position to back bias the gradiometer as shown in Figure 14 below. With the magnet centered, you should be able to find a magnet position in which the resistance is almost the maximum measured without a magnet. (In this position the component of the magnetic field along the sensitive direction of the resistors is near zero.) 4. Move the magnet towards the end of the sensor until the resistance reads halfway between the previously measured values. The gradiometer is biased to approximately 125 Oe. Biasing to other field levels can be achieved by moving the magnet until other values of resistance between maximum resistance (zero field) and minimum resistance (maximum field) values. The resistors in the bridge are being used as GMR field sensors in a non-bridge configuration. Page 22

23 Gradiometer Magnet V+ OUT B NVE AB N S OUT A V- Axis of Sensitivity FIGURE 13 - FINDING MINIMUM RESISTANCE N S Move magnet FIGURE 14 - GRADIOMETER MAGNET POSITIONING Page 23

24 TYPICAL APPLICATIONS MEASURING DISPLACEMENT Basic concepts Because of their high sensitivity, GMR Magnetic Field Sensors can effectively provide positional information of actuating components in machinery, proximity detectors, and linear position transducers. Figure's 15A and 15B illustrate two simple sensor/permanent magnet configurations used to measure linear displacement. In figure 15A, displacement along the y-axis varies the Bx field magnitude detected by the sensor that has its sensitive plane lying along the x-axis. The configuration in figure 15B has the direction of displacement and the sensitive plane along the x-axis. N S N S y y X (axis of sensitivity) X (axis of sensitivity) FIGURE(S) 15 A&B - CONFIGURATIONS FOR MEASURING LINEAR DISPLACEMENT Application examples Hydraulic/pneumatic pressure cylinder stroke position Suspension position Fluid level Machine tool slide position Aircraft control-surface position Vehicle detection Page 24

25 ANGULAR POSITION/SPEED MEASUREMENT Basic concepts GMR sensors offer a rugged, low cost solution to rotational reference detection. High sensitivity and DC operation afford the GMR Magnetic Field Sensor an advantage over inductive (variable reluctance) sensors. Inductive sensors have very low outputs at low frequencies and can generate large noise signals when subjected to high frequency vibration. Figure 16 shows a configuration for sensing the magnetic fluctuations caused by the teeth of a ferrous gear as it rotates. Y (axis of sensitivity) x N S FIGURE 16 - ANGULAR POSITION SENSING The above configuration orientates the GMR sensor in the y-axis. The position of the permanent magnet is such that its magnetic field lies in the x-axis and biases the sensor at the zero point. As the gear rotates, the teeth will perturb the magnetic field in the y direction creating a signal in the sensitive axis of the bridge. Application examples Industrial gear tooth sensing Fine-pitch gear tooth sensing Automotive timing (crankshaft and camshaft) Automotive transmission speed sensing Automotive ABS wheel speed sensing Page 25

26 CURRENT MEASUREMENT Basic concepts GMR Magnetic Field Sensors can effectively sense the magnetic field generated by a current. Figure 17 illustrates the sensor package orientation for detecting the field from a current-carrying wire. This application allows for current measurement without breaking or interfering with the circuit of interest. Note the wire can be located above or below the chip as long as it is orientated perpendicular to the sensitive axis. Figure 18 shows an another configuration where a current trace on a PCB is under the sensor when the sensor is board mounted. Axis of sensitivity Direction of current flow FIGURE 17 - SENSING MAGNETIC FIELD FROM A CURRENT-CARRYING WIRE Page 26

27 NVE AAxxx-xx Axis of Sensitivity Current carrying PCB trace PCB FIGURE 18 - SENSING MAGNETIC FIELD FROM A CURRENT CARRYING PCB TRACE Current Sensing - Detail Considerations Care must be taken in interpreting the output wave-form when using GMR as current sensors. Reason being, GMR sensors function as omnipolar sensors by producing positive output regardless of the magnetic field direction. In the case of AC excitation, the bipolar field created by a sinusoidal AC current will produce an output that will look like a full-wave rectified sinusoid. Biasing the sensor part-way up the curve will restore a sinusoidal output with a DC component (see Figure 12). As an example of the expected output, NVE has calculated the output for three configurations: a current path on the PCB the sensor, a wire whose center is inch from the sensor, and one with center 0.1 inches from the sensor. These are approximations since they do not take the length of the sensor and the behavior of the flux concentrators in this very non-uniform field into account. NVE also measured the output of a AA sensor with 12 V applied and up to 1.2 A. Figures 19 and 20 show the results. The bipolar output is due to the Earth s field, the offset in the bridge, and the fact that the field excursions did not pass the zero-region in the negative direction. The maximum field generated at the sensor was of the order of 1 Oe. The wire diameters match the current to chip distances in Figure 19. The current sensitivities are within a factor of 2 of the approximation. Page 27

