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Summary of Contents for Philips Magnetoresistive Sensor
- Page 1 DISCRETE SEMICONDUCTORS General Magnetoresistive sensors for magnetic field measurement 2000 Sep 06...
- Page 2 Operating principles Philips magnetoresistive sensors Flipping Effect of temperature on behaviour Using magnetoresistive sensors Further information for advanced users Appendix 1: The magnetoresistive effect Appendix 2: Sensor flipping Appendix 3: Sensor layout. 2000 Sep 06 Fig.1 Philips magnetoresistive sensors. General...
- Page 3 , as given by: and R are material parameters and to achieve optimum sensor characteristics Philips use Fe19Ni81, which has a high R value and low magnetostriction. With this material, R is of the order of 3%.
- Page 4 Philips Semiconductors Magnetoresistive sensors for magnetic field measurement handbook, halfpage Fig.3 The resistance of the permalloy as a function of the external field. handbook, full pagewidth 2000 Sep 06 In this basic form, the MR effect can be used effectively for...
- Page 5 Section “Further information for advanced users” later in this chapter. Philips magnetoresistive sensors Based on the principles described, Philips has a family of basic magnetoresistive sensors. The main characteristics of the KMZ sensors are given in Table 2.
- Page 6 Philips Semiconductors Magnetoresistive sensors for magnetic field measurement Flipping The internal magnetization of the sensor strips has two stable positions. So, if for any reason the sensor is influenced by a powerful magnetic field opposing the internal aligning field, the magnetization may flip from one position to the other, and the strips become magnetized in the opposite direction (from, for example, the ‘+x’...
- Page 7 Philips Semiconductors Magnetoresistive sensors for magnetic field measurement Figure 7 also shows that the flipping itself is not instantaneous, because not all the permalloy strips flip at the same rate. In addition, it illustrates the hysteresis effect exhibited by the sensor. For more information on sensor flipping, see Appendix 2 of this chapter.
- Page 8 Philips Semiconductors Magnetoresistive sensors for magnetic field measurement handbook, full pagewidth (mV/V) Fig.9 Output voltage ‘V ’ as a fraction of the supply voltage of a KMZ10B sensor as a function of transverse field ‘H ’ for several temperatures. 2000 Sep 06...
- Page 9 Philips Semiconductors Magnetoresistive sensors for magnetic field measurement Figure 10 is similar to Fig.9, but with the sensor powered by a constant current supply. Figure 10 shows that, in this case, the temperature dependency of sensitivity is significantly reduced. This is a direct result of the increase in bridge resistance with temperature (see Fig.8), which...
- Page 10 MR sensors are briefly described below. NALOG APPLICATION CIRCUITRY In many magnetoresistive sensor applications where analog signals are measured (in measuring angular position, linear position or current measurement, for example), a good application circuit should allow for sensor offset and sensitivity adjustment.
- Page 11 Philips Semiconductors Magnetoresistive sensors for magnetic field measurement Further information for advanced users EFFECT In sensors employing the MR effect, the resistance of the sensor under the influence of a magnetic field changes as it is moved through an angle...
- Page 12 Philips Semiconductors Magnetoresistive sensors for magnetic field measurement handbook, halfpage Fig.13 The resistance of the permalloy as a function of the external field H after linearization (compare with Fig.6). For sensors using Barber poles arranged at an angle of +45 to the strip axis, the following expression for the...
- Page 13 The circuit can be divided into two stages: a differential amplifier stage that produces a symmetrical output signal derived from the magnetoresistive sensor, and an output stage that also provides a reference to ground for the amplification stage. H (kA/m)
- Page 14 -------- - The positive temperature coefficient (TC) of the amplification is: 2000 Sep 06 KTY82-110 For the given negative ‘TC’ of the magnetoresistive sensor and the required amplification of the input stage ‘A1’, the resistance ‘R (10) where TC sensor and TC amplifier.
- Page 15 Fig.17. MBH616 where the demagnetizing factor N magnetization M 0 = 4 The field H range of a magnetoresistive sensor, as f is given by: + DR cos f(2) becomes: ----- - 1 ------ - cos...
