WO2007096318A1

WO2007096318A1 - Sensor device for detecting a magnetic field variable

Info

Publication number: WO2007096318A1
Application number: PCT/EP2007/051544
Authority: WO
Inventors: Gabriel Daalmans
Original assignee: Siemens Aktiengesellschaft
Priority date: 2006-02-20
Filing date: 2007-02-19
Publication date: 2007-08-30
Also published as: DE102006007770A1

Abstract

The sensor device (1) detects a magnetic field variable. It comprises a transducer unit (2), a source (3) which feeds the transducer unit (2), and an amplifier (4) which is connected to the transducer unit (2). The transducer unit (2) contains at least one transducer bridge circuit having two bridge paths (5, 6) which are connected in parallel and each of which comprises a first magnetic field transducer (7, 8) and a further path element (9, 10) which is connected in series. The amplifier (4) is connected to two tap nodes (11, 12), a respective one of which is provided between the first magnetic field transducer (7, 8) and the further path element (9, 10) of each bridge path (5, 6). The first magnetic field transducer (7, 8) of each bridge path (5, 6) is a respective tunnel magnetoresistor which may be composed of a plurality of tunnel elements (13) which are electrically connected in series. A magnetic antenna (29; 37, 38; 47; 55, 56) which increases a local magnetic flux density at this first magnetic field transducer (8; 33, 34; 46; 51, 54) and a modulation unit (41; 48; 59) which is magnetically connected to the antenna (37, 38; 47; 55, 56) by means of a modulation coil (39, 40; 49; 57, 58) are provided.

Description

description

Sensor device for detecting a magnetic field size

The invention relates to a sensor device for detecting a magnetic field size. It comprises at least one converter unit, a source supplying the converter unit and an amplifier connected to the converter unit.

Such a magnetic field-sensitive sensor device is known, for example, from the article by R. Stolz et al. "Integrated SQUID Gradiometer System for Magneto-Cardiography Without Magnetic Shielding", IEEE Transactions on Applied Superconductivity, 2003, Vol 13 (2), pages 356 to 359. Such sensor devices are suitable for high-resolution detection even of very weak magnetic field distributions. At present, they usually have a magnetic field converter designed as a SQUID (superconducting quantum interference p-detector) sensor as the main component of the converter unit.

SQUID sensors allow a magnetic field resolution in the order of 10 fT / VHz to 10 pT / VHz, the high sensitivity even for very weak magnetic fields but only by means of a costly cooling to -269 ° C (liquid

Helium) or at -196 ° C (liquid nitrogen) is achieved. In addition, additional components such as the magnetic flux concentrating antennas and / or reference sensors including electronic units for compensating homogeneous interference magnetic fields can be provided. This results in a high sensitivity for gradient fields with simultaneous insensitivity to homogeneous fields. Overall, these known sensor devices are complex and often relatively large.

In addition to the above, rather basic-oriented state of the art are known from DE 102 44 889 Al, DE 10 2004 017 191 Al, the US 6 82 443 Bl and DE 10 2004 040 079 B3 technical devices for measuring magnetic field quantities known. Such devices contain, as magnetic field sensors, in particular so-called XMR elements, which are realized in modern technology as GMR (Giant Magnetic Resonance), AMR (Anisotropic Magnetic Resonance) or TMR (Tunnel Magnetic Resonance) elements. For example, in DE 102 44 889 A1 for measuring the magnetic field, chains are formed from individual TMR sensor elements. The DE 10 2004 017 191 Al shows in detail the use of magnetic field sensors for

Direction Determination of Moving Objects On the other hand, US 6 82 443 B1 describes sensors for accurately measuring and displaying ("mapping") electromagnetic fields Finally, DE 10 2004 040 079 B3 discloses bridge arrangements of magnetic field sensors.

Based on the above prior art, it is an object of the invention to provide a sensor device which is highly soluble and can be produced or operated more cost-effectively than the comparable known sensor devices.

This object is achieved by the features of independent claim 1. Further developments of the invention are specified in the dependent claims.

The invention relates to a sensor device for detecting a magnetic field size comprising at least one transducer unit, a source feeding the transducer unit and an amplifier connected to the transducer unit, wherein the transducer unit comprises first magnetic field transducer and another branch element connected in series in which at least one of the first magnetic field transducers is arranged a magnetic antenna which increases a local magnetic flux density at this first magnetic field transducer and in that there is a modulation unit which is magnetically connected to the antenna by means of a modulation coil. In the measuring device according to the invention, a feedback path is preferably provided between the amplifier and at least one of the first magnetic field converter. The feedback branch is preferably connected to the first magnetic field converter by means of a magnetic antenna and a modulation coil.

In the context of the invention, such a sensor device is advantageously realized, in which a) the converter unit at least one

Transducer bridge circuit comprising two bridge branches connected in parallel, b) each of the bridge branches comprises a first magnetic field transducer and a series-connected further branch element.

In the latter context, advantageously c) the amplifier is connected to two tap nodes, one of which is provided between the first magnetic field converter and the other branch element of each bridge branch, and is preferably d) the first magnetic field converter each bridge branch each a tunnel magnetoresistor, the is composed of several electrically connected in series tunnel elements.

The sensor device according to the invention can therefore be realized to the effect that as a magnetic field transducer advantageously a tunnel magnetoresistor (= TMR) is provided with a plurality of tunnel elements. Depending on the number and in particular surface of the tunnel elements, the field or field gradient resolution can be set. It was recognized that the resolution can be improved with increasing number of tunnel elements. An optimization of

Resolution is therefore also possible in particular without any individual tunnel element being designed for optimal resolution. At the to be detected Magnetic field size may in particular be a magnetic field or a gradient of a magnetic field.

Furthermore, based on the number of tunnel elements, the noise of the converter unit can be increased, so that it is preferably at least as large as the noise of the (pre-) amplifier. Then the noise of the amplifier has a negligible and in particular much smaller impact on the finally achievable resolution than in other solutions in which the amplifier noise outweighs that of the transducer unit. The amplifier is preferably low noise, but still inexpensive, so for example, in a "low cost" variant executed.

The multiplicity of tunnel elements can preferably be integrated on a common chip without further ado, so that the sensor device according to the invention can be realized essentially in a very compact manner. In particular, the converter unit is very space-saving. It can be realized without voluminous components.

The converter bridge circuit included in the converter unit improves the temperature behavior. The output of the sensor device according to the invention is therefore also very stable in temperature. In addition, the bridge circuit can help to prevent the influence of disturbing, in particular homogeneous magnetic fields, which are superimposed on the measured magnetic field magnitude.

Overall, a very high field or field gradient resolution can be achieved by means of the sensor device according to the invention, which is comparable to that of a SQUID sensor, but no part of the sensor device according to the invention is to be cooled to a low temperature. The sensor device according to the invention thus works at room temperature, so that in particular no expensive cooling devices are required. It is advantageous to use standard combined in a novel way. This reduces both the manufacturing and the operating costs.