28 H(Oe) = I(A)/(5 r(cm)) Field a distance r from the center of a wire AA board wire wire current to chip (in) current to chip (m) current (A) H (Oe) sensitivity (mv/v/oe) current sensitivity (mv/v/a) FIGURE 19 - CURRENT SENSOR WIRE POSITION COMPARISONS AA Output vs. Current (wire placed over SO8 package) Output (mv/v) " wire 3.13 mv/v/a " wire 2.33 mv/v/a Current (A) FIGURE 20 - CURRENT SENSOR OUTPUT COMPARISONS (bipolar response due to earth s field as offset) Page 28

29 Current Sensing Application Examples Non-intrusive AC or DC current detection or sensing PCB mounted current detection or sensing (PCB trace or strap current carrier) Toroidal Hall effect current detector or sensor replacements Industrial instrumentation Industrial process control Current probes Additional information on this application can be found in Appendix APP 003. In addition, NVE has a current sensor evaluation kit, AG003-01, which has a variety of different size traces complete with on-board sensors, available through its distributors. MAGNETIC MEDIA DETECTION General discussion GMR Magnetic Field Sensors can be used for detecting different types of magnetic media. In this situation, NVE defines magnetic media as material that has a distinct magnetic signature. The media is typically a non-magnetic substrate with magnetic material placed in or on the substrate. Typically, GMR sensors are used to read the magnetic signature by sweeping the substrate and the sensor past each other. Depending upon the application, the magnetic parts of the substrate can be detected indirectly by sensing a perturbation of an externally applied field or sensed directly due to the part s own field. The output of the sensor will be a function of: (1) the magnetic properties of the media; (2) the working gap; and (3) the type of sensor used. For more information, see Appendix APP 002. Application examples Magnetic ink detection Magnetic stripe reading Fine magnetic particle detection Media magnetic signature detection Magnetic anomaly detection in substrates Page 29

30 EXTERNAL FLUX CONCENTRATORS L gap L concentrator External flux concentrator NVE AAxxx-xx External flux concentrator Axis of Sensitivity Flux concentration factor: FC ~ L concentrator/l gap Basic concepts FIGURE 21 - EXTERNAL FLUX CONCENTRATOR EXAMPLE Applications in which minimum size is not of prime importance, external flux concentrators can be used to increase the sensitivity of NVE s sensors. These external flux concentrators function in the same manner as the flux concentrators within the sensor. Elongated pieces of soft magnetic material gather external magnetic flux and expose the sensor to a magnetic field that is larger than the external magnetic field. For best results, two pieces of soft magnetic materials of the same size are used. The concentration factor is approximately, the ratio of one flux concentrator s length to the gap between the two flux concentrators. The long dimension of the flux concentrators should be aligned with the sensitive axis of the NVE sensor. To minimize the gap, the flux concentrators should butt up against the NVE sensor package. Refer to Figure 21 for an SO8 package example. Since the effective permeability of the flux concentrators is equal to the concentration factor, material with permeability 100 or more times the concentration factor, will be more than sufficient. Hot rolled iron wire or even cut off iron nail's will work. Flux concentrators can be round or rectangular in cross section for mounting considerations. The NVE sensor must be centered within the flux concentrators cross sectional area. The diameter of the flux concentrators should be an appreciable fraction of the gap length or flux spreading in the gap will reduce the concentration factor. The flux concentration achieved will depend on all dimensions. However, it will depend Page 30

31 primarily on the ratio of the concentrator length to gap. The best calculation; however, is an experimental measurement made with an actual sensor and flux concentrators. For prototyping and production, the external flux concentrator can be placed down on the PCB or other substrate. The top of the metal strip must be at least as high as the sensor package to be truly effective. It should be noted that a flux concentrator that increases the sensitivity of a GMR sensor by a factor of 5 will also reduce the maximum field to which the sensor can respond to 1/5 its original value. Application examples Earth s field measurements Optimization of sensor magnetic circuits (various applications) Vehicle detection Page 31

