Philips KMZ10A Скачать руководство пользователя страница 25 | Manualshive

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2000 Sep 06

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Philips Semiconductors

Magnetoresistive sensors for
magnetic field measurement

General

Fig.30 Timing diagram for flipping circuit (a) output voltage; (b) filtered output voltage; (c) output voltage filtered

and demodulated.

handbook, full pagewidth

MBH618

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internal
magnetization

flipping current IF

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T

time

time

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time

VO

Hy

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Содержание KMZ10A

Страница 1: ...2000 Sep 06 DISCRETE SEMICONDUCTORS General Magnetoresistive sensors for magnetic field measurement ...

Страница 2: ...l field measurement 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 Fig 1 Philips magnetoresistive sensors ...

Страница 3: ...istics and how their behaviour may be affected by external influences and by their magnetic history Operating principles Magnetoresistive MR sensors make use of the magnetoresistive effect the property of a current carrying magnetic material to change its resistivity in the presence of an external magnetic field the common units used for magnetic fields are given in Table 1 Table 1 Common magnetic...

Страница 4: ...KMZ series of sensors four permalloy strips are arranged in a meander fashion on the silicon Fig 4 shows one example of the pattern on a KMZ10 They are connected in a Wheatstone bridge configuration which has a number of advantages Reduction of temperature drift Doubling of the signal output The sensor can be aligned at the factory Fig 3 The resistance of the permalloy as a function of the externa...

Страница 5: ...cribed Philips has a family of basic magnetoresistive sensors The main characteristics of the KMZ sensors are given in Table 2 Fig 5 Bridge configuration with offset trimmed to zero by resistors RT handbook halfpage MLC129 2 1 GND VO VCC VO RT RT 3 4 Table 2 Main characteristics of Philips sensors Notes 1 In air 1 kA m corresponds to 1 25 mT 2 Data given for operation with switched auxiliary field...

Страница 6: ...greater the field Hy the smaller the field Hx This follows naturally since the greater the field Hy the closer the magnetization s rotation approaches 90 and hence the easier it will be to flip it into a corresponding stable position in the x direction Looking at the curve in Fig 7 where Hy 0 5 kA m for such a low transverse field the sensor characteristic is stable for all positive values of Hx a...

Страница 7: ... KMZ10B sensor The data sheets show also the spread in this variation due to manufacturing tolerances and this should be taken into account when incorporating the sensors into practical circuits In addition to the bridge resistance the sensitivity also varies with temperature This can be seen from Fig 9 which plots output voltage against transverse field Hy for various temperatures Figure 9 shows ...

Страница 8: ...urement General Fig 9 Output voltage Vo as a fraction of the supply voltage of a KMZ10B sensor as a function of transverse field Hy for several temperatures handbook full pagewidth 3 0 15 3 2 2 MLC134 5 10 10 5 15 0 1 1 H kA m y VO mV V T 25 C amb o 25 C o 75 C o 125 C o operating range ...

Страница 9: ...result of the increase in bridge resistance with temperature see Fig 8 which partly compensates the fall in sensitivity by increasing the voltage across the bridge and hence the output voltage Figure 8 demonstrates therefore the advantage of operating with constant current Fig 10 Output voltage Vo of a KMZ10B sensor as a function of transverse field Hy for several temperatures handbook full pagewi...

Страница 10: ...ired for compensation Philips KTY series are well suited for this purpose as their positive Temperature Coefficient TC matches well with the negative TC of the MR sensor The degree of compensation can be controlled with the two resistors R7 and R8 and special op amps with very low offset and temperature drift should be used to ensure compensation is constant over large temperature ranges Please re...

Страница 11: ...xis see Fig 12 As aluminium has a much higher conductivity than permalloy the effect of the Barber poles is to rotate the current direction through 45 the current flow assumes a saw tooth shape effectively changing the rotation angle of the magnetization relative to the current from α to α 45 A Wheatstone bridge configuration is also used for linearized applications In one pair of diagonally oppos...

Страница 12: ...he body of the chapter Fig 7 shows that flipping is not instantaneous and it also illustrates the hysteresis effect exhibited by the sensor This figure and Fig 14 also shows that the sensitivity of the sensor falls with increasing Hx Again this is to be expected since the moment imposed on the magnetization by Hx directly opposes that imposed by Hy thereby reducing the degree of bridge imbalance a...

Страница 13: ... Hy In Fig 15 for the KMZ10B sensor the extension for Hy 1 kA m is shown TEMPERATURE COMPENSATION With magnetoresistive sensors temperature drift is negative Two circuits manufactured in SMD technology which include temperature compensation are briefly described below The first circuit is the basic application circuit already given see Fig 11 It provides average sensor to sensor compensation of se...

Страница 14: ...dge resistance of the magnetoresistive sensor The amplification of the complete amplifier can be calculated by 10 The positive temperature coefficient TC of the amplification is 11 For the given negative TC of the magnetoresistive sensor and the required amplification of the input stage A1 the resistance RA and RB can be calculated by 12 13 where TCKTY is the temperature coefficient of the KTY sen...