In particular, with the features of the dependent claims of claim 1, particularly advantageous embodiments of the sensor device according to the invention result:

A variant is expedient in which the tunnel elements assigned to one of the tunnel magnetoresistors are connected to one another in the form of a chain structure. This chain-like arrangement is particularly space-saving and can be produced in particular integrated on a chip.

Furthermore, it is preferred that the further branch element of each bridge branch is designed as a magnetoresistance-insensitive resistor, in particular as an ohmic metal resistor. This results in a half bridge with particularly low production costs. A magnetic field insensitive conventional ohmic resistance is less expensive than a tunnel magnetoresistor. In such a half-bridge then only two differently oriented easy magnetization axes ("easy axis") occur, namely one for each of the two electrodes of a tunnel junction of the first magnetic field converter.

On the other hand, it is also possible that the further branch element of each bridge branch is designed as a second magnetic field converter, in particular in the form of a tunnel magnetoresistor. This results in particular in a full bridge with a total of four magnetic field converters. Such a full bridge provides a higher by a factor of 2 output signal level at the converter unit and thus also in the sensor device as a whole.

In another preferred variant is one of the first magnetic field converter or at least one of the other Branch elements provided with a magnetic shield. Preferably, a linear magnetic field converter is also provided. Such a tunnel magnetoresistor, which is linearly dependent on the magnetic field quantity to be detected, has in particular two easy axes of magnetization ("easy axis") which are perpendicular to one another and to the direction of the magnetic field quantity to be detected whereas magnetic field gradients can be detected by means of alternative variants realized without shielding, for example.

Preferably, it is also possible that at least one of the first magnetic field transducer, a magnetic antenna is arranged, which increases a local magnetic flux density at this first magnetic field transducer. In particular, however, this flux density increase by the antenna takes place only at the first magnetic field transducer assigned to it, whereas the flux ratios at the other first magnetic field transducer remain unaffected by this antenna. There may be provided a further antenna. The antenna leads to an increase in field sensitivity and / or resolution. This is especially true when the antenna has a sufficiently large thickness to act as a flux concentrator. For space and efficiency reasons, it is favorable to integrate the antenna and the associated magnetic field transducer together on a chip.

A further variant is expedient in which, in particular for noise suppression, a modulation unit, in particular by means of a modulation coil, is magnetically connected to the antenna. In this case, a non-linear magnetic field converter is preferably provided, in which the easy axes of the magnetization ("easy axis") in the relevant magnetic layers of the tunnel magnetoresistance are therefore not oriented perpendicular to each other but parallel. By means of the modulation specified according to the invention, it is possible to suppress a low-frequency noise component of the converter unit in the output signal of the sensor device.

Advantageously, a feedback branch may also be provided between the amplifier and at least one of the first magnetic field transducers. Such feedback stabilizes and linearizes the output signal of the sensor device.

Moreover, it is favorable if the feedback branch is connected to the first magnetic field converter by means of a magnetic antenna and a coil. The antenna and the coil can then also be used otherwise, for example for a modulation for low-frequency noise suppression. Then the number of components required is reduced. The antenna is used for several purposes.

The invention can be used for a wide variety of technical applications. Particularly advantageous is the application for a magnetic field measurement in magnetic resonance tomographs.

Further details and advantages of the invention will become apparent from the following description of the figures

Embodiments with reference to the drawing in conjunction with the claims.

Show it:

1 shows a block diagram of a sensor device for

Detecting a magnetic field gradient with two each comprising a plurality of tunnel elements

Magnetic transducers 2 shows an embodiment of a magnetic field converter according to FIG. 1 with a chain-like arrangement of the tunnel elements, FIG.

3 shows a block diagram of a transducer unit for detecting a magnetic field gradient comprising antennas assigned to the magnetic field converters,

4 shows an equivalent circuit diagram of the magnetic fluxes which occur in one of the antenna-provided magnetic field converters according to FIG. 3, FIG. 5 shows a block diagram of a sensor device for

Detecting a magnetic field comprising a magnetically shielded magnetic field transducer,

6 shows a block diagram of a sensor device for

Detecting a magnetic field comprising a magnetically shielded magnetic field transducer and a feedback branch,

7 shows a block diagram of a transducer unit for detecting a magnetic field gradient comprising antennas assigned to the magnetic field converters and one connected to the antennas

Modulation unit,

8 shows diagrams for courses of a measuring signal of

Converter unit according to FIG 7 over the magnetic field and over time for a non-linear magnetic field transducer with and without external

Magnetic field, as well

9 and

10 shows block diagrams of sensor devices for

Detection of a magnetic field comprising a transducer unit in half or full bridge circuit and a modulation unit.

In the figures, corresponding parts or units are provided with the same reference numerals. FIGS. 1 to 10 will be described below in conjunction with the measurement of magnetic field quantities. Subsequently, a specific application for MRI scanners will be discussed. FIG. 1 shows a block diagram of a sensor device 1 for detecting a gradient .theta.B / .theta.x of a magnetic field B (x). It contains a magnetic field-sensitive converter unit 2, a source 3 supplying the converter unit 2 with a bias voltage E and an amplifier 4 connected to the converter unit 2. The converter unit 2 is realized as a bridge circuit which has two bridge branches 5 connected in parallel and 6 in each case with a series connection of a first magnetic field converter 7 or 8 and a further branch element 9 or 10. Between the magnetic field converters 7 and 8 on the one hand and the branch elements 9 and 10 on the other hand, in each case a tap node 11 or 12 is present. The tap nodes 11 and 12 form a center tap to which the amplifier 4 is connected.

The magnetic field converters 7 and 8 are each designed as tunnel magnetoresistors with a magnetic field-dependent resistance value Ri (B) or R ₂ (B). In contrast, the branch elements 9 and 10 are designed as simple ohmic resistors, in the exemplary embodiment as metal resistors, with a magnetic field-independent resistance value Ro. The bridge circuit is thus executed in the embodiment of FIG 1 as a half bridge. In the field-free case, the tunnel magnetoresistors of the magnetic field converters 7 and 8 also have a resistance of R ₀ . Otherwise, their resistance value Ri (B) or R ₂ (B) depends on the magnetic field B (xi) or B (x ₂ ) pending at their location Xi or x ₂ . The magnetic field transducers 7 and 8 are arranged in the direction of an x-spatial coordinate with a distance I Xi-X ₂ I to each other.

