Current sensor

CROSS-REFERENCE TO A RELATED APPLICATION

0001This application is a 35 U.S.C. §371 national phase application of PCT/CN2012/083742, filed on Oct. 30, 2012, which claims priority to a Chinese Patent Application No. 201110452834, filed on Dec. 30, 2011, incorporated herein by reference in its entirety.

FIELD OF THE TECHNOLOGY

0002The present invention relates to a current sensor, in particular a magnetic tunnel junction current sensor utilizing a magnetic tunnel junction resistor for temperature compensation.

BACKGROUND ART

0003The Hall Effect is commonly used in the sensing element of current sensors. Additionally but less commonly, the anisotropic magnetoresistance (AMR) and the giant magnetoresistance (GMR) effects are also used in the sensing elements of current sensors. What these effects have in common is they can be used to build a magnetic sensor in order to measure the magnetic field generated by a current flowing through a conductor, which is proportional to the value of the current flowing through the conductor.

0004Because the sensitivity of Hall elements is very low, when Hall elements are used in current sensors, a magnetic flux concentrator is usually used to amplify the magnetic field, in order to increase the sensitivity of the Hall sensor. Unfortunately this increases the size and weight of the current sensor, and it degrades linearity. Additionally, Hall sensors have high power consumption, which is undesirable. Although AMR sensors have much higher sensitivity than Hall sensors, AMR sensors have narrower linear range, and they require a set/reset coil, resulting in manufacturing process complexity, increased size, and increased power consumption. GMR sensors also have higher sensitivity than Hall sensors, but the linear range is also narrow.

0005Magnetic tunnel junctions (MTJ) are beginning to find application as magnetoresistive sensors for industrial applications. They use the tunneling magnetoresistance (TMR) effect of a multilayered magnetic material stack to sense the magnitude and direction of a magnetic field, with significantly larger sensitivity than AMR, Hall, and GMR sensors, while also providing better temperature stability. Thus a current sensor utilizing MTJ sensing elements provides improved temperature stability, higher sensitivity, lower power consumption, and better linearity than a Hall effect based current sensor with no additional magnetic flux concentrator structure; compared to AMR current sensors it provides improved temperature stability, higher sensitivity, wider linear range without additional set/reset coil structure; and, compared to GMR based current sensors, it provides better temperature stability, higher sensitivity, lower power consumption, and wider linear range.

0006Although temperature characteristics of a TMR based current sensor are superior to Hall Effect, AMR, and GMR based current sensors, temperature compensation is still necessary for high accuracy.

SUMMARY OF THE INVENTION

0007The present invention provides a current sensor using MTJ elements both to sense the magnetic field from the current and to compensate temperature drift thereby providing high sensitivity, wide linear range, low power consumption, small size, and good temperature performance characteristics.

0008To achieve the above object, the present invention provides a single chip current sensor comprising a MTJ sensor bridge containing one or more MTJ elements, an electric conductor in the vicinity of the sensor bridge that carries a test electric current, and at least one MTJ temperature compensation resistors, wherein magnets are placed on the sides of the one or more MTJ temperature compensation resistors producing a magnetic field to bias the free layer of the MTJ elements in the temperature compensation resistors such that the direction of the magnetization of the free layer of each of the MTJ temperature compensation elements is anti parallel to the direction of the magnetization of its pinned layer, such that the one or more MTJ temperature compensation resistors are in a high resistance state that only changes as the function of temperature in the measurement range of the current sensor, wherein the sensor bridge is connected in series with the one or more MTJ temperature compensation resistors so as to stabilize the sensor bridge voltage output, and the sensor bridge determines the test electric current by measuring the magnetic field produced by the test electric current in the electric conductor which is provided as an output voltage.

0009Preferably, the MTJ sensor bridge and the one or more MTJ temperature compensation resistors respectively comprise one or more MTJ elements connected in series, wherein said MTJ elements have the same temperature coefficient, R_H, and R_L.

0010Preferably, the sensor bridge is a half bridge.

0011Preferably, the sensor bridge is a full bridge.

0012The present invention uses the above structure to compensate for temperature drift of the current sensor while providing the benefits of high sensitivity, wide linear range, low power consumption, small size, and excellent thermal stability.

BRIEF DESCRIPTION OF THE FIGURES

0013<figref idref="DRAWINGS">FIG. 1</figref> is a diagram of a magnetic; tunnel junction element.

