Hall cell sensor with temperature compensation

Abstract

A Hall cell magnetic field sensor (2) comprising a Hall cell (4), a bias current source (8) connected to the Hall cell configured to generate a bias current (IBIAS) to drive the Hall cell, and an analog temperature compensation current generator (10) connected to the Hall cell, characterized in that the temperature compensation current generator is configured to generate a quadratically varying compensation current (ICOMP) which is added to the bias current (IBIAS) for compensation of a temperature dependent measurement output error relative to a measurement output at a selected reference temperature (T0).

Description

[0001] HALL CELL SENSOR WITH TEMPERATURE COMPENSATION

[0002] The present invention relates to a Hall cell magnetic field sensor with a circuit for compensation of the effect of temperature variations on the output of the Hall cells. The sensor may be implemented in various magnetic field sensing applications, including in particular in a current transducer.

[0003] Analog temperature compensation methods used in the Hall sensor applications are typically based on piecewise-linear approximations of the sensor circuit's nonlinear temperature variation profile. A typical solution involves a pair of breakpoints splitting the temperature axis into three ranges, each of which having a different first-order temperature coefficient. Because compensation is linear in every range, it cannot truly fit the nonlinear behaviour of the sensor circuit's gain, which gives rise to errors.

[0004] Other conventional methods, for instance as described in KR101918108B1 , use analog-to- digital converters to digitize temperature in order to generate from the digital domain a nonlinear temperature profile, usually having a polynomial law, of arbitrary shape. Such methods however are costly because of the extra hardware needed for digitization and D-to-A conversion.

[0005] In view of the foregoing, it is an object of this invention to provide a Hall cell sensor with a circuit for compensation of the effect of temperature variations, that is accurate yet economical to produce and implement.

[0006] It is advantageous to provide a Hall cell sensor with a circuit for compensation of the effect of temperature variations, that is easy to integrate in various applications.

[0007] It is advantageous to provide a current transducer with a Hall cell magnetic field sensor that is accurate over a large range of operating temperatures and that is economical to produce and implement.

[0008] Objects of this invention have been achieved by providing a Hall cell sensor according to claim 1. Objects of this invention have been achieved by providing a current transducer with a Hall cell according to claim 10. Dependent claims set forth various advantageous features of embodiments of the invention. Disclosed herein is a Hall cell magnetic field sensor comprising a Hall cell, a bias current source connected to the Hall cell configured to generate a bias current IBIAS to drive the Hall cell, and an analog temperature compensation current generator connected to the Hall cell. The analog temperature compensation current generator is configured to generate, in an analog domain, an analog quadratically varying compensation current ICOMP which is added to the bias current IBIAS for compensation of a temperature dependent measurement output error relative to a measurement output at a selected reference temperature To.

[0009] In an advantageous embodiment, the analog temperature compensation current generator comprises a trim circuit configured to adjust a gain and / or a temperature offset of the compensation current ICOMP.

[0010] In an advantageous embodiment, the trim circuit comprises a current mirror circuit with an input branch and a plurality of output branches each selectably connected to the input branch via a digitally controllable switch.

[0011] In an advantageous embodiment, the analog compensation current generator comprises at least one generator channel, each generator channel comprising a first order current generator circuit configured to generate a first order linearly varying current h, and a second order current generator circuit connected to an output of the first order current generator circuit, the second configured to generate a quadratically varying second order current I2 from the first order linearly varying current h.

[0012] In an advantageous embodiment, the 2nd order current generator comprises or consists of a current squarer circuit.

[0013] In an advantageous embodiment, the current squarer circuit is based on a translinear loop circuit comprising bipolar transistors (Qo, Q , Q2, Q3)

[0014] In an advantageous embodiment, each generator channel comprises a trim circuit.

[0015] In an advantageous embodiment, the first and second order current generators are connected together by a connection circuit comprising or consisting of a current mirror circuit.

[0016] In an advantageous embodiment, the analog compensation current generator comprises a pair of said generator channels, one for generation of a portion of the temperature dependent compensation current for temperatures below the selected reference temperature To and the other for generation of a portion of the temperature dependent compensation current for temperatures above the selected reference temperature To. Also disclosed herein is a current transducer comprising a Hall cell magnetic field sensor according to any preceding embodiment.

