Device for correcting the stress shift of a Wheatstone bridge

Abstract

Electronic device comprising a Wheatstone bridge circuit (PW) and a correction circuit (CC) coupled to the Wheatstone bridge circuit (PW), configured to correct a shift in the output voltage of the Wheatstone bridge (PW), characterized in that the correction circuit (CC) has an input interface (7) for receiving a first voltage, a supply module (1) configured to supply the Wheatstone bridge circuit (PW) with a second voltage derived from the first voltage and with a first current (11) regulated based on the current value of the resistors of the Wheatstone bridge circuit (PW), and to generate a second current (12) proportional to the first current (I1), and a digital-to-analog current converter (CNA) configured to provide a correction current at the outputs (2,3) to supply the Wheatstone bridge circuit (PW) from a digital correction signal (SNC) and the second current (12).

Description

[0001] Embodiments of the invention relate to Wheatstone bridge circuits and, more precisely, the correction of their voltage shift ("offset").

[0002] The invention relates in particular to Wheatstone bridge sensors, which are used, for example, to measure low-speed physical phenomena such as pressure, temperature, gas detection, etc.

[0003] The following are known in the state of the art - DE 196 53 592 A1 - DE 601 09 660 T2 - US 2013 / 0 319 136 A1, which each show bridge circuits for acquiring sensor signals, whereby unwanted error signals are corrected or compensated..

[0004] Wheatstone bridge sensors generally feature a Wheatstone bridge circuit and a high-impedance preamplifier to amplify the output voltage of the Wheatstone bridge, which is proportional to the supply voltage of the Wheatstone bridge.

[0005] The full-scale output voltage of this type of Wheatstone bridge sensor is often very low, for example 10 to 50 mV / V. It then requires high-precision pre-amplification before any digital conversions and a shift correction of the input signals of the pre-amplifier, i.e., the output signals of the Wheatstone bridge.

[0006] This shift can result from a separation of the Wheatstone bridge resistors or from temperature fluctuations, and can be of the same order of magnitude, for example 10 to 30 mV / V, as the full-scale output voltage. Therefore, an increase in this output signal shift of the Wheatstone bridge should be avoided as much as possible.

[0007] Furthermore, since the typical bandwidth of physical values ​​at low speed is generally between 0.1 and 100 Hz, the lowest possible noise level is desirable.

[0008] According to one embodiment, a device is provided for correcting the voltage shift of a Wheatstone bridge, which is independent of the gain of the amplifier chain and simultaneously ensures that the shift correction takes into account any fluctuations in the supply voltage of the Wheatstone bridge as well as any fluctuations in the values ​​of the resistances of the Wheatstone bridge with respect to temperature.

[0009] According to another embodiment, a correction of the displacement is provided which is insensitive to the 1 / f noise of the amplifier chain.

[0010] One possible solution involves - to control the Wheatstone bridge with a current source and a follower amplifier in order to reproduce the supply voltage or a voltage derived from that supply voltage on the Wheatstone bridge itself, - to generate a second current proportional to the current supplied by the power source and to use this second current as the reference current of a digital-to-analog current converter, and - to adjust the output current of the converter, for example during a calibration phase, using a digital correction signal in order to suppress the voltage shift as much as possible.

[0011] According to one aspect, an electronic device is therefore provided which has a Wheatstone bridge circuit and a correction circuit which is coupled to the Wheatstone bridge circuit and is designed to correct a shift in the output voltage of the Wheatstone bridge.

[0012] According to a general feature of this aspect, the correction circuit has an input interface for receiving a first voltage, a supply module designed to supply the Wheatstone bridge circuit with a second voltage derived from the first voltage and with a first current regulated based on the current value of the resistors in the Wheatstone bridge circuit, as well as generating a second current proportional to the first current, and a digital-to-analog current converter designed to supply a correction current at the outputs of the Wheatstone bridge circuit based on a digital correction signal and the second current.

[0013] The second voltage can be essentially equal to or essentially proportional to the first voltage.

[0014] The first current is typically inversely proportional to the current value of the resistors in the Wheatstone bridge circuit. Therefore, this regulated first current can compensate for any temperature fluctuations in the Wheatstone bridge resistors.

[0015] Furthermore, the digital-to-analog converter has, for example, a first converter input for receiving the second current, a second converter input for receiving the digital correction signal, and a differential current output that is coupled to the two outputs of the Wheatstrom bridge circuit to supply the correction current.