0014<figref idref="DRAWINGS">FIG. 2</figref> shows a plot of the transfer curve of a MTJ linear magnetic field sensing element with magnetic field applied along the hard axis direction.

0015<figref idref="DRAWINGS">FIG. 3</figref> shows how several MTJ elements can be connected in series to form a magnetoresistor.

0016<figref idref="DRAWINGS">FIG. 4</figref> shows the magnetoresistive transfer curve of an MTJ element at different temperatures.

0017<figref idref="DRAWINGS">FIG. 5</figref> is a schematic diagram of a push-pull half-bridge MTJ sensor.

0018<figref idref="DRAWINGS">FIG. 6</figref> shows the typical output of a push-pull half-bridge MTJ sensor.

0019<figref idref="DRAWINGS">FIG. 7</figref> shows the simulated output of a push-pull half-bridge MTJ sensor at different temperatures.

0020<figref idref="DRAWINGS">FIG. 8</figref> is a schematic diagram of a push-pull full-bridge MTJ sensor.

0021<figref idref="DRAWINGS">FIG. 9</figref> shows the typical output of a push-pull full-bridge MTJ sensor.

0022<figref idref="DRAWINGS">FIG. 10</figref> shows the simulated output of a push-pull full-bridge MTJ sensor at different temperatures.

0023<figref idref="DRAWINGS">FIG. 11</figref> illustrates the concept of temperature compensation resistors applied to a half-bridge TMR current sensor.

0024<figref idref="DRAWINGS">FIG. 12</figref> shows another arrangement for implementing a temperature compensation resistor within a half-bridge MTJ current sensor.

0025<figref idref="DRAWINGS">FIG. 13</figref> illustrates a concept for using a temperature compensation resistor within a full-bridge MTJ current sensor.

0026<figref idref="DRAWINGS">FIG. 14</figref> shows a plot of two different curves for the temperature dependence of the output of temperature compensated and uncompensated MTJ current sensors.

SPECIFIC EMBODIMENTS

0027<figref idref="DRAWINGS">FIG. 1</figref> schematically illustrates the multilayer film and operational concept of an MTJ element. An MTJ element <b>1</b> generally includes a synthetic antiferromagnet (SAF) <b>3</b>, and another ferromagnetic layer or <b>5</b>, between on opposing sides of a tunnel barrier layer <b>4</b>. In this structure, the upper layer is the free layer whose magnetization direction is changed in response to the external magnetic field. The lower ferromagnetic layer (SAF layer) <b>3</b> is a fixed magnetic layer, and its magnetization direction is rigidly pinned in one direction, so that under normal conditions, it will not change. The pinning layer <b>2</b> is usually an antiferromagnetic material that can be deposited above or below a pinned layer SAF. The MTJ structure is usually deposited on top of a conductive seed layer <b>10</b> for use as the bottom electrode, while another conductive capping layer <b>6</b> is deposited on top of the MTJ structure. The resistance <b>11</b> of the MTJ element is measured between the seed layer <b>10</b> and the capping layer <b>6</b>, and it represents the relative orientation of the free layer <b>5</b> and the pinned layer <b>3</b> magnetization directions. When the upper ferromagnetic free layer <b>5</b> and the lower the lower ferromagnetic pinning layer magnetization directions are parallel to each other, the resistance <b>11</b> of the MTJ element will be representative of the low resistance state. When the upper ferromagnetic free layer <b>5</b> lower ferromagnetic pinned layer <b>3</b> magnetization directions are antiparallel to each other, the overall resistance of the element <b>11</b> will be representative of the high resistance state. By known techniques, MTJ element <b>1</b>, can be fabricated such that the change in resistance between the low and high resistance states is linearly dependent on magnetic field.

0028<figref idref="DRAWINGS">FIG. 2</figref> shows the linear response of a MTJ magnetoresistive element <b>1</b>. When saturated, the output curve will be in either the low resistance <b>12</b> or high resistance <b>13</b> states. When the magnetization direction of the pinned layer <b>8</b> is parallel to the magnetization direction of the free layer <b>7</b>, the MTJ element <b>1</b> is a low-resistance state <b>12</b>; when the magnetization direction of the pinned layer <b>8</b> is anti-parallel to the magnetization direction of the free layer <b>7</b>, the MTJ element <b>1</b> is a high-resistance state <b>13</b>. Before saturation is reached, the output curve is linearly dependent on the applied magnetic field H. The output curve is usually not symmetric about H=0. Typically the saturation fields <b>21</b> and <b>22</b> are offset with respect to H=0, such that the low-resistance state <b>12</b> saturation point is closer to H=0. The offset value is often referred to as “orange peel coupling” or “Neel coupling,” the typical value of this offset is usually between 1 and 25 Oe, and the roughness of the MTJ element <b>1</b>, the influence the effect, making it dependent on materials and manufacturing processes. Between the saturated regions, the output curve equation can be approximated as:

0000<maths id="MATH-US-00001" num="00001"><math overflow="scroll"><mtable><mtr><mtd><mrow><mrow><mi>R</mi><mo></mo><mrow><mo>(</mo><mi>H</mi><mo>)</mo></mrow></mrow><mo>=</mo><mrow><mrow><mfrac><mrow><msub><mi>R</mi><mi>H</mi></msub><mo>-</mo><msub><mi>R</mi><mi>L</mi></msub></mrow><mrow><mn>2</mn><mo></mo><mstyle><mspace width="0.3em" height="0.3ex" /></mstyle><mo></mo><msub><mi>H</mi><mi>s</mi></msub></mrow></mfrac><mo></mo><mrow><mo>(</mo><mrow><mi>H</mi><mo>-</mo><msub><mi>H</mi><mi>o</mi></msub></mrow><mo>)</mo></mrow></mrow><mo>+</mo><mfrac><mrow><msub><mi>R</mi><mi>H</mi></msub><mo>+</mo><msub><mi>R</mi><mi>L</mi></msub></mrow><mn>2</mn></mfrac></mrow></mrow></mtd><mtd><mrow><mo>(</mo><mn>1</mn><mo>)</mo></mrow></mtd></mtr></mtable></math></maths>

0029<figref idref="DRAWINGS">FIG. 3</figref> shows several MTJ elements <b>1</b> is connected in series to form a MTJ magnetoresistor. This series connected MTJ magnetoresistance structure helps to reduce noise and improve stability of the sensor. In the MTJ magnetoresistor, the bias voltage of each of the MTJ element <b>1</b>, decreases as the number of magnetic tunnel junctions increases. This reduces the current required to produce a large output voltage, thus reducing shot noise, along with the increasing ESD immunity. In addition, as the number of MTJ elements <b>1</b> is increased, the noise of the MTJ is correspondingly reduced, because the uncorrelated random variation of each MTJ element <b>1</b> averages out.

0030<figref idref="DRAWINGS">FIG. 4</figref> shows MTJ output curves acquired at various different temperatures, it shows that as the temperature rises, the MTJ element low resistance does not change significantly, but the high resistance value changes significantly, such that overall, MTJ magnetoresistance decreases with increasing temperature.

0031<figref idref="DRAWINGS">FIG. 5</figref> is a schematic diagram of a push-pull half-bridge MTJ sensor. As shown, the two MTJ resistors R<b>1</b>, R<b>2</b> have anti-parallel pinned layer magnetization directions, and the magnetization direction of the free layer <b>7</b> changes in response to the external magnetic field, a steady voltage, Vbias, is applied between the Vcc and GND terminal pads, and the Vout pad is the bridge output. When applying an external magnetic field to the push-pull half-bridge sensor, the magnetic field component along the sensitive direction <b>9</b> increases the resistance of resistor R<b>1</b> while decreasing the resistance of resistor R<b>2</b>, when applied in the opposite direction the magnetic field decreases the resistance of resistor R<b>1</b> while increasing the resistance of resistor R<b>2</b>. The typical push-pull half-bridge sensor output curve is shown in <figref idref="DRAWINGS">FIG. 6</figref>.

0032<figref idref="DRAWINGS">FIG. 7</figref> shows the simulated output of a push-pull half-bridge MTJ sensor at different temperatures. As shown, the output voltage of the half-bridge circuit is reduced as the temperature rises.

0033<figref idref="DRAWINGS">FIG. 8</figref> schematically illustrates a push-pull full-bridge MTJ sensor. As shown n the figure four MTJ resistors R<b>11</b>, R<b>12</b>, R<b>21</b>, R<b>22</b> are connected in a full bridge configuration, a steady bias voltage is applied between the Vbias and GND terminal pads, the pads V+ and V− are the output terminals, the magnetization direction of the free layer <b>7</b> of the four resistors changes in response to an externally applied magnetic field, resistors R<b>11</b> and R<b>22</b> (R<b>12</b> and R<b>21</b>) have a pinned layer magnetization <b>8</b> pointing in the same direction, the adjacent resistors R<b>11</b> and R<b>12</b> (R<b>11</b> and R<b>21</b>, etc.) have their pinned layer magnetizations <b>8</b> pointing in opposite directions, the sensitive direction <b>9</b> of the bridge circuit is along the axis parallel and antiparallel to the pinned layer direction <b>8</b>.