32 APPENDIX APP 002 CURRENCY DETECTION NVE s sensors have been used for detecting the magnetic material in paper bills. NVE does not know very much about the physical structure of paper currency. The US Government does not give out information in this regard. What we have been told, and confirmed, is the black ink has ferrous materials in it. NVE believes this magnetic material can be modeled as very small bar magnets. The magnetic field emitted by the bar magnets has two detectable orientations. The first is the vertical field and second, the horizontal component of the field. The sensitive axis of the sensor should be parallel with the component of field desired, i.e., horizontal axis to pickup the horizontal component of the field. Vertical component Horizontal component FIGURE 22 - FERRITE BAR MAGNET NVE concluded the bill can be magnetized and demagnetized by the application of an external field. With only the earth s field present, the bill s magnetic properties were seen. However, the signal increases when using an external field. The amount of signal increase with an external biasing field has not been quantified. This external magnetic field can be set up in a few different orientations, either in a front/back biasing configuration or positioned up stream of the sensor. The back biasing configuration typically consists of a electro/permanent magnet glued to the under side of the sensor or to the under side of the PCB. The magnet should be aligned as to produce the minimum output from the sensor. The ferromagnetic particles in the bill will magnetize and thus distort the field to produce a field in the sensitive direction at the sensor. Front biasing works in much the same manner except in this case the source of external field is coming from the other side of the bill. Bill Magnet Flux AA Passive mounting card 4 leads, V+,V- Out+, Out- Front biasing FIGURE 23 - FRONT AND BACK BIASING Magnet Back biasing Page 32

33 Positioning the magnet up stream from the sensor consists of magnetizing the bill before it reaches the sensor-- this terminology comes from an application where the bills are moving past the sensor on a conveyer belt or rollers. With a magnet placed near the moving bill, the bill is magnetized before passing the sensor. The application s geometrical requirements, strength of magnet, as well as sensor-bill distance will determine which configuration works the best in each application. The following are some graphs that show the characteristics of back biasing an AA002 sensor. To determine the sensitivity of back biasing positioning, the magnet was moved along the sensors sensitive axis. The general objective was to position the magnet so that there is minimum field in the sensitive direction. To obtain this, one pole of the magnet must be directly under the sensing resistors of the bridge. Note the steep slope around the zero output. This steep slope along with zero field hysteresis, makes exact positioning a difficult task. Characteristc of magnet moving in sensitive direction 4.5 Offset voltage (V) Relative postion (in) FIGURE 24 - BIASING MAGNET POSITIONING NVE also experimented with non-sensitive axis magnet displacement. As expected, the results showed that positioning in this dimension is not nearly as sensitive as positioning in the sensitive axis. This graph shows the near zero output when the majority of field in not in the sensitive direction. Page 33

34 Output V Characteristic of magnet moving in non-sensitive direction Displacement FIGURE 25 - BIASING MAGNET POSITIONING NVE performed analysis of the proximity from bill to the GMR sensor. The back biased sensor was moved away from a highly magnetic spot on the bill. The distance from the bill has a negative component due to possible compression of the sensor into the bill s substrate. The position is relative with possible systematic error making its accuracy questionable. This shows the advantages of close proximity. Characteristic of moving sensor further from bill 7 Voltage out from amp (10x) (mv) V (mv) trial 1 V (mv) trial 2 V avg Distance from bill to sensor package (in) FIGURE 26 - GAP PERFORMANCE In most situations, the signal from the bill was so weak that it may not be seen without proper signal conditioning -- a simple differential amplifier with filtering capacitors is usually sufficient. NVE s Application Notes show some signal conditioning circuits. If static field measurements are not needed, AC coupling is beneficial in many ways. Page 34

35 For most magnetic media detection applications, including US bills, the magnetic field is not very large and thus NVE s most sensitive sensor is necessary. NVE s AA sensor saturates at 15 Oe and has sensitivity levels between 3.0 and 4.2 mv/v/oe. This part utilizes large flux concentrators to achieve this level of sensitivity. These flux concentrators also contribute to the magnetic hysteresis of the sensor. The hysteresis shows up mainly in a changing DC offset of the part. By AC coupling the output of the sensor, the changes in magnetic field, are seen at the output and not the DC offset. Another feature of AC coupling is its ability to give a consistent output independent of the sensor's orientation with the earth s field. Since the device is so sensitive, it also picks up the earth s DC field. By AC coupling, the Earth s DC offset is not seen at the output. The magnetic hysteresis in the sensor also contributes to what we at NVE call Bat Ears around the zero field area (see Figure 2 in NVE s Engineering and Application Notes). The output from fields in this area is very susceptible to the hysteresis effect. The hysteresis also gets greater if the field is bipolar, i.e., if it crosses the zero field point. To get away from this area, biasing methods can be used. By applying a field in the sensitive direction, the sensor will be biased part way up the curve. This point will now be the pseudo zero field point. External fields will now add or subtract from this biasing field and the output will move around this pseudo zero field point. With proper biasing, the output will not go around the true zero point, and will be biased from the zero-field area. A permanent magnet or electric current may be used for biasing purposes. Permanent magnets placed above, below, or to the side of the sensor will utilize the biasing technique. The size and strength of the permanent magnet will change the location for biasing purposes. Current biasing is achieved by running a current near the sensor as shown in Figure 20 of this booklet. By wrapping a coil around the device with the coilaxis parallel to the axis of sensitivity, substantial field will be produced. For currency detection, 0.1A through 10 turns of wire around a AA002 sensor would be sufficient. Magnetic media detection is a very interesting application and NVE s sensors provide a reliable, non-contact, static or dynamic detection of the magnetic ink on paper currency. NVE s AA002 sensor can resolve the distinct features printed on US and other currency. NVE s magnetic field sensors have been used for counterfeit detection, sorting, and simple bill presence detection. If you have any questions or comments on this, or any other application of NVE s GMR Magnetic Field Detectors, please call the Applications Engineering Dept. of NVE at , apps@nve.com, or call and ask for the Applications Engineering. Page 35