[0016] Since the second current is proportional (the proportionality factor can be equal to or not equal to 1) to the first current, it is possible to take into account the possible fluctuation of the resistances of the Wheatstone bridge circuit, and this second current serves as a reference current in the digital-to-analog converter.

[0017] Given a Wheatstone bridge circuit and temperature, it is therefore possible during a calibration phase to adjust the value of the digital correction signal to provide a differential current signal that can suppress the voltage shift at the output of the Wheatstone bridge circuit. This output current is inherently directly related to the second current and thus indirectly to the first current, the first voltage, and the second voltage. The shift correction therefore accounts for any fluctuations in the supply voltage and resistance of the Wheatstone bridge, particularly with temperature. Technological shortcomings can thus be corrected automatically and easily without the need for a sophisticated and expensive level sensor.

[0018] Furthermore, such an electronic device can suppress the shift of the output signals of the Wheatstone bridge before the preamplification stage, as it is insensitive to the gain and 1 / F noise of the preamplifier.

[0019] According to one embodiment, the supply module has the following features: - a first current source for supplying the first current, which is controlled by a follower amplifier whose input is coupled to the input interface, the output of the first current source, which is coupled to a feed terminal of the Wheatstone bridge circuit, which is coupled to the other input of the follower amplifier, and - a second current source for supplying the second current, which is controlled by the follower amplifier, with the output of the second current source coupled to the first converter input.

[0020] The follower amplifier makes it possible to supply a simulation of the first voltage to the feed terminal of the Wheatstone bridge circuit.

[0021] The first current source can have at least one first bipolar transistor whose base is coupled to the output of the follower amplifier and whose collector is coupled to the feed terminal of the Wheatstone bridge circuit, and the second current source can have at least one second bipolar transistor whose base is coupled to the output of the follower amplifier and whose collector is coupled to the first converter input.

[0022] By using bipolar transistors, for example of the PNP type, in the current sources, it is possible to fundamentally limit the 1 / F noise of these current sources and to further improve the performance of the Wheatstone bridge.

[0023] In one variant, the supply module can have the following features: - at least one first PMOS transistor whose gate is coupled to the output of the follower amplifier and whose drain is coupled via a first chopper circuit to the feed terminal of the Wheatstone bridge circuit and via a second chopper circuit to the first converter input, - at least one second PMOS transistor, whose gate is coupled to the output of the follower amplifier and whose drain is coupled via a third chopper circuit to the feed terminal of the Wheatstone bridge circuit and via a fourth chopper circuit to the first converter input, and - Control means designed to control the chopper circuits, such that the first current source alternatively comprises at least one first PMOS transistor or at least one second PMOS transistor, and the second current source alternatively comprises at least one second PMOS transistor or at least one first PMOS transistor.

[0024] The 1 / F noise of the transistor(s) of the first current source looped back to the follower amplifier is attenuated by the presence of the loop. This is not the case for the MOS transistor(s) that are not in the loop.

[0025] The presence of chopper circuits, which function as a switching network, makes it possible to distribute over time the first and / or second transistors that are looped back to the follower amplifier or connected to the first input of the converter, thus reducing the 1 / F noise generated by the current sources.

[0026] Further advantages and features of the invention will become apparent upon careful reading of the detailed description of embodiments, which are cited as non-limiting examples and illustrated by the accompanying drawings, on which: - the Fig. 1 and Fig. 2 relate to different embodiments of an electronic device according to the invention.

[0027] Reference is now being made to Fig. 1, to illustrate an embodiment of an electronic device DIS according to the invention, which is embodied, for example, in a Wheatstone bridge sensor.

[0028] The electronic device DIS has a correction circuit CC and a Wheatstone bridge circuit PW.

[0029] The correction circuit CC has a supply module 1 and a digital-to-analog current transformer CNA, which has a differential current output BS1 and BS2, which is coupled to the outputs 2 and 3 of the Wheatstone bridge circuit PW.

[0030] The power supply module 1 includes a follower amplifier 4, a first current source 5 which has a first bipolar transistor PNP 50, and a second current source 6 which has a second bipolar transistor PNP 60.

[0031] An input interface 7 is coupled to the non-inverting input of the follower amplifier 4.

[0032] The bases of the two bipolar transistors 50 and 60 of the two current sources 5 and 6 are coupled together with the output 8 of the follower amplifier 4.

[0033] The collector of the first bipolar transistor 50 is coupled to a feed terminal 9 of the Wheatstone bridge circuit PW, which is coupled to the inverting input 10 of the follower amplifier 4.