- Page 16 Philips Semiconductors Magnetoresistive sensors for magnetic field measurement ------------------------- - ------------ where |H | and H and H of the external field. In the simplest case H ages U and U become: ----- - 1 ------ - 1 – ------ -...
- Page 17 field measurement ARBER POLE SENSORS A number of Philips’ magnetoresistive sensors use a ‘barber-pole’ construction to linearize the R-H relationship, incorporating slanted strips of a good conductor to rotate the current. This type of sensor has the widest range of...
- Page 18 figures are only approximations as the exact values depend on a number of variables such as thickness, deposition and post-processing. Table 3 Comparison of magnetoresistive sensor (13) NiFe 81:19 NiFe 86:14 , which can be esti-...
- Page 19 Philips Semiconductors Magnetoresistive sensors for magnetic field measurement this also considerably enlarges H coefficient of is required, NiCo alloys are preferable. The amorphous alloy CoFeB has a low slightly worse thermal stability but due to the absence of grain boundaries within the amorphous structure, exhibits excellent magnetic behaviour.
- Page 20 Philips Semiconductors Magnetoresistive sensors for magnetic field measurement handbook, full pagewidth (mV) Fig.23 Sensor output ‘V A Safe Operating ARea (SOAR) can be determined for magnetoresistive sensors, within which the sensor will not flip, depending on a number of factors. The higher the auxiliary field, the more tolerant the sensor becomes to...
- Page 21 Magnetoresistive sensors for magnetic field measurement APPENDIX 3: SENSOR LAYOUT In Philips’ magnetoresistive sensors, the permalloy strips are formed into a meander pattern on the silicon substrate. With the KMZ10 (see Fig 25) and KMZ51 series, four barber-pole permalloy strips are used while the KMZ41 series has simple elements.
- Page 22 Philips Semiconductors Magnetoresistive sensors for magnetic field measurement handbook, halfpage V CC Fig.26 KMZ10 and KMZ11 bridge configuration. 2000 Sep 06 In one pair of diagonally opposed elements the barber-poles are at +45˚ to the strip axis, with the second pair at 45˚.
- Page 23 A technique called ‘flipping’ (patented by Philips) can be used to control the sensor. Comparable to the ‘chopping’ technique used in the amplification of small electrical signals, it not only stabilizes the sensor but also eliminates the described offset effects.
- Page 24 Philips Semiconductors Magnetoresistive sensors for magnetic field measurement coil sensor Fig.28 Flipping coil. handbook, full pagewidth FLIPPING SOURCE 2000 Sep 06 Flipping causes a change in the polarity of the sensor output signal and this can be used to separate the offset signal from the measured signal.
- Page 25 Philips Semiconductors Magnetoresistive sensors for magnetic field measurement handbook, full pagewidth Fig.30 Timing diagram for flipping circuit (a) output voltage; (b) filtered output voltage; (c) output voltage filtered and demodulated. 2000 Sep 06 flipping current I F time internal magnetization...
- Page 26 Philips Semiconductors Magnetoresistive sensors for magnetic field measurement ENSOR TEMPERATURE DRIFT The sensitivity of MR sensors is also temperature dependent, with sensitivity decreasing as temperature increases (Fig.31).The effect on sensor output is certainly handbook, full pagewidth (mV/V) Fig.31 Output voltage ‘V ’...
- Page 27 Philips Semiconductors Magnetoresistive sensors for magnetic field measurement The simplest form of temperature compensation is to use a current source to supply to the sensor instead of a voltage source. In this case, the resulting reduction in sensitivity due to temperature is partially compensated by a corresponding increase in bridge resistance.
- Page 28 Philips Semiconductors Magnetoresistive sensors for magnetic field measurement The optimal method of compensating for temperature dependent sensitivity differences in MR measurements of weak fields uses electro-magnetic feedback. As can be seen from the sensor characteristics in Figs 31 and 32,...
- Page 29 Philips Semiconductors Magnetoresistive sensors for magnetic field measurement The influence of other disturbing fields can also be eliminated provided they are well known, by adding a second current source to the compensating coil. Such fields might be those arising from the set-up housing, ferromagnetic components placed close to the sensor or magnetic fields from electrical motors.