The magnetic field converters 7 and 8 are linear, ie their output signal depends linearly on the magnetic field B (xi) or B (x ₂ ) to be detected. In the tunnel magnetoresistors of the magnetic field converters 7 and 8, the linear behavior is caused by the magnetizations Mi and M ₂ oriented perpendicular to each other in the direction of the easy axes ("easy axis") and the magnetizations Mi and M ₂ are field-free Case symmetrical with respect to the magnetic field direction to be detected. So they are each at an angle of about 45 ° to the x direction. Under the influence of a magnetic field B to be measured, at least one of the magnetizations Mi and M _{2 is} rotated with respect to its original orientation.

According to the embodiment shown in FIG. 2, the tunnel magnetoresistors of the magnetic field converters 7 and 8 are composed of a number N of individual tunnel elements 13, which are electrically connected in series. Each tunnel element 13 has a lower and an upper electrode 14 and 15, between which a tunnel junction 16 is arranged. At least one of the electrodes 14 and 15 is realized in the embodiment with a soft magnetic material.

The N tunnel elements 13 of a tunnel magnetoresistor are connected to one another like a chain structure. Each lower and each upper electrode 14 and 15 contacts two adjacent tunnel elements 13, wherein the tunnel element pairs contacted by the upper electrodes 15 are each offset by one tunnel element 13 from the tunnel element pairs contacted by the lower electrodes 14. Adjacent tunnel elements 13 are therefore always in the opposite direction, ie from top to bottom or from bottom to top, passed by the charge carriers.

The amplifier 4 is a low-cost, low-noise preamplifier whose noise is, for example, at a value of 5 nV / VHz. Depending on the frequency range covered in the specific case of application, however, even higher noise values than 5 nV / VHz may still be regarded as low in noise.

The mode of operation of the sensor device 1 according to FIG. 1 as well as particularly favorable dimensioning specifications for individual components of the sensor device 1 are described in more detail below: The actual detection of the magnetic field gradient θB / θx to be measured takes place by means of the magnetic field converters 7 and 8, the half-bridge circuit serving for temperature stabilization of a voltage difference U1-U2 (= output voltage) present at the tap nodes 11 and 12 as the original measurement signal. This measurement signal is further processed in the amplifier 4 low noise, so that a total of a very high magnetic field gradient resolution results. The latter can be adjusted in particular by a suitable choice of the number N of the tunnel elements 13.

The nominally identical magnetic field-dependent resistance values Ri (B) and R ₂ (B) of the magnetic field converters 7 and 8 are calculated when the magnetic field B (xi) and B (x ₂ ) are present according to:

Ri (B) = R ₀ + (öRi / δB) B (xi), (1)

R ₂ (B) = R ₀ + (ÖR ₂ / ÖB) B (x ₂ ) (2)

With

Ro = NR ₃ , (3)

(δRi _{/ 2} / δB) = N (δR _: / δB), (4)

where R-, the resistance of each of the tunnel elements 13 and j is a serial number of the tunnel elements 13. This results in the voltage difference Ui-U ₂ present at the center tap of the half-bridge as a measuring signal to:

Ui-U ₂ = [E / 2R ₀ ] [(ÖRi / ÖB) B (xi) - (ÖR ₂ / ÖB) B (x ₂ )] = ≡ [NE ₃ ] [(1 / R _:) (ÖR _: / ÖB)] (Xi-X ₂ ) (öB (x) / δx), (5)

wherein with E the voltage drop across the complete half-bridge bias, with E ₃ a falling across each of the tunnel elements 13 partial bias and with (l / R-,) (OR ₁ ZdB) a field sensitivity of each of the tunnel elements 13 is designated. The latter field sensitivity is a technology- and material-sensitive parameter. It depends primarily on an energetic given in eV Barrier height of the tunnel junction of each of the tunnel elements 13 and of a tunnel barrier thickness t _B , but not of a transition cross-sectional area A of each of the tunnel elements 13. Overall, the measurement signal U1-U2 according to equation (5) thus linearly dependent on the number N of the tunnel elements 13, not but from the lateral dimensions of the tunnel crossing.

A noise voltage density u _n at the center tap of the half-bridge can be approximated by the following equation:

u ² _n = Nu ² _: + 0.5Nu ² _: (f), (6)

being with

u ² _: = 4k _b TR _: (7)

the white thermal noise of a transition, so one of the tunnel elements 13 and with

u ² _: (f)> αE ² _: / (Vf), (8)

is the low-frequency, due to electrical influences 1 / f noise of a transition is called.

Moreover, in equations (6) to (8) f stands for the frequency, k _b for the Boltzmann constant, T for a temperature of the magnetic field transducers 7 and 8, α for a proportionality constant dependent on the technology and the material and V for an active volume of the tunnel junction of each of the tunnel elements 13.

The active volume V included in equation (8) is linearly proportional to the transversal cross-sectional area A, whereas the white noise u ² _: according to equation (7) is linearly proportional to the resistance R- of the tunneling elements 13 and thus inversely proportional to the cross-sectional area A of the transition. Thus, the relationship for the noise voltage density u ² _{n can be} transformed to:

u ² _n = (N / A) [4k _b T (pt _B ) + 0.5αE ² _: / (λ f)], (9)

wherein p denotes the resistivity of the tunnel elements 13, t _B the thickness of the tunnel barrier of the tunnel elements 13 and λ thermalisation length within the charge carriers on the average after tunneling through one of the tunnel elements 13.

From the equations (5) and (9), a total magnetic field gradient resolution (θB / θx) _min results:

(3B / 3x) = _min [1 / V (NA)] [4k _b T (pt _B) + 0, 5αE ^₂ / (λf)] ° ^'5 /

[E ₁ {(1 / R _:) (OR ₁ ZdB)} (Xi-X ₂ )] = = (1 / VN) (3B / 3x) _: , _min , (10)

with (.theta..sub.B / θx) - _j, mm as a Magnetfeldgradientenauflösung a single of the tunnel elements 13. Equation (10) shows that the Magnetfeldgradientenauflösung (.theta..sub.B / θx) _min of the converter unit 2 in total inversely proportional to the square root of the product of the transitional cross-sectional area A one of the tunnel elements 13 and the number N of the tunnel elements 13 is. By means of a change of these two sizes, the resolution can thus be adjusted.

In addition, the equation (10) shows that the magnetic field gradient resolution (θB / θx) _{min of} the transducer unit 2 as a whole versus the magnetic field gradient resolution (θB / θx) - |, mm of a single one of the tunnel elements 13 is a factor of the square root of the number N of the tunnel elements 13 can be improved. However, this statement is valid only when the noise u ² _n of the half bridge, so the transducer unit 2, _A is greater than a noise amplifier ² u _n, the amplifier 4. Accordingly, the condition:

u ² _n = (N / A) [4k _b T (pt _B ) + 0.5αE ² _: / (λ f)]> u ² _n , _A (11) to fulfill. Equations (10) and (11) are the design rules for the transducer unit 2, in particular for the tunneling magnetoresistors of the magnetic field converters 7 and 8.