[0017] The proposed invention offers a means to compensate for the nonlinear temperature variations of an electric current in a Hall cell readout circuit, through addition of a compensation current that varies quadratically with temperature. The invention exploits the fact that the Hall sensor gain is proportional to its bias current. The temperature compensation current is adjustable in amplitude and in sign, and so is its location on the temperature axis. Several such circuits may be combined to form piecewise-quadratic temperature-dependent quantities.

[0018] Further objects and advantageous features of the invention will be apparent from the claims, from the detailed description, and annexed drawings, in which:

[0019] Figure 1a is a schematic circuit diagram of a Hall sensor according to the prior art;

[0020] Figure 1 b is a schematic plot of a non-linear current output behaviour of a Hall cell as a function of temperature;

[0021] Figure 2 is a schematic circuit diagram of a Hall sensor according to an embodiment of the invention;

[0022] Fig. 3a is a schematic block diagram of a compensation current generator of a Hall sensor according to an embodiment of the invention;

[0023] Fig. 3b are schematic plots of 1st order, 2nd order and output currents of the compensation current generator according to a embodiment of the invention, as a function of temperature;

[0024] Fig. 4a is a schematic circuit diagram of a 1st order current generator of a Hall sensor according to an embodiment of the invention;

[0025] Fig. 4b is a schematic circuit diagram of a 2nd order current generator of a Hall sensor according to an embodiment of the invention;

[0026] Fig. 4c is a schematic circuit diagram of a circuit for interconnecting the 1st and second order current generators of figures 4a and 4b of a Hall sensor according to an embodiment of the invention; Fig. 5 is a schematic circuit diagram of an adjustable gain circuit (magnitude trim circuit) for a compensation current generator of a Hall sensor according to an embodiment of the invention;

[0027] Figure 6a is a plot of compensation current as a function of temperature, showing plots for magnitude trim settings (gain), of a Hall sensor according to an embodiment of the invention;

[0028] Figure 6b is a plot of compensation current as a function of temperature, showing plots for different reference temperature To settings, namely different centre temperature trim settings, of a Hall sensor according to an embodiment of the invention.

[0029] Referring to the figures 1a and 1 b, a conventional Hall sensor has a Hall cell 4 connected to a current source generating a bias current IBIAS to drive the Hall cell, the Hall cell having a Wheatstone bridge circuit layout with voltage outputs VOUT-, VOUT+ for the Hall cell measurement readout. Temperature variations on the Hall cell 4 generate a dependence on temperature in the bridge output that is not linear and leads to an error in the measurement readout. This is illustrated in figure 1b, that shows the variation with temperature of a bridge output voltage Vout for some fixed applied magnetic field. Conventional analog temperature compensation methods based on piecewise-linear approximations seek to compensate for the temperature based measurement error, however they are usually not very accurate over the typical operating range of the Hall sensors. In effect, in Hall cells it is observed that the variation with temperature of a bridge output voltage Vout has a substantially parabolic or quadratic form, as illustrated in the plot of figure 1 b, whereby the piece-wise linear approximations do not fit well the nonlinear behaviour of the sensor circuit's gain.

[0030] Referring to figure 2, a Hall sensor 2 according to embodiments of the present invention, comprises a Hall cell 4 connected to a current source generating a bias current IBIAS to drive the Hall cell, similar to a conventional Hall sensor (as described above), and further an analog temperature compensation current generator 10 connected to the Hall cell, the analog temperature compensation current generator configured to generate a quadratically varying compensation current ICOMP which is added to the bias current IBIAS.

[0031] The functioning principle of a first embodiment of the analog temperature compensation current generator 10 is illustrated in figures 3a to 3d. In the embodiment of figure 3a, 3b, a first order linearly varying current h is generated by a first order current generator circuit 12, this first order current h input in second order current generator circuit 14 that outputs a quadratically varying current I2. The quadratically varying second order current I2 may be adjusted by trim circuit 18 that applies a gain G and a DC offset, thus adjusting the output current in amplitude and setting the zero compensation current value to a selected reference temperature To value. The analog temperature compensation current generator 10 thus generates an adjustable quadratically varying output compensation current "ICOMP".