0034When applying a magnetic field to the full-bridge push-pull, the component of the magnetic field along the sensitive direction <b>9</b> increases the resistance of resistors R<b>11</b> and R<b>22</b> while decreasing the resistance of the other two resistors R<b>12</b> and R<b>21</b>, when the polarity of the applied magnetic field is changed the resistance of R<b>11</b> and R<b>22</b> is decreased while the resistance of R<b>12</b> and R<b>21</b> increases accordingly, using two pairs of resistors with the opposite response—one pair of resistors has increasing response while another has decreasing response—can increase the sensitivity of the bridge circuit output. This arrangement is called a “push-pull” full-bridge circuit. The output voltage of each half is defined:

0000<maths id="MATH-US-00002" num="00002"><math overflow="scroll"><mtable><mtr><mtd><mrow><mrow><mi>V</mi><mo></mo><mstyle><mspace width="0.3em" height="0.3ex" /></mstyle><mo></mo><mn>1</mn><mo></mo><mrow><mo>(</mo><mi>H</mi><mo>)</mo></mrow></mrow><mo>=</mo><mrow><mfrac><mrow><mi>R</mi><mo></mo><mstyle><mspace width="0.3em" height="0.3ex" /></mstyle><mo></mo><mn>21</mn><mo></mo><mrow><mo>(</mo><mi>H</mi><mo>)</mo></mrow></mrow><mrow><mrow><mi>R</mi><mo></mo><mstyle><mspace width="0.3em" height="0.3ex" /></mstyle><mo></mo><mn>11</mn><mo></mo><mrow><mo>(</mo><mi>H</mi><mo>)</mo></mrow></mrow><mo>+</mo><mrow><mi>R</mi><mo></mo><mstyle><mspace width="0.3em" height="0.3ex" /></mstyle><mo></mo><mn>21</mn><mo></mo><mrow><mo>(</mo><mi>H</mi><mo>)</mo></mrow></mrow></mrow></mfrac><mo></mo><msub><mi>V</mi><mi>bias</mi></msub></mrow></mrow></mtd><mtd><mrow><mo>(</mo><mn>2</mn><mo>)</mo></mrow></mtd></mtr><mtr><mtd><mrow><mrow><mi>V</mi><mo></mo><mstyle><mspace width="0.3em" height="0.3ex" /></mstyle><mo></mo><mn>2</mn><mo></mo><mrow><mo>(</mo><mi>H</mi><mo>)</mo></mrow></mrow><mo>=</mo><mrow><mfrac><mrow><mi>R</mi><mo></mo><mstyle><mspace width="0.3em" height="0.3ex" /></mstyle><mo></mo><mn>22</mn><mo></mo><mrow><mo>(</mo><mi>H</mi><mo>)</mo></mrow></mrow><mrow><mrow><mi>R</mi><mo></mo><mstyle><mspace width="0.3em" height="0.3ex" /></mstyle><mo></mo><mn>12</mn><mo></mo><mrow><mo>(</mo><mi>H</mi><mo>)</mo></mrow></mrow><mo>+</mo><mrow><mi>R</mi><mo></mo><mstyle><mspace width="0.3em" height="0.3ex" /></mstyle><mo></mo><mn>22</mn><mo></mo><mrow><mo>(</mo><mi>H</mi><mo>)</mo></mrow></mrow></mrow></mfrac><mo></mo><msub><mi>V</mi><mi>bias</mi></msub></mrow></mrow></mtd><mtd><mrow><mo>(</mo><mn>3</mn><mo>)</mo></mrow></mtd></mtr></mtable></math></maths>

0035The combined bridge output is defined as:

0000<br /><i>V</i>(<i>H</i>)=<i>V</i>1(<i>H</i>)−<i>V</i>2(<i>H</i>)  (4)

0036In the bridge circuit, the MTJ resistance values may be defined as:

0000<maths id="MATH-US-00003" num="00003"><math overflow="scroll"><mtable><mtr><mtd><mrow><mrow><msub><mi>R</mi><mn>11</mn></msub><mo></mo><mrow><mo>(</mo><mi>H</mi><mo>)</mo></mrow></mrow><mo>=</mo><mrow><mrow><mi>R</mi><mo></mo><mstyle><mspace width="0.3em" height="0.3ex" /></mstyle><mo></mo><mn>22</mn><mo></mo><mrow><mo>(</mo><mi>H</mi><mo>)</mo></mrow></mrow><mo>=</mo><mrow><mrow><mfrac><mrow><msub><mi>R</mi><mi>H</mi></msub><mo>-</mo><msub><mi>R</mi><mi>L</mi></msub></mrow><mrow><mn>2</mn><mo></mo><mstyle><mspace width="0.3em" height="0.3ex" /></mstyle><mo></mo><msub><mi>H</mi><mi>s</mi></msub></mrow></mfrac><mo></mo><mrow><mo>(</mo><mrow><mrow><mo>-</mo><mi>H</mi></mrow><mo>-</mo><msub><mi>H</mi><mi>o</mi></msub></mrow><mo>)</mo></mrow></mrow><mo>+</mo><mfrac><mrow><msub><mi>R</mi><mi>H</mi></msub><mo>+</mo><msub><mi>R</mi><mi>L</mi></msub></mrow><mn>2</mn></mfrac></mrow></mrow></mrow></mtd><mtd><mrow><mo>(</mo><mn>5</mn><mo>)</mo></mrow></mtd></mtr><mtr><mtd><mrow><mrow><mi>R</mi><mo></mo><mstyle><mspace width="0.3em" height="0.3ex" /></mstyle><mo></mo><mn>12</mn><mo></mo><mrow><mo>(</mo><mi>H</mi><mo>)</mo></mrow></mrow><mo>=</mo><mrow><mrow><mi>R</mi><mo></mo><mstyle><mspace width="0.3em" height="0.3ex" /></mstyle><mo></mo><mn>21</mn><mo></mo><mrow><mo>(</mo><mi>H</mi><mo>)</mo></mrow></mrow><mo>=</mo><mrow><mrow><mfrac><mrow><msub><mi>R</mi><mi>H</mi></msub><mo>-</mo><msub><mi>R</mi><mi>L</mi></msub></mrow><mrow><mn>2</mn><mo></mo><mstyle><mspace width="0.3em" height="0.3ex" /></mstyle><mo></mo><msub><mi>H</mi><mi>s</mi></msub></mrow></mfrac><mo></mo><mrow><mo>(</mo><mrow><mi>H</mi><mo>-</mo><msub><mi>H</mi><mi>o</mi></msub></mrow><mo>)</mo></mrow></mrow><mo>+</mo><mfrac><mrow><msub><mi>R</mi><mi>H</mi></msub><mo>+</mo><msub><mi>R</mi><mi>L</mi></msub></mrow><mn>2</mn></mfrac></mrow></mrow></mrow></mtd><mtd><mrow><mo>(</mo><mn>6</mn><mo>)</mo></mrow></mtd></mtr></mtable></math></maths>

0037Such that:

0000<maths id="MATH-US-00004" num="00004"><math overflow="scroll"><mtable><mtr><mtd><mrow><mrow><mi>V</mi><mo></mo><mrow><mo>(</mo><mi>H</mi><mo>)</mo></mrow></mrow><mo>=</mo><mrow><mrow><mo>(</mo><mfrac><mrow><msub><mi>R</mi><mi>H</mi></msub><mo>-</mo><msub><mi>R</mi><mi>L</mi></msub></mrow><mrow><msub><mi>R</mi><mi>H</mi></msub><mo>+</mo><msub><mi>R</mi><mi>L</mi></msub></mrow></mfrac><mo>)</mo></mrow><mo></mo><mrow><mo>(</mo><mfrac><mi>H</mi><msub><mi>H</mi><mi>s</mi></msub></mfrac><mo>)</mo></mrow><mo></mo><msub><mi>V</mi><mi>bias</mi></msub></mrow></mrow></mtd><mtd><mrow><mo>(</mo><mn>7</mn><mo>)</mo></mrow></mtd></mtr></mtable></math></maths>

0038This is the output of the push-pull full-bridge, and a simulated output curve is shown in <figref idref="DRAWINGS">FIG. 9</figref>.

0039<figref idref="DRAWINGS">FIG. 10</figref> shows a simulation of the MTJ push-pull full-bridge circuit output at different temperatures. It clearly indicates that as the temperature increases, the output voltage of the bridge circuit is reduced.