To arrive at a low-cost solution, comes as amplifier 4 of the low-cost preamplifier with an amplifier noise of at least 5 nV / VHz used. Given the required magnetic field gradient resolution (θB / θx) _{min of} the transducer unit 2 as a whole and with known material and technology parameter (1 / R _:) (3R ₁ ZdB), it is then possible to derive from equations (10) and (11 ) specify a condition for the number N of tunnel elements 13:

N> (u " _n , _A ) ^u 'V [(3B / 3x) _min (E _D ) {(1 / R _:) (3R ₁ ZdB)} (X ₁ -X ₂ ) 1: i2)

In the following Table 1, minimum values of the number N determined for various specifications of the magnetic field gradient resolution (θB / θx) _min , the amplifier noise u ² _n , _A , the distance (X1-X2) of the magnetic field converters 7 and 12 are determined from equation (12) 8, of the material and technology parameter (1 / R-,) (3R ₁ ZdB) and the partial bias voltage E ₃ dropping across each of the tunnel elements 13:

The number N thus determined may be substituted into equations (10) and (11) to derive a condition for the transition area A of each of the tunnel elements 13:

A <[N / ² u, _n _A] [4 k T _b (pt _B) + 0, _5αE: ² / λf] (13) For tunnel elements 13 with alumina (Al 2 O 3) barrier, the expressions (pt _B ) and (α / λ) usually take the following values:

This reduces equation (13) for Al ₂ O 3 technology to:

A <[N / UV A] [10 ^" " + 10 ^" " E ₁ V f]; i3a)

It can be deduced from this, up to which cut-off frequency f _c the low-frequency 1 / f noise predominates. Then the second addend in the last parenthetical expression of equation (13a) is greater than the first addend. As can be seen from the following Table 2, this frequency range with dominant 1 / f noise depends very much on the partial bias E ₃ :

From Equation (13a) and in accordance with the specifications of Table 1, (minimum) values for the transitional cross-sectional area A of tunneling elements 13 realized in Al ₂ O ₃ technology can be determined. In the following Table 3, values for the transition cross-sectional area A thus determined are listed as examples and assuming a resolution of 1 pT / cmVHz required for the converter unit 2 and the use of the above-specified low-cost pre-amplifier:

1 3 - 1 0 ³

40 5 0, 3

0, 0 1 1, 5 - 1 0 ⁵

1 1, 6 - 1 0 ³

80 10 0, 3

0, 0 1 8 - 1 0 ⁴

1 1, 6 - 1 0 ⁴

800 10 0, 3

0, 0 1 8 - 1 0 ⁵

The total cross-sectional area of the tunnel junctions of all tunnel elements 13 of one of the magnetic field transducers 7 and 8 is of the order of 10 ⁵ μm ² . The tunnel elements 13 are integrated in particular on a chip. Consequently, the magnetic field converter 7 and 8 and thus also the transducer unit 2 and the sensor device 1 overall have a very compact design with a small footprint. In addition, the sensor device 1 can be produced with comparatively low production costs.

In Figure 3, another embodiment of a converter unit 17 is shown in block diagram representation. The converter unit 17, like the converter unit 2, is a gradiometer. It detects a magnetic field gradient. you

The construction is essentially similar to that of the converter unit 2. The main difference consists in additionally provided magnetic antennas 18 and 19, one of which is assigned to one of the two magnetic field converters 7 and 8 respectively. The antennas 18 and 19 are made of a soft magnetic

Material and serve to increase the local magnetic flux density at the location of each associated magnetic field transducer

7 and 8, respectively. On the other hand, the antennas 18 and 19 do not cause any influence on the flux density at the respectively non-associated magnetic field converter 8 or 7. Ultimately, the two antennas 18 and 19 lead to an improvement of the sensitivity and the resolution. The antennas 18 and 19 are laterally adjacent to the respective magnetic field transducer 7 and

8, so that a small air gap 20 or 21 remains. The influence of the antennas 18 and 19 by means of a flux analysis of the unit of antenna 18 or 19, air gap 20 or 21 and soft magnetic electrodes 14 and 15 of the tunnel elements 13 is also examined below with reference to FIG.

This analysis is based on the definition of magnetic flux conductors or of magnetic flux resistances, which are reproduced in the equivalent circuit diagram according to FIG. The antenna 18 or 19 is converted into an impedance R _A , the air gap 20 or 21 in an impedance R _G and the soft magnetic electrodes 14 and 15 of the tunnel junctions in an impedance R _τ . These impedances R _A , R _G and R _τ are connected in series. Parallel to this series connection, a further impedance Rp is arranged, which describes the interacting volume, in which the flux density is influenced by the antenna 18 or 19. The main task of the antenna 18 or 19 is to increase this interacting volume.

The forming in the air gap 20 and 21 magnetic

The flux Φi can be expressed by the total magnetic flux Φ in the interacting volume in combination with the mentioned magnetic impedances:

Φ i = Φ / [1+ (R _A + R _G + R _T ) / RP] (14)

With

Φ = A _A B ₀ = (w _A L _A ) B ₀ (15a) Φ i = (hw _A ) B _G (15b)

where with A _A an antenna cross-section perpendicular to the magnetic field, with w _A an antenna width, with L _A an antenna length, with h an air gap width, with B _G a local flux density in the air gap 20 and 21 and B ₀ a

Flux density of the output measuring field is designated. These variables are entered in the illustration in FIG 3 with. The different magnetic impedances are calculated according to:

R _P = 1 / (μ _o w _A ); i6a)

RA = L _A / (μ _o μAW _A t _A : i6b)

RG = h / (μ _o w _A t _A ): i6c) w _τ / (μ _o μ _τ w _A t _τ : i6d)

where μ o the magnetic permeability of the vacuum, with μ _A an effective relative permeability of the antennas 18 and 19, with μ _τ an effective relative permeability of the soft magnetic electrodes 14 and 15 of the tunnel elements 13, with w _τ an electrode width, with t _{A is} an antenna layer thickness and t t denotes an electrode _{layer thickness} . Substituting equations (15) and (16) into equation (14) yields the following relationship for antenna gain:

B _G / B ₀ = (L _A / h) / [l + {l + L _A / (μ _A t _A) + _τ w / (μ _τ _τ t)}] (17)

It is clear that the maximum gain in the flux density is equal to (L _A / 2h). However, this maximum profit can only be achieved if the conditions:

L _A / (μ _A t _A ) «1; i8a) W _τ / (μ _τ t _τ )« 1; i8b)

are fulfilled. This is especially great for

Permeability values and large layer thicknesses of the case. Example numerical values are listed in Table 4 below, yielding a gain of approximately 10 when taking equation (17) into consideration:

Magnetic field transformers 17 thus dimensioned make it possible to improve the magnetic field gradient resolution up to about 100 fT / cmVHz at frequencies f which are above the limit frequency f _c mentioned in table 2. Thus, in the frequency range of the white noise, a resolution comparable to that of the SQUID sensors can be achieved. In contrast to the SQUID sensors, however, all components in the magnetic field converter 17 can be at room temperature. Cooling to low temperatures is not required.