Страница 15: ... be obtained from equation 1 R R0 DR cos2f 2 and with a constant current Ι the voltage drop in the x direction Ux becomes Ux ρ Ι 3 Besides this voltage which is directly allied to the resistance variation there is a voltage in the y direction Uy given by Uy ρ Ι 4 This is called the planar or pseudo Hall effect it resembles the normal or transverse Hall effect but has a physically different origin ...

Страница 16: ...n insufficient value will produce an open characteristic hysteresis of the sensor An easy axis in the y direction leads to a sensor of higher sensitivity as then Ho Hk Hd Linearization As shown the basic magnetoresistor has a square resistance field R H dependence so a simple magnetoresistive element cannot be used directly for linear field measurements A magnetic biasing field can be used to solv...

Страница 17: ...e seen that for small values of Hy relative to H0 the R H dependence is linear In fact this equation gives the same linear R H dependence as the planar Hall effect sensor but it has the magnitude of the magnetoresistive sensor Barber pole sensors require a certain magnetization state A bias field of several hundred A m can be generated by the sensing current alone but this is not sufficient for se...

Страница 18: ...A magnetic shunt parallel to the magnetoresistor or only having a small field component in the sensitive direc tion can also be employed with very high field strengths A high signal voltage Ux can only be produced with a sensor that can tolerate a high supply voltage Uo This requires a high sensor resistance R with a large area A since there are limits for power dissipation and current density The...

Страница 19: ...ensor magnetization and hence the output characteristic depends on the magnitude of the transverse field Hy The greater this field the more the magnetization rotates towards 90 and therefore it becomes easier to flip the sensor into the corresponding stable position in the x direction This means that a smaller Hx field is sufficient to cause the flipping action As can be seen in Fig 22 for low tra...

Страница 20: ... not irreversibly demagnetize the sensor If Hd is negative and much larger than the stabilising field Hx the sensor will flip This effect is reversible with the sensor returning to the normal operating mode if Hd again becomes negligible see Fig 24 However the higher Hx the greater the reduction in sensor sensitivity and so it is generally recommended to have a minimum auxiliary field that ensures...

Страница 21: ...y strips are used while the KMZ41 series has simple elements The patterns used are different for these three families of sensors in every case the elements are linked in the same fashion to form the four arms of a Wheatstone bridge The meander pattern used in the KMZ51 is more sophisticated and also includes integrated compensation and flipping coils see chapter on weak fields the KMZ41 is describ...

Страница 22: ...ridge imbalance is then a linear function of the amplitude of the external magnetic field in the plane of the permalloy strips normal to the strip axis This layout largely eliminates the effects of ambient variations e g temperature on the individual elements and also magnifies the degree of bridge imbalance increasing sensitivity Fig 26 indicates two further trimming resistors RT which allow the ...

Страница 23: ...applications incorporate this coil on the silicon However when measuring weak fields second order effects such as sensor offset and temperature effects can greatly reduce both the sensitivity and accuracy of MR sensors Compensation techniques are required to suppress these effects OFFSET COMPENSATION BY FLIPPING Despite electrical trimming MR sensors may have a maximum offset voltage of 1 5 mV V I...

Страница 24: ...ection plus the offset is measured in the second half This results in two different outputs symmetrically positioned around the offset value After high pass filtering and rectification a single continuous value free of offset is output smoothed by low pass filtering See Figs 29 and 30 Offset compensation using flipping requires additional external circuitry to recover the measured signal Fig 28 Fl...

Страница 25: ...eld measurement General Fig 30 Timing diagram for flipping circuit a output voltage b filtered output voltage c output voltage filtered and demodulated handbook full pagewidth MBH618 offset internal magnetization flipping current IF VO T time time VO time VO time VO Hy T T a b c ...

Страница 26: ...t negligible as it can produce a difference of a factor of three within a 25 C to 125 C temperature range for fields up to 0 5 kA m This effect is not compensated for by the flipping action described in the last section Fig 31 Output voltage Vo as a fraction of the supply voltage for a KMZ10B sensor as a function of transverse field Hy at several temperatures handbook full pagewidth 3 0 15 3 2 2 M...

Страница 27: ...ance Thus a current source not only improves the stability of the output voltage Vo and reduces the variation in sensitivity to a factor of approximately 1 5 compared to a factor of three using the voltage source However this method requires a higher supply voltage due to the voltage drop of the current source Fig 32 Output voltage Vo of a KMZ10B sensor as a function of transverse field Hy using a...

Страница 28: ...ortional to the output voltage change caused by the change in measured field The magnetic field produced by the compensation coil is in the opposite direction to the measured field so when it is added to the measured field it compensates exactly for the change in the output signal regardless of its actual temperature dependent value This principle is called current compensation and because the sen...

Страница 29: ...ts the individual requirements of an application should be carefully analysed in terms of the performance gains versus relative costs Table 4 Summery of compensation techniques TECHNIQUE EFFECT Setting avoids reduction in sensitivity due to constant stabilization field Flipping avoids reduction in sensitivity due to constant stabilization field as well as compensating for sensor offset and offset ...

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