36 APP 003 GMR CURRENT SENSING Introduction NVE s AC Current Sensor provides a non-intrusive way to analyze the current through a PCB trace or any other current carrying conductor. The sensor utilizes GMR materials to sense the magnetic field produced by current flowing through a conductor. The sensor has AC and DC sensing capabilities although mainly DC current sensing will be covered. Principles of Operation The current sensor has a definite axis of sensitivity that runs through the center of the package from left to right when reading the markings. The current of interest should flow perpendicular to this axis as shown in NVE s Sensor Bulletin 008. The magnetic field created by the current surrounds the conductor radially. As the magnetic field effects the GMR material in the sensor, a differential output is produced at the Out pins of the sensor as shown in Sensor Bulletin 008. The magnetic field strength is directly proportional to the current flowing through the conductor. As the current increases, the magnetic field surrounding it will also increase and thus increase the output from the sensor. Likewise, as the current decreases, the magnetic field and output from the sensor will decrease. How It Performs Since the current is not measured directly, a correlation must be built to get the current information from the output of the sensor. The following data and graphs are based upon analysis of NVE s Current Sensor Evaluation Board AG The PCB contains four traces of three different widths: 90 mils, 60 mils, and 10 mils as seen in Sensor Bulletin 010. Page 36

37 DATA ANALYSIS- One To Ten Amps Most analysis at this time has been done with larger currents (1-10 A) through the 90 and 60 mil traces found on NVE s Current Sensor Evaluation Board AG An AC sensor was placed over the 90 and 60 mil traces and different levels of DC current were run through the traces. This current and the corresponding output from the sensors are shown in Figures 27 and 28 respectively. 400 AC over 0.090"wide "thick trace Output (mv) mv Out = 29.8 ± 0.2A ± Current(A) FIGURE 27 - FUNCTION FOR 90 MIL TRACE The sensor was supplied with 8.33V and was hand-soldered over the trace. The trace is wide and ± thick. The marks on the graph are output error bars which cover the expected error from this part due to intrinsic hysteresis and measurement errors. The current was swept from zero to ten amps and back to zero multiple times. The output voltage at specific current levels was analyzed and an output voltage precision was determined to have a relative error of approximately ± 0.7% with errors of up to 2% possible at low currents. A linear fit on the data above shows a 29.8 ± 0.2 mv/a correlation in this configuration. The sensor utilizes a Wheatstone bridge and thus the applied voltage across the bridge is directly related to the output. By dividing the slope by 8.33V, we get a more useful number of 3.57±0.02 mv/v/a. With this number, the user can determine the expected output for any applied voltage. Page 37

38 The same analysis was given to a wide trace of the same thickness. A voltage of 8.33V was applied and the resulting graph is shown below in Figure AC over 0.060"wide "thick trace Output(mV) mv Out = 33.6 ± 0.8(A) ± 8.3V Current(A) FIGURE 28 - FUNCTION FOR 60 MIL TRACE The current in this trace was swept from zero to 9 amps in the same fashion as the 90 mil analysis. The output to current correlation from this graph is 4.0 ± 0.1 mv/v/a. The differences between the 90 and 60 mil traces can be thought of by looking at the field distribution/density differences between the two due to the width differences. Resolution The resolution of the sensor is a function of environmental electromagnetic noise, intrinsic noise, and hysteresis. In most applications, the environmental noise is the limiting factor in resolution. Data and information in this section are based upon nonfiltered, non-amplified, non-shielded output. In this raw configuration, a resolution of better than 1 ma was found. With proper filtering, amplifying and shielding, the noise level can be decreased and thus the usable resolution will increase. For more information on noise, amplification, and filtering, see NVE s Engineering and Application Notes. Hysteresis and repeatability All magnetic materials have an effect called magnetic hysteresis. This hysteresis contributes greatly to the error values given above. Simply put, hysteresis creates a potential that the same current can produce two different voltage outputs. The hysteresis, and thus the error, is largest when the current changes direction. If the current changes direction, the precision of the output at low currents decreases Page 38