FIG. 5 shows a further exemplary embodiment of a sensor device 22 as a block diagram. In contrast to the preceding exemplary embodiments, the magnetic field gradient B is not detected by means of the sensor device 22, but the magnetic field B is detected. Also, the sensor device 22 comprises a transducer unit 23, which is designed as a half-bridge.

In addition to the unchanged in the bridge branch 6 maintained magnetic field transducer 8 a shielded magnetic field transducer 24 is provided in the other bridge branch 5, which apart from a magnetic shield 25 as well as the magnetic field converters 7 and 8 is constructed. In particular, he is also a tunnel magnetoresistor with a variety in

Series switched tunnel elements 13 realized. The linear behavior of the magnetic field converter 24 and 8 is analogous to the embodiment of FIG 1 by the use of magnetic layers with mutually perpendicular easy magnetization axes.

The shield 25 is designed as a low-sized soft magnetic resistance. In a basically possible alternative embodiment as a full bridge, three of the then four provided magnetic field converter would be shielded. For deriving the voltage difference U1-U2 present at the center tap of the half-bridge of the sensor device 22 according to FIG. 5, a further exemplary embodiment of a sensor device not shown in FIG.

This sensor device, not shown, differs from that of the sensor device 22 shown in FIG 5 only in that the magnetic field converter is magnetically not shielded in the left bridge branch 5 and that in the right bridge branch 6 of the (also unshielded) magnetic field transducer 8 and the ohmic resistance executed branch element 10 are reversed in order. The magnetic field sensitive components are thus arranged diagonally in the bridge. In addition, the bridge of the sensor device, not shown, is built very close, so that both magnetic field sensitive components detect the same magnetic field B. Then the following relationships apply.

The voltages Ui and U ₂ present at the tap nodes 11 and 12 in the left and right bridge branch 5 and 6 respectively result in:

U ₂ = ER ₀ / (R ₀ + R ₂ ) (20)

and thus the voltage difference U1-U2 which can be tapped off at the center tap:

Ui-U ₂ = E [Ri / (Ri + R ₀₎ -R ₀ / (R ₀ + R _2)] (21)

Ideally, according to equations (1) and (2):

Ri = R ₂ = R ₀ + (ER / OEB) B (X), (22)

wherein the change of the magnetic field B between the mounting locations Xi and X _{2 of} the two magnetic field sensitive components is negligible due to the compact design. Equation (22) simplifies equation (21) to:

Ui-U ₂ = E (ÖR / ÖB) B (X) / [2R ₀ + (ÖR / ÖB) B (X)] (23)

and assuming a small magnetic field B to:

Ui-U ₂ = E (öR / δB) B (x) / 2R ₀ (24)

Taking into account that the sensor means, not shown, due to the double number of unshielded, magnetic field sensitive components also provides twice the output signal as the sensor device 22 shown in FIG 5, so calculated at the center tap of the half bridge of the sensor device 22 shown in FIG 5 as a measurement signal voltage difference Ui-U ₂ to:

Ui-U ₂ = (E / 4R ₀ ) (ÖR / ÖB)] B (x ₂ ) (25)

Thus, a measurement signal is detected for the magnetic field B (x ₂ ) present at the location x _{2 of} the right-hand magnetic field converter 8.

In a field detection, in contrast to a gradient detection, locally homogeneous interference fields become more noticeable. If the transducer unit 23 is designed to be extremely sensitive, such homogeneous interference fields can lead to an operating point shift, so that the linear operating range is left.

In order to prevent this and to stabilize the operating point, an additional feedback branch 27 is provided in the exemplary embodiment of a sensor device 26 shown in FIG. A converter unit 28 of the sensor device 26 differs from the converter unit 23 of the sensor device 22 only by a magnetic antenna 29 additionally provided on the magnetic field converter 8, which is surrounded by a modulation coil 30. The feedback branch 27 leads from the output of Amplifier 4, for example, via an optional inverting isolation amplifier 31 to the modulation coil 30. Ggf. For example, the feedback branch 27 may also contain other or further components such as a summing element and / or a modulation unit.

Analogous to the equation (12), which for the

Magnetic field gradient θB / θx detecting sensor device 1 applies, can also for the magnetic field B sensing sensor devices 22 and 28 specify a sizing rule for the number N of the tunnel elements 13 of the magnetic field converters 25 and 8. It is:

N> (u ² _n, _A) ° ^'5 / [B _min _(E:) (1 / _R:) (8R ZdB _1)], (26)

where B _{min is} a desired and thus therefore predefinable

Magnetic field resolution is designated. In contrast, the other applies

Sizing rule for the

Transitional cross-sectional area A of each of the tunnel elements 13 according to equation (13) equally for magnetic field gradients θB / θx and detecting magnetic field B detected

Sensor devices.

FIG. 7 shows a block diagram of a further exemplary embodiment of a transducer unit 32 for detecting a magnetic field gradient .theta.B / .theta.x. It is designed as a full bridge. There are a total of four magnetic-field converter 33, 34, 35 and 36 are provided which are each designed as a tunnel magneto resistors having a resistance value Ri, R.2, R3 and R _4. Also the

Tunnel magnetoresistors of the magnetic field converters 33 to 36 have the chain-type structure shown in FIG. 2 with a plurality of tunnel elements 13.

In each bridge branch 5 and 6, a shielded and a field-sensitive sensor is provided. Magnetically shielded are therefore the magnetic field transducers 35 and 36, whereas the Meßgrößenempfindlichen and for the purpose of Gradient detection at the same height (ie, perpendicular to the x direction) arranged magnetic field converters 33 and 34 each have a magnetic antenna 37 and 38 is assigned. Each of the antennas 37 and 38 is wound with a modulation coil 39 and 40, respectively, implemented in planar technology, which is electrically connected to a modulation unit 41.

The modulation unit 41 allows in the range of the measuring variable-sensitive magnetic field converters 33 and 34, the targeted additional generation of a magnetic

Modulation flux Φ _moc ι and a modulation _{magnetic field} B _moc ι with a modulation frequency f _mOd - The then at the center _tap pending measurement signal .DELTA.U = U1-U2 has an amplitude modulation with the modulation frequency f _mod on. This amplitude modulation is shown in the right-hand diagram according to FIG. 8, in which characteristics of the resulting measurement signal .DELTA.U = U1-U2 over time t with and without external magnetic field B are shown. As can be seen, the measurement information about the external magnetic field B and the magnetic field gradient θ B / θ x of interest is contained in the amplitude of the measurement signal ΔU = U 1 -U 2. It can be extracted by means of phase-sensitive demodulation not shown in detail.