[0034] The first converter input EC1 is coupled to the collector of the second bipolar transistor 60 of the second current source 6. The converter CNA receives a digital correction signal SNC via the second converter input EC2.

[0035] The emitters of transistors 50 and 60 are connected to a supply voltage VCC.

[0036] In this example, the Wheatstone bridge circuit has four theoretically identical resistors R1-R4, which have a resistance value equal to R. pont exhibit and are connected between the power supply terminal 9 and ground GND.

[0037] If a Wheatstone bridge circuit PW is balanced, there is zero voltage at outputs 2 and 3 of the Wheatstone bridge circuit.

[0038] Due to technological limitations, such as the isolation of the resistors in the Wheatstone bridge circuit PW, fluctuations in the resistance values ​​are possible. Consequently, the Wheatstone bridge circuit PW becomes unbalanced, resulting in a voltage shift at its output.

[0039] During a calibration phase, which takes place, for example, after the integrated circuit has been manufactured in the factory, the displacement is corrected to restore the balance of the Wheatstone bridge circuit PW.

[0040] The supply voltage V pont The input signal of the Wheatstone bridge circuit PW at input interface 7 is replicated at the feed terminal 9 of the Wheatstone bridge circuit PW by the follower amplifier 4. The total resistance of the Wheatstone bridge circuit is approximately R pontTaking into account the technological shortcomings, and the first current I1 supplied by the first power source 5 to supply the Wheatstone bridge circuit PW is approximately V pont / R pont .

[0041] The second current source 6 supplies a second current I2 at the first converter input EC1, which is proportional to the first current I1, as a reference current. The proportionality factor depends on the dimensional ratio between transistors 50 and 60.

[0042] During the calibration phase, the differential current outputs BS1 and BS2 of the digital-to-analog converter CNA are adjusted until the voltage shift at the output of the Wheatstone bridge is suppressed by adjusting the value of the digital correction signal SNC received at the second converter input EC2.

[0043] Once the balance of the Wheatstone bridge is restored (zero voltage at outputs 2 and 3 of the Wheatstone bridge circuit), the value of the digital correction signal SNC is frozen and stored for the converter CNA and is used in the subsequent operation of the converter CNA.

[0044] Since the reference current of the digital-to-analog converter CNA is proportional to the first current I1, which in turn is related to the supply voltage of the Wheatstone bridge circuit PW and the current value of the resistors R1-R4 of the Wheatstone bridge PW, the differential current output of the converter CNA, which is designed to suppress the voltage shift at the output of the Wheatstone bridge PW, takes into account the possible fluctuation of the supply voltage V. pont and the resistances R pont the Wheatstone bridge PW at temperature.

[0045] By using bipolar transistors in the power supply module 1 and possibly in the digital-to-analog converter CNA, it is possible to limit the 1 / F noise of these power sources and possibly of the digital-to-analog current converter CNA.

[0046] Now reference is made to Fig. 2, to illustrate another embodiment of the electronic device according to the invention.

[0047] The following describes only the differences between the two embodiments.

[0048] Firstly, current sources 5 and 6 use PMOS transistors instead of bipolar transistors in the previous embodiment.

[0049] Furthermore, a network of chopper circuits 11, which are usually referred to by the English acronym "chopper" by those skilled in the art, is added to the supply module 1.

[0050] The power source 5 has a first PMOS transistor 51, whose gate is coupled to the output 8 of the follower amplifier 4 and the drain is coupled to the feed terminal 9 of the Wheatstone bridge circuit PW via a first chopper circuit H1 and to the first converter input EC1 via a second chopper circuit H2.

[0051] Likewise, the power source 6 has a second PMOS transistor 61, whose gate is coupled to the output 8 of the follower amplifier 4 and the drain is coupled to the feed terminal 9 of the Wheatstone bridge circuit PW via a third chopper circuit H3 and to the first converter input EC1 via a fourth chopper circuit H4.

[0052] The electronic device DIS further comprises control means MC, which are, for example, made from logic circuits or by software in a microcontroller, which are designed to control the chopper circuit such that when the chopper circuits H1 and H4 are closed, the chopper circuits H2 and H3 are open and vice versa.

[0053] When the chopper circuits H1 and H4 are closed, the current source 5 supplies a first current I1 to supply the Wheatstone bridge, and the current source 6 supplies a second current I2 at the first converter input EC1 as a reference current.

[0054] Power source 5 then forms the first power source and power source 6 forms the second power source.