[0032] In the embodiment of figures 3c, 3d, the analog temperature compensation current generator 10 generates a pair of second order currents, labeled "I2 COLD" and "I2 HOT", following each a quadratic temperature law in a dedicated temperature range, then summed together after some weighting to form an adjustable quadratically (parabolically) varying output compensation current "ICOMP".

[0033] Both second order currents "I2 COLD" and "I2 HOT" are generated out of a pair of linearly varying first order currents "h COLD" and "h HOT", of first-order temperature coefficients -01 and Pi, which are then independently squared by analog means.

[0034] In variants, the above principle may be extended to three or a greater number of currents channels, each with a first order and second order generator and trim control, the outputs of which are then summed at the output of the current generator. This would allow to combine three or more quadratically varying and independently trimmed currents to form a more complex compensation current profile for fitting with a real temperature dependent measurement error behaviour of a Hall or other type of magnetic field sensor.

[0035] There are various perse well known ways to generate first-order varying currents with analog circuitry.

[0036] Referring to figure 4a, an example of a 1storder current generator 12 of a compensation current generator according to an embodiment of the invention is shown. To generate a current Ioutthat is linear with temperature above some threshold temperature value To(with Iout= 0 for T < To), one sets the first current equal to a reference current = IREFand second current equal to a variable current I2= IN, where IREFrepresents a current that is constant over T, and INis a current that decreases linearly with T.

[0037] At temperatures less than the threshold temperature T < To, with a proper choice of resistor and current values, I2= INis larger than = IREF, so node B is higher than node A, causing C to stay near ground and keeping transistor Mooff. One gets Iout= 0. At T = Toand above, I2has decreased and is no longer large enough to pull node B above node A, so that a current has to flow through Moin order to keep the amplifier balanced with identical levels at B and A.

[0038] Applying Kirchhoff's laws results in the following formula for this current : specified above.

[0039] Given expressions for i! and I2, threshold Tomay be calculated from Iout= 0. To generate a current Ioutthat is linear with temperature below some threshold To, one sets = INand I2= IREF, and similar reasoning applies.

[0040] Referring to figure 4b, an embodiment of a 2nd order current generator 14 of a current generator according to an embodiment of the invention is shown. The 2nd order current generator circuit may be connected to the first order current generator by a current mirror circuit 16, for instance as shown in figure 4c.

[0041] In the embodiment of figure 4b, the 2nd order current generator comprises or consists of a current squarer based on a translinear loop.

[0042] In an embodiment, the translinear loop comprises bipolar transistors Qo, Q , Q2, Q3. In another embodiment, the translinear loop may also be based on MOS transistors operating in weak inversion. Translinear circuits exploit the exponential characteristic of bipolar transistors or MOS transistors operating in weak inversion. For example, consider the bipolar transistor embodiment:

[0043] Ic= JsoeVbe / VTwhere Icis the collector current, Isois a (constant) saturation current, kT

[0044] Vbeis the base-emitter voltage and VT= — is the so-called thermal voltage.

[0045] Owing to this exponential law, adding, respectively subtracting base-emitter voltages amounts to multiplying, respectively dividing collector currents.

[0046] Accordingly, we can write : c=B ~ ^bel + ^beOand Vc= VB— Vbe3+ Vbe2, so that Vbel- Vbe0= Vbe3- Vbe2.

[0047] By the translinear principle

[0048] If we have equal input curr

[0049] If in addition Iois chosen to be constant, then the output current Ioutis simply proportional to the square of the input current Iin. Choosing some reference, temperature-independent current for Io, the above circuits thus output a current that is proportional to the square of its input. Advantageously, with the above embodiments, it is possible to generate the desired currents with simple analog means.