In the exemplary embodiment according to FIG. 7, a nonlinear dependency of the voltages Ui or U2 present at the middle tap, as can be seen from the left-hand diagram of FIG. 8, is provided by the magnetic field B. The nonlinear behavior of at least the measuring variable-sensitive magnetic field converters 33 and 34 is achieved by magnetizations M ₃ and M ₄ in the direction of the easy axes in the relevant magnetic layers are not perpendicular to each other, but oriented parallel or anti-parallel. With respect to the direction of the external magnetic field B, the magnetizations M ₃ and M _{4 are} preferably oriented vertically.

The local total magnetic field consists of the modulation magnetic field B _MO ci = IB _Moc ι I expj (2πf _Mθ dt) and to be measured Magnetic field B (x) together, which is lower frequency compared to the modulation frequency f _Mod . The essential frequency content of the magnetic field B (x) is thus below the modulation frequency f _Mθd -

The modulation magnetic field B _Mod is equal to the two magnetic field converters 33 and 34. Without the low-frequency magnetic field B (x) to be measured, it generates a change in the respectively associated resistance values Ri and R2 according to:

Ri = R ₀ + ΔRiexpj (4πf _Mθ dt) (27)

R ₂ = R ₀ + ΔR ₂ expj (4πf _Mθ dt) (28)

An additional low-frequency magnetic field B (x) to be measured modulates the resistance changes ΔRi and ΔR ₂ according to FIG

Ri - R ₀ = .DELTA.R _Moc ιexpj (4πf _Mθ dt) + (δRi / OEB) B (x _x) expj (2.pi.f _Mod t) (29) R ₂ - R ₀ = .DELTA.R _Mod expj (4πf _Mod t) + (ÖR ₂ / ÖB) B (x ₂ ) expj (2πf _mod t) (30)

where ΔR _{Mod is} an amplitude of the modulation-induced change in the nominally equal resistance values Ri and R _{2 of} the magnetic field converters 33 and 34. With nominally identical resistance values Ri and R ₂ and without gradient field:

ΔRi = ΔR ₂ = ΔR _Mod (31)

The measuring signal ΔU = Ui-U _{2 of} the converter unit 32 is then:

Ul - U2 = (E / 2R ₀ ) (Ri - R2) =

= (E / 2R ₀ ) <δR / δB> {B (xi) -B (x ₂ )} expj (2πf _Mod t) (32)

where <ÖR / ÖB> stands for the average of the magnitudes aδRi / öB and ÖR ₂ / ÖB averaged over the magnetic field converters 33 and 34. From equation (32) it can be seen that the output voltage ΔU = Ui-U ₂ deviates from zero only if there is a field gradient of the magnetic field B (x) to be measured. Due to manufacturing tolerances in the magnetic field converters 33 to 36, the bridge circuit of the converter unit 32 may not be perfectly symmetrical, so that in the output voltage and a proportion with twice the modulation frequency f _Mod may occur. Due to the phase-sensitive detection by f _Mod , which is anyway required for demodulation, this component is automatically filtered out with twice the modulation frequency f _Mθd .

Due to the combination of non-linear magnetic field converters 33 and 34 and additional magnetic field modulation, a low-frequency noise component due to the 1 / f noise of the magnetic field converters 33 to 36 can be suppressed.

The modulation magnetic field B _mOd generated by the modulation is superimposed on the external magnetic field B to be measured. As a result, the field sensitivity of the affected magnetic field converters 33 and 34 is modulated between a maximum and a minimum, eg a vanishing, sensitivity. Thus, the influence of the magnetic field B to be measured is measured in a time window which is much smaller than the time period within which the low-frequency fluctuations (= 1 / f noise) of the

Magnetic field converters 33 and 34 make noticeable. The detection of the magnetic field B to be measured is consequently not affected by these low-frequency fluctuations. Thus, at least in the frequency range below the cutoff frequency f _c , the converter unit 32 has an improved behavior with regard to sensitivity and resolution compared to the converter unit 2 according to FIG.

The advantageous magnetic field modulation for suppressing low-frequency noise components can be used not only in the detection of the magnetic field gradient θB / θx, but also in the detection of the magnetic field B. Corresponding embodiments magnetic field detecting Sensor devices 42 and 43 are shown as block diagrams in FIGS. 9 and 10, respectively.

The sensor device 42 according to FIG. 9 has a converter unit 44 realized as a half-bridge with the branch elements 9 and 10 designed as simple ohmic resistors and with a shielded and a measuring-variable-sensitive magnetic field converter 45 and 46, respectively. The latter are non-linear tunnel magnetoresistors each having a plurality in FIG Series of switched tunnel elements 13. The measuring variable-sensitive magnetic field transducer 46 is provided as a magnetic flux concentrator acting magnetic antenna 47 which is surrounded by a modulation unit 48 is electrically connected to a modulation coil 49.

The sensor device 43 according to FIG. 10 has a converter unit 50 in the form of a full bridge with four non-linear magnetic field converters 51, 52, 53 and 54 which are again constructed as tunnel magnetoresistors each with a multiplicity of tunnel elements 13 connected in series.

In each bridge branch 5 and 6, a shielded and a field-sensitive sensor is provided. Accordingly, the magnetic field transducers 52 and 53 are magnetically shielded, whereas magnetic field transducers 51 and 54 each have a magnetic antenna 55 or 56 associated therewith for the purpose of field detection perpendicular to the x-direction at different heights. Each of the antennas 55 and 56 is wound with a modulation coil 57 and 58, respectively. Modulation coils 57 and 58 are electrically connected in series and connected to a modulation unit 59. The magnetic field modulations produced on both magnetic field converters 51 and 54 are preferably in phase.

The sensor device 43 results in a signal amplitude of the measurement signal U1-U2 that is twice as large compared to the sensor device 42. Both sensor device 42 and 43 have the favorable suppression of low-frequency noise components.

All of the above-described sensor devices 1, 22, 26, 42 and 43 as well as transducer units 17 and 32 offer a high magnetic field or magnetic field gradient resolution in the case of a small volume of construction. The respective converter units 2, 17, 23, 28, 32, 44 and 50 can be integrated in particular on a chip. At an achievable resolution of a few pT / cm, the chip has

Edge dimensions of a few mm by 10 mm. This is very small for such a high resolution. With the inclusion of antennas, even an even higher frequency range between about 1 kHz (= typical value for the cutoff frequency f _c ) and the ferromagnetic resonance frequency

Achieve resolution of better than 1 pT / VHz or 1 pT / cmVHz.

The sensor devices described above can be used for a wide variety of applications with magnetic field measurements. In particular, metrological problems with nuclear spin tomographs that work according to the magnetic resonance method can thus be solved.