[0055] When the chopper circuits H2 and H3 are closed, the first current I1 is supplied by the current source 6 and the second current I2 is supplied by the current source 5.

[0056] Power source 6 then forms the first power source and power source 5 forms the second power source.

[0057] The 1 / f noise of the MOS transistor of the current source 5, which is looped back to the input 10 of the follower amplifier 4, can be attenuated by the presence of the loop. This is not the case for the MOS transistor that is not in the loop.

[0058] To limit the 1 / f noise emanating from the current sources 5 and 6, the control devices MC control the chopper circuits H1 to H4 so that they alternately act as a switching network to distribute over time which of the PMOS transistors is fed back to the follower amplifier 4 or connected to the first input of the converter EC1.

[0059] Therefore, the 1 / f noise of the power sources can be significantly reduced with the help of this network of chopper circuits 11.

[0060] The invention is not limited to the described embodiments, but encompasses all variants.

[0061] Thus, the power sources can have several transistors connected in parallel.

[0062] If the transistors are MOS transistors, chopper circuits can be added to distribute over time the number of MOS transistors forming the first looped-back current source and those forming the second current source.

[0063] Finally, for example, a voltage divider or amplifier can be provided between the inverting input of the amplifier and the supply terminal 9 to supply terminal 9 with a second voltage proportional to the voltage V. pont , to provide.

Claims

[1] Electronic device comprising a Wheatstone bridge circuit (PW) and a correction circuit (CC) coupled to the Wheatstone bridge circuit (PW) and designed to correct a shift in the output voltage of the Wheatstone bridge (PW), characterized by, that the correction circuit (CC) has an input interface (7) for receiving a first voltage, a supply module (1) designed to supply the Wheatstone bridge circuit (PW) with a second voltage taken from the first voltage and with a first current (11) regulated on the basis of the current value of the resistors of the Wheatstone bridge circuit (PW), as well as to generate a second current (12) proportional to the first current (I1), and a digital-to-analog current converter (CNA) designed to supply a correction current at the outputs (2, 3) of the Wheatstone bridge circuit (PW) from a digital correction signal (SNC) and the second current (12). [2] Device according to claim 1, wherein the second voltage is substantially equal to or substantially proportional to the first voltage. [3] Device according to claim 1 or 2, wherein the digital-to-analog converter (CNA) has a first converter input (EC1) for receiving the second current (12), a second converter input (EC2) for receiving the digital correction signal (SNC) and a differential current output (BS1, BS2) which is coupled to the two outputs (2, 3) of the Wheatstrom bridge circuit (PW) to supply the correction current. [4] Device according to one of claims 1 to 3, wherein the supply module (1) has a first current source (5) for supplying the first current (11), which is controlled by a follower amplifier (4) whose input is coupled to the input interface (7), wherein the output of the first current source is coupled to a feed terminal (9) of the Wheatstone bridge circuit (PW), which is coupled to the other input (10) of the follower amplifier (4), and a second current source (6) for supplying the second current (12), which is controlled by the follower amplifier (4). [5] Device according to claims 3 and 4, wherein the first current source (5) has at least one first bipolar transistor (50) whose base is coupled to the output (8) of the follower amplifier (4) and whose collector is coupled to the feed terminal (9) of the Wheatstone bridge circuit (PW), and the second current source (6) has at least one second bipolar transistor (60) whose base is coupled to the output (8) of the follower amplifier (4) and whose collector is coupled to the first converter input (EC1). [6] Device according to claims 3 and 4, wherein the supply module comprises the following: at least one first PMOS transistor whose gate is coupled to the output (8) of the follower amplifier (4) and whose drain is coupled to the feed terminal (9) of the Wheatstone bridge circuit (PW) via a first chopper circuit (H1) and to the first converter input (EC1) via a second chopper circuit (H2), at least one second PMOS transistor, whose gate is coupled to the output (8) of the follower amplifier (4) and whose drain is coupled to the feed terminal (9) of the Wheatstone bridge circuit (PW) and is looped back via a third chopper circuit (H3) to the other input (8) of the follower amplifier (4) and is coupled to the first converter input (EC1) via a fourth chopper circuit (H4), and Control means (MC) designed to control the chopper circuits such that the first current source (5) alternatively comprises the at least one first PMOS transistor (51) or the at least one second PMOS transistor (61), and the second current source (6) alternatively comprises the at least one second PMOS transistor (61) or the at least one first PMOS transistor (51).

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