39 significantly. The specified error of 0.7% will not be obtained again until the current is above 2A or so. This guideline is very rough and each application will vary. Another magnetic contribution to the error can be overcome by an initialization current. Often, depending on the magnetic history (hysteresis) of the sensor, the initial outputs are different from subsequent outputs as seen in Figure 29 below. 300 Initialization of AC Output(mV) Initial sweep Series2 Series3 Series4 Series5 Series Current(A) FIGURE 29 - HYSTERESIS INITIALIZATION The initial sweep data has deviated from the other series of current sweeps. After the first sweep was completed, the subsequent five sweeps fell right on each other. This shows that a lower error can be obtained by initializing the sensor. After initialization, the error will be much lower until the working current range is exceeded in either direction. Saturation of the device (currents in the 20A range) as well as changing the applied current direction will increase the hysteresis/error. An example of this can be seen in Figures 27 and 28 above. Figure 27 shows the output after an initialization sweep had been done. The lower hysteresis can be seen by looking at the 0.7% error in slope as compared to Figure 28 that has a 2% error in slope due to non-initialization hysteresis. For currents of approximately 2 Amps and smaller, the output repeatability is nominally 2% while higher currents produce output repeatability errors of less than 1%. Low current measurements of an initial current sweep may exceed 15% error in repeatability. Page 39

40 DATA ANALYSIS- Low Current Sensing The low current analysis is handled here separately from the higher current analysis due to special considerations that must be made, although much of the same hysteresis and resolution considerations from high field sensing apply here. For low current sensing, two configurations of wide traces were used. The first analysis will be with a AC sensor over a single 10 mil trace and the second analysis will consist of a AC sensor over seven 10 mil traces. With these traces, milliamp and sub-milliamp currents are of interest. Due to the hysteresis at low currents as discussed above, a biasing magnet was used to set the parts to approximately half of their linear range, or approximately 20 mv/v. This bias point can be seen as the Y intercept in the Figures below. In this way, the output will not be near the natural zero current range, and thus, repeatability is increased. With this configuration an alternating sense current will produce a bipolar output with a DC offset in an AC application. More information on biasing is given later. AC over single 0.010"wide "thick trace Output(mV) mv Out = ± 0.08(A) ± Current(A) FIGURE 30 - FUNCTION FOR 10 MIL TRACE The sensor that was used to take this data was supplied with 8.06V. The marks on the graph are output error bars which cover the expected error from this part due to the parts natural hysteresis and measurement errors. The current was swept from zero to 100 ma and back to zero multiple times. In this biased state, the sensor is extremely linear and hysteresis is low. A weighed linear fit shows a ± 0.08 mv/a correlation with 8.06V supplied which results in a sensitivity of 3.70 ± 0.01 mv/v/a. Page 40

41 The same analysis was given to the seven wide traces of the same thickness. A voltage of 8.06V was again applied and the resulting graph is shown below in Figure 31. Output(mV) AC over seven 0.010"wide "thick traces mv Out = ± 0.06(A) ± Current(A) FIGURE 31 - FUNCTION FOR 7 X 10 MIL TRACE Seven traces were run under the part so that the magnetic fields from the seven traces add constructively at the sensor thus getting a much higher output with less applied current. To a first order approximation theory predicts that the field will be increased seven fold from just a single trace. The sensitivity from the single trace above is 3.7 mv/v/a, seven times this is 25.9 mv/v/a which is not quite achieved. Presumably this discrepancy is due to the different current distributions. This loss would not be as extreme if seven times the current went through the single trace. Resolution The sensitivity and resolution are a function of environmental noise. By shielding, amplifying, and filtering, the low limit and usable resolution can be greatly increased. The data for the analysis done here was with a raw setup, no amplification or filtering. In a zero gauss chamber, single microamps were detected but the measurement equipment limited any in depth analysis. Effects of Biasing For the analysis done above, a small ceramic magnet was used to supply a magnetic field in a direction which is parallel to the sensitive axis of the sensor. This magnetic field pushes the output to a certain value which is now a psuedo zero field point. The Page 41