There are two types of magnetic feeder in the magnetic resonance method. For one thing, there are DC magnetic fields that serve to magnetically mark the diagnostic room. The local field determines the frequency of the nuclear resonance and thus the diagnostic space is uniquely marked by the frequency. The DC field is composed of a large homogeneous field and of gradient fields in the three coordinate directions. The homogeneous field is typically between 0.3 and 3 Tes- Ia. In contrast, the gradient fields are typically in the range of about 0.1 mT / cm.

As is known, in the magnetic resonance method, the nuclear spins of the protons (H ⁺ ) are transmitted through the DC magnetic field aligned. A high-frequency field (42 MHz at 1 Tesla) deflects the spins and brings them to precision around the DC field. Switching off the high-frequency field then leads to a relaxation of the spins and an inductive spin echo in the high-frequency coil. The relaxation is tissue-specific and thus causes the contrast in the subsequent image representation during the evaluation.

In the MRI scanner, two magnetic fields must therefore be detected, namely the large homogeneous field on the one hand and the gradient fields on the other hand. The homogeneous field is so large that, in particular, Hall probes are also suitable for the measurement, since the soft magnetic layers in fields of the order of magnitude of 1 Tesla usually become magnetically hard and no longer show any sensor effect. Thus, magnetoresistive sensors are eliminated for this purpose. Gradient fields of typically 0.1 mT / cm lead over a length of 2 m for a symmetrical application to a maximum of 10 mT at the axial end faces.

The latter fields can be measured very well, if one uses a magnetoresistive sensor with feedback circuit according to the facilities described above. Decisive here is the feedback circuit integrated in the sensor device, which makes it possible to measure even larger fields - provided that

- the field is increased from zero,

the feedback loop is technologically correctly designed and suitably dimensioned, and the homogeneous field is switched off during the measurement of the gradient field.

In such a sensor device, the chain of tunnel elements described in detail with reference to FIG. 2 is not absolutely necessary. Other magnetoresistive resistors are possible. Particularly in the problem with a DC magnetic field in the case of the MRI scanner, the low-frequency noise of the sensor is very important. Especially tunnel elements are rushing heavily, which is why the

Significance is related to the invention. If AMR resistors are used, there is less low frequency noise, so modulation may be unnecessary for some of the applications.

In the latter context, however, a measuring bridge composed of chains of magnetoresistive transducers can be useful, whereby the transducers can be both GMR (Giant Magnetic Resonance), AMR (Anisotropic Magnetic Resonance) or TMR (Tunnel Magnetic Resonance) elements. In particular, a chain with AMR transducers is an extended element: Resistors are interconnected with one structural element, which can advantageously be done without additional connection technology.

If the complete magnetic field distribution in the MRI scanner is to be measured, hybrid sensors are required. For the homogeneous DC-FeId sensors with Hall elements, sensors with XMR elements are used for the gradient fields.

Such a hybrid measuring device can be realized with a sensor chip. Such a sensor chip is repeatedly used in the MRI scanner in a plane perpendicular to the longitudinal axis and in two positions which are rotated with respect to each other by 90 ° about the longitudinal axis. In this case, two such measurement levels can be provided. This allows a complete measurement of the field distribution in all three coordinate directions simultaneously. With the latter approach, the following field components can be measured:

B (z) with a Hall sensor in a measuring plane;

- θB (z) / θz with a Hall sensor in two measurement planes;

B (x) or dB (x) / θx with an XMR sensor in a measuring plane;

- B (y) or θB (y) / θy with an XMR sensor in a measuring plane.

A universal chip for the measurement of the fields in nuclear resonance thus consists in the present case at least of a Hall element and an AMR bridge. The chip can thus measure B (z) and θB (y) / θy simultaneously in the event that the chip plane is the xy plane. A rotation of the chip 90 ° about the z-axis allows the simultaneous measurement of B (z) and θB (x) / θx. For this reason, two 90 ° twisted chips in the same xy plane are capable of measuring the magnetic field quantities B (z), θB (x) / θx and θB (y) / θy. If the poles are arranged differently, the measuring bridge can also directly measure the components B (x) and B (y) instead of their gradients.

The measurement in two levels also allows the detection of θB (z) / θz by interconnecting the Hall sensors of the two levels.

The XMR sensors have - as described - the property of feedback and can thus detect the magnetic fields as linear or non-linear sensors with great accuracy. However, this results in the need for substrate positioning.

The combination of XMR sensors and Hall sensors in a room with very high fields is metrologically only meaningful. full if the very high field is oriented perpendicular to the substrate plane. Because only in this case, the magnetization of the magnetic layers is not affected by the axial field. It is therefore necessary for meaningful use of the chip to have a separate device that allows for proper alignment of the chip. Such a device must therefore have two orthogonal axes of rotation, which are perpendicular to the direction of the large axial field. In this case, the substrate plane is spanned by coordinates x _s , y _s and z _s . With an optimal orientation of the substrate, x _s and y _{s would be} infinite. The normal vector n _s may be aligned in the z direction.

An optimal alignment is achieved by that

- By rotation about the x-axis of the normal vector of the substrate plane in the x-z plane is aligned. The optimal alignment coincides with a signal maximum of the Hall sensor; - By rotation about the y-axis of the normal vector of the substrate plane is aligned in the z-direction. The optimal alignment, in turn, coincides with a signal maximum of the Hall sensor.

Once the alignment is completed, the magnetic field gradient sensors are powered by electrical power and operated so that the gradient fields θB _x / θx and θB _y / θy can be measured. Two measurement planes are used for the measurement of θB _z / θz. For this purpose, the differential voltage of the two Hall sensors is evaluated, whereby the principle of the positioning device is realized.

A feedback of magnetic fields that lie in the substrate plane is not self-evident in practice. The return Coupling can be realized with wire wound coils in the feedback circuit. The chip is then positioned between two macroscopic coils. However, as far as the feedback of the sensor is done for fields in the plane of the substrate according to the prior art, such an arrangement is large and thus limits the spatial resolution in the measurement. Such an arrangement is also expensive, since the coils must be wound and several components must be assembled. Furthermore, such an arrangement can feed back only relatively small fields, since the magnetic circuit has a large air gap. The product of current and number of turns must be large. The feedback coils are thick film coils fabricated in planar technology, which are located a few microns away from the magnetoresistors on the substrate.

For example, assuming s / b << 1 and R _M (A) << R _M (air), a feedback coil having dimensions b and s becomes more effective by a factor of b / s, by adding the above-described magnetic antenna as a core can be done.

The magnetic resistance of the antenna R _M (A) is represented as: R _M (A) = b / [μ _o μ _eff dw] (33)

The magnetic resistance of vacuum R _M (air) is represented as:

R _M (air) = b / [μobw] (34)

The demand R _M (A) << R _M (air) thus means: d »(b / μ _ef f) (35) The technical teaching described above thus includes the instruction that the length b of the antenna is not simply arbitrarily increased in order to improve the feedback matrix, but that at the same time the cross section of the antenna is also increased. Advantageously, for this reason, the antenna will be designed in thick film technology, in the range of about one micron (1 μm) to several tens of microns (nxlO μm).