[0050] The current IOUT output by each second order current generator may be trimmed by a trim circuit 18 of the analog temperature compensation current generator 10 to output the summed compensation current lComPthat is added to the bias current IBIAS. An example of a trim circuit 18 that may be included in the analog temperature compensation current generator 10 for each current channel is illustrated in figure 5. In this embodiment, the trim circuit comprises a current mirror circuit with an input branch IN and a plurality N of output branches each connected selectively to the output OUT of the trim circuit via a switch controllable by a digital signal Bo to BM. The digitally controlled switches are selectively closed or opened depending on the selected gain G of the second order current generator. An example of output currents with different amplification is illustrated in figure 6a. The reference temperature To may be independently trimmed by using a similar (programmable mirror) arrangement to set either h or I2. An example of output currents with different current DC offsets for setting the reference temperature at which the compensation current is nul, is illustrated in figure 6b.

[0051] In embodiments having a current generator with a plurality of channels, for instance in the embodiment of figures 3c, 3d, each channel is connected to a trim circuit, the outputs of the trim circuits then being summed by a summing circuit 20 to form the output compensation current I comp.

[0052] The Hall sensor according to embodiments of the invention may comprise a plurality of Hall cells. The plurality of Hall cells may be connected to the same current generator, or may each be connected to a respective different current generator.

[0053] In an advantageous application, the Hall sensor according to embodiments of the invention may be incorporated in a current transducer. List of references used

[0054] Hall cell magnetic field sensor 2

[0055] Hall cell 4

[0056] Hall elements

[0057] Wheatstone bridge circuit arrangement

[0058] Measurement readout Vout-, Vout+

[0059] Bias current source 8

[0060] Analog temperature compensation current generator 10

[0061] First order current generator 12

[0062] Second order current generator 14

[0063] Connection circuit 16

[0064] Current mirror circuit

[0065] Trim circuit 18

[0066] Programmable current mirror circuit

[0067] Switches S for digital control inputs BO....Bn

[0068] Summing circuit 20

Claims

Claims1. A Hall cell magnetic field sensor (2) comprising a Hall cell (4), a bias current source (8) connected to the Hall cell configured to generate a bias current (IBIAS) to drive the Hall cell, and an analog temperature compensation current generator (10) connected to the Hall cell, characterized in that the analog temperature compensation current generator is configured to generate in an analog domain an analog quadratically varying compensation current (ICOMP) which is added to the bias current (IBIAS) for compensation of a temperature dependent measurement output error relative to a measurement output at a selected reference temperature (To).

2. The sensor of any preceding claim wherein the analog temperature compensation current generator comprises a trim circuit (18) configured to adjust a gain and / or a temperature offset of the compensation current (ICOMP).

3. The sensor of any preceding claim wherein the trim circuit (18) comprises a current mirror circuit with an input branch and a plurality of output branches each selectably connected to the input branch via a digitally controllable switch.

4. The sensor of any preceding claim wherein the analog temperature compensation current generator comprises at least one generator channel, each generator channel comprising a first order current generator circuit (12) configured to generate a first order linearly varying current (h), and a second order current generator circuit (14) connected to an output of the first order current generator circuit (12), the second configured to generate a quadratically varying second order current (I2) from the first order linearly varying current (h).

5. The sensor of any preceding claim wherein the 2nd order current generator comprises or consists of a current squarer circuit.

6. The sensor of the preceding claim wherein the current squarer circuit is based on a translinear loop circuit comprising bipolar transistors (Qo, Q , Q2, Q3)7. The sensor of any of the three directly preceding claims wherein each generator channel comprises a trim circuit according to claim 2 or 3.

8. The sensor of any of the four directly preceding claims wherein the first and second order current generators are connected together by a connection circuit comprising or consisting of a current mirror circuit (16).

9. The sensor of any of the five directly preceding claims wherein the analog temperature compensation current generator comprises a pair of said generator channels, one for generation of a portion of the temperature dependent compensation current for temperatures below the selected reference temperature (To) and the other for generation of a portion of the temperature dependent compensation current for temperatures above the selected reference temperature (To).

10. A current transducer comprising a Hall cell magnetic field sensor according to any preceding claim.

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