0040It is not difficult to see, the MTJ bridge output drifts with temperature because the MTJ elements resistance changes leading to a change in output voltage. To compensate the temperature drift, a temperature compensating resistor can be used. <figref idref="DRAWINGS">FIG. 11</figref> schematically illustrates a temperature compensation resistor <b>16</b> on a chip connected with a MTJ half-bridge circuit. As shown, the half-bridge circuit <b>14</b> is connected in series with the temperature compensation resistor <b>16</b>. Temperature compensation resistor <b>16</b> is biased by the magnetic field of permanent magnet <b>17</b> setting the magnetization direction of the free layer <b>7</b>, so that it is antiparallel to the magnetization direction of the pinned layer <b>8</b> and therefore in the high resistance state <b>13</b>, and the external field does not change the resistance <b>11</b> within the measurement temperature range. The chip is provided with a conductor <b>20</b> in which the measured current <b>19</b> flows between the I_in+ and I_in− pads, the sensor bridge circuit <b>14</b> detects the magnetic field <b>21</b> produced by the current <b>19</b>. Ideally temperature compensation resistor <b>16</b> has the same temperature coefficient as half-bridge arm resistors R<b>1</b>, R<b>2</b> such that the effect of temperature cancels out, when the magnetoresistance changes with temperature, the temperature compensation resistor and the bridge circuit move higher and lower respectively at the same time, such that the output of the bridge is stable against temperature, providing the temperature compensation.

0041<figref idref="DRAWINGS">FIG. 12</figref> shows another single-chip MTJ half-bridge plus temperature compensation resistor <b>16</b> arrangement. As shown in the figure, the bridge resistors R<b>1</b> and R<b>2</b>, have pinned layer magnetization <b>8</b> pointing in the same direction, while the magnetization direction of the free layer <b>7</b> varies in response to the external field, half bridge circuit <b>14</b> and the temperature compensation resistor <b>16</b> are connected in series, temperature compensation resistor <b>16</b> is biased with a strong permanent magnet <b>17</b> to set the magnetization direction of the free layer <b>7</b>, antiparallel to the magnetization direction of the pinned layer <b>8</b> such that the resistance is in the high resistance state <b>13</b>, and the external field does not change the resistance <b>11</b> within the measurement temperature range. The chip is provided with a U-shaped conductor <b>20</b> above or below resistors R<b>1</b> and R<b>2</b> in which the measured current <b>19</b> flows between the I_in+ and I_in− pads, the sensor bridge circuit <b>14</b> detects the magnetic field <b>21</b> produced by the current <b>19</b>.

0042<figref idref="DRAWINGS">FIG. 13</figref> shows another single-chip MTJ full-bridge plus temperature compensation resistor <b>16</b> arrangement. As shown, the four MTJ resistors R<b>11</b>, R<b>12</b>, R<b>21</b>, and R<b>22</b> are interconnected as full-bridge and connected in series with temperature compensation resistor <b>16</b>, the four MTJ resistors have magnetization direction <b>8</b> of the pinned layer rigidly aligned in the same direction and the free layers <b>7</b> change in response to the external magnetic field, compensation resistor <b>16</b> is biased with a strong permanent magnet <b>17</b> to set the magnetization direction of the free layer <b>7</b>, antiparallel to the magnetization direction of the pinned layer <b>8</b> such that the resistance is in the high resistance state <b>13</b>, and the external field does not change the resistance <b>11</b> within the measurement temperature range. The chip is provided with a U-shaped conductor <b>20</b> in which the measured current <b>19</b> flows between the I_in+ and I_in− pads, the sensor bridge circuit <b>14</b> detects the magnetic field <b>21</b> produced by the current <b>19</b>.

0043The output of the bridge circuit Vout(T) and the actual output voltage Vout of the bridge are linearly related by the following function:

0000<br /><i>V</i>_OUT(<i>T</i>)<b>32</b><i>V</i>_OUT(1<i>+k</i>_T<i>ΔT</i>)  (8)

0044The “kT” is the temperature coefficient of the bridge, where in Hall current sensors kT is commonly expressed in terms of PPM/C. <figref idref="DRAWINGS">FIG. 14</figref> shows testing results for the temperature coefficient of a MTJ push-pull full-bridge with and without a temperature compensating resistor, we can clearly see, before removing the temperature compensating resistor, the temperature coefficient is much smaller, therefore the effect of the temperature compensating resistor is obvious.

0045While specific implementations of the present invention are illustrated, obviously, various modifications of the invention may be made without departing from the spirit or scope of the invention. Those skilled in the art of the present invention understand that the spirit and intent of the present invention is not changed by such modifications.