Claims

claims

A sensor device for detecting a magnetic field magnitude (B, θB / θx) comprising at least one transducer unit (2; 17; 23; 28; 32; 44; 50), a transducer unit (2; 17; 23; 28 ; 32; 44; 50) source (3) and an amplifier (4) connected to the transducer unit (2; 17; 23; 28; 32; 44; 50), the transducer unit (2; 17; 23, 28, 32, 44, 50) comprise at least one first magnetic field transducer (7, 8, 24, 33, 34, 45, 46, 51, 54) and a series-connected further branch element (9, 10, 35, 36) 52, 53), characterized in that a magnetic antenna (29, 37, 38, 47, 55, 56) is arranged on at least one of the first magnetic field transducers (8, 33, 34, 46, 51, 54) having a local magnetic flux density at this first magnetic field transducer (8; 33, 34;

46; 51, 54), and in that there is a modulation unit (41; 48; 59) which is magnetically coupled to the antenna (37, 38; 47; 55, 56) by means of a modulation coil (39, 40; 49; 57, 58). connected.

2. Sensor device according to claim 1, characterized in that between the amplifier (4) and at least one of the first magnetic field transducer (8), a feedback branch (27) is provided.

3. Sensor device according to claim 2, characterized in that the feedback branch (27) by means of the magnetic antenna (29) and a modulation coil (30) to the first magnetic field transducer (8) is connected.

A sensor device according to claim 1, characterized in that the magnetic field magnitude to be detected is a magnetic field gradient (θB / θx), and that a number (N) of tunneling tunnel elements (13) connected in series is a tunnel magnetoresistance according to the relationship

N> (u ² _n , _A ) ° ' ⁵ / [(θ B / θ x) _min (E ₃ ) {(1 / R _:) ( ₃ R _: / a B)} (X ₁ -X ₂ )] with u ² _n , _{A is} the noise of the amplifier (4), with (θB / θx) _min a predeterminable magnetic field gradient resolution, with E-, one over each of the tunnel elements (13) sloping partial bias, with (l / R- ,) (3R _: / θB) a material- and technology-dependent parameter of the tunnel elements (13) and (X1-X2) a distance between the two first magnetic field converters (7, 8, 33, 34) are designated.

A sensor device according to claim 1, characterized in that the magnetic field quantity to be detected is a magnetic field (B) and a number (N) of the series-connected tunnel elements (13) of a tunnel magnetoresistance according to the relationship

N> (u ² _n , _A ) ° ' ⁵ / [B _min (E _:) (1 / R _:) OR ₃ ZdB)]

with u ² _n , _A the noise of the amplifier (4), with B _min a predeterminable magnetic field resolution, with E ₃ one over each of the tunnel elements (13) sloping partial bias and with (l / R-,) (θR- _| / θB) a material- and technology-dependent parameter of the tunnel elements (13) are designated.

A sensor device according to claim 1, characterized in that the magnetic field quantity to be detected is a magnetic field gradient (θB / θx) or a magnetic field (B) and a junction cross-sectional area (A) of each tunnel element (13) of a tunnel magnetoresistance according to the relationship

A <[N / u ² _n , _A ] [4k _b T (pt _B ) + 0.5αE ² _: / λf]

where N is a number of tunneling tunneling elements (13) connected in series, u ² _n , _{A is} the noise of the amplifier (4), k _{B is} the Boltzmann constant, T is an ambient temperature, p is the specific one ohmic resistance of the tunnel elements (13), with t _B a tunnel barrier thickness of each of the tunnel elements (13) of a tunneling magnet with a constant, with E ₃ a partial bias voltage dropping across each of the tunnel elements (13), thermalising λ within the charge carriers on average after tunneling through one of the tunnel elements (13), and with f a frequency of detecting magnetic field magnitude are designated.

7. Sensor device according to claim 1, characterized in that a) the converter unit (2; 17; 23; 28; 32; 44; 50) contains at least one converter bridge circuit with two bridge branches (5, 6) connected in parallel, and b) each the bridge branches (5, 6) from the first magnetic field converter (7, 8, 24, 33, 34, 45, 46, 51, 54) and the branched branch element (9, 10, 35, 36, 52, 53) ) consists .

8. Sensor device according to claim 1, characterized in that c) the amplifier (4) is connected to two tap nodes (11, 12), of which one between the first magnetic field converter (7, 8, 24, 33, 34; 45, 46, 51, 54) and the further branch element (9, 10, 35, 36, 52, 53) of each bridge branch (5, 6), and d) the first magnetic field converter (7, 8, 24; 33, 34, 45, 46, 51, 54) of each bridge branch (5, 6) is in each case a tunnel magnetoresistor composed of a plurality of tunnel elements (13) connected electrically in series.

9. Sensor device according to claim 8, characterized in that one of the tunnel magnetoresistors associated tunnel elements (13) are connected together in the form of a chain structure.

10. Sensor device according to claim 7, characterized in that the further branch element (9, 10) of each bridge branch (5, 6) as magnetfeldunempfindlicher Resistance, in particular as an ohmic metal resistor, is formed.

11. Sensor device according to claim 7, characterized in that the further branch element each

Bridge branch (5, 6) as a second magnetic field transducer (35, 36, 52, 53), in particular in the form of a tunnel magnetoresistor (TMR) is formed.

12. Sensor device according to one of the preceding

Claims, characterized in that one of the first magnetic field converter (24; 45) or at least one of the further branch elements (35, 36; 52, 53) is provided with a magnetic shield (25).

13. Sensor device according to one of the preceding

Claims, characterized in that a measuring chip of at least one XMR sensor and a Hall sensor is formed.

14. Sensor device according to claim 13, characterized in that at least one magnetoresistive measuring bridge for detecting a field or a field gradient in the plane of the substrate together with an on-chip feedback circuit, which compensates for the field strength or the gradient of the measuring field by feedback , is provided.

15. Sensor device according to claim 14, characterized in that the chip contains means for suppressing the low-frequency noise by modulation.

16. Sensor device according to claim 14 or claim 15, characterized in that on the chip two XMR sensors, which are rotated with respect to each other by 90 °, are arranged.

17. Sensor device according to claim 16, characterized in that with the sensor chip either the gradient or the field strengths of two orthogonal fields in the substrate plane can be measured and fed back.

18. Sensor device according to one of claims 13 to 17, characterized in that a magnetic field vector is completely characterized by the chip with the magneto-resistive sensors on the one hand and Hall sensors on the other hand, in particular for measuring a magnetic field in a magnetic resonance tomograph.