FIELD OF THE INVENTION

The invention relates to the field of sensors. More specifically it relates to a circuit and method for biasing of a bridge sensor structure and reading out a signal therefrom, such as for biasing and reading out a Wheatstone bridge or a Hall sensor element.

BACKGROUND OF THE INVENTION

It is known in the art of sensor technology to use resistive structures, such as Hall plates or Wheatstone bridges, for detecting physical quantities. For example, Wheatstone bridges may be used in pressure sensors, e.g. using piezo-resistive elements. Likewise, Hall elements may be used for measuring a magnetic field in a current sensor or in an angular position sensor. The passive resistive structures used in such sensors may require the application of an excitation signal, e.g. in the form of a voltage or electrical current, in order to detect an output signal. The term “biasing” refers to a process in which such excitation signal, e.g. a voltage or current, is applied, while the term “readout” refers to retrieving a sensor signal from the resistive structure that is indicative of the physical quantity to be detected by the sensor or indicative of an intermediate physical quantity used for determining the physical quantity to be detected.

Spinning is a technique used in state-of-the-art sensor biasing and readout methods, e.g. for suppressing or compensating for an offset of the resistive bridge structure, e.g. an offset of a Hall plate. In such spinning techniques, nodes of the resistive structure that were first used for biasing, in combination with reading out a sensor signal from different nodes, can thereafter be used for sensing, in combination with biasing different nodes. For example, an excitation signal may alternate between being applied to a first set of nodes and a second set of nodes, while detecting the output signal, e.g. a Hall signal, over the other set of nodes. It is known in the art to implement Hall spinning by abruptly switching a constant bias voltage or current from the first set of nodes to a second set of nodes, while simultaneously using the other set for readout.

For example, such spinning techniques may be applied for offset compensation of Hall plates. Furthermore, spinning of bridges can be used for implementing a continuous self-testing procedure, e.g. for pressure sensors, such as disclosed in the UK patent application no. GB2489941.

Resistive structures, such as Hall plates, may have a significant parasitic capacitance due to a relatively large size of the element. Therefore, the resistive element may behave as an RC circuit, which can limit the dynamic response of the device, thus also limiting the achievable spinning speed. Furthermore, since nodes previously used for biasing can be used for sensing in a following step of applying a spinning technique, a time substantially larger than the RC time constant may be needed to enable the bias voltage to settle to a value that is substantially lower than the biasing voltage, e.g. from a bias voltage of 2V to below 1 μV. Furthermore, low-noise amplification methods known in the art may require a continuous operation over some time to prevent noise aliasing caused by sampling effects. Therefore, after stabilizing to a low voltage, the output signal must be sensed over a sufficiently large time span, thus further constraining the maximum achievable spinning speed while maintaining noise at an acceptable level.

In particular applications, a large bandwidth for the detected signal as function of time may be required. For example, in current sensors, a fast-changing electrical current may be monitored by detecting an associated magnetic field using a Hall sensor. The desirable signal bandwidth may be, for example, of the same order of magnitude as the spinning frequency used to compensate for sensor offsets. Furthermore, flexible digital processing, e.g. post-processing, of the sensor front-end signals may be advantageous. For particular applications, it may also be desirable to sample the readout of each individual spin phase. By sampling the output of each spinning phase, a highly flexible and well-controlled sampling-based processing circuit can be provided for further back-end processing of the sensor signal.

SUMMARY OF THE INVENTION

It is an object of embodiments of the present invention to provide good, efficient and/or fast biasing and read-out of a bridge sensor structure, such as a resistive bridge, e.g. a Wheatstone bridge or a Hall plate.

The above objective is accomplished by a method and device according to the present invention.

It is an advantage of embodiments of the present invention that node switching in a spinning operation of a resistive bridge sensor structure can occur near a zero crossing of the excitation signal, e.g. of a bias voltage, such that a stabilization time of the readout signal after this switching, e.g. due to a parasitic capacitance of the sensor structure, can be reduced or avoided. For example, when spinning operations are applied to the sensor structure, transient signals between different spinning phases can be avoided or reduced when switching is performed near a zero-crossing of the excitation signal. Thus, the resistive bridge sensor structure can be more quickly read out after switching of a set of nodes from an excitation function to a sensing function since less stabilization time is required for avoiding an integration or averaging operation of a monitored readout signal during an uncontrolled transient period, e.g. a time frame after switching in which the readout signal could be characterized by unpredictable and/or stochastic behaviour.

It is an advantage of embodiments of the present invention that fast node switching in a spinning operation of a resistive bridge sensor structure can be achieved, e.g. at switching frequencies in the range of 100 kHz to 50 MHz, e.g. in the range of 500 kHz to 10 MHz, e.g. in the range of 1 MHz to 5 MHz, e.g. about 3 MHz.

It is an advantage of embodiments of the present invention that a low level of readout noise can be achieved, even in a high-speed node spinning operation of a resistive bridge sensor structure.

It is an advantage of embodiments of the present invention that a low level of readout noise can be achieved using a continuous-time readout front-end, e.g. using integration-based low-noise readout, such as charge integration.

It is an advantage of embodiments of the present invention that a reproducible and/or predictable sensor sensitivity can be achieved, e.g. by preventing, compensating and/or controlling bias-source uncertainties, such as stochastic variation of a voltage or electrical current bias signal, errors from dynamic response limitations and/or a drift in readout interval.

It is an advantage of embodiments of the present invention that transient spikes in the excitation signal supplied to, and the readout signal obtained from, a resistive bridge sensor structure can be reduced or avoided.

It is an advantage of embodiments of the present invention that a readout offset of a resistive bridge sensor structure can be controlled, avoided and/or compensated for in a high-speed spinning operation of the sensor structure.

It is an advantage of embodiments of the present invention that a high temporal resolution in monitoring a readout signal obtained from a resistive bridge sensor structure can be achieved while maintaining noise at an acceptable level.

In a first aspect, the present invention relates to a circuit for biasing a bridge sensor structure, e.g. a resistive bridge sensor structure, and for reading out a sensor signal from the bridge sensor structure. The circuit comprises at least two pairs of connection terminals, in which each pair of connection terminals is adapted for connecting to complementary terminals of the bridge sensor structure. For example, the complementary terminals may refer to opposite contacts of the bridge sensor, e.g. opposite with respect to a center of the bridge, e.g. of a cross-shaped or star-shaped bridge.

The circuit further comprises an excitation signal generator for generating an electrical excitation signal for biasing, e.g. for exciting, the bridge sensor structure, in which the excitation signal is generated based on, e.g. provided as, a non-constant periodic continuous function of time. ‘Non-constant’ may refer to the exclusion of a substantially constant signal, e.g. excluding a signal having no substantial time-dependence. ‘Continuous’ may refer to the mathematical sense of the term, e.g. such that sufficient small changes in time can result in arbitrarily small changes of the signal value with respect to each point of time.

Thus, the circuit further comprises an excitation signal generator for generating an electrical excitation signal for biasing, e.g. for exciting, the bridge sensor structure, in which the excitation signal does not change abruptly, e.g. stepwise.

For example, the electrical excitation signal may vary less than 50% of its signal range in any time window that has a duration of less than 25% of the period of the periodic function, e.g. less than 25% in any time window having a duration of less than 15% of the period, e.g. less than 20% in any time window having a duration of less than 5% of the period.

The circuit also comprises a detection circuit for obtaining the sensor signal from the bridge sensor structure by electrically connecting the detection circuit to any pair of the at least two pairs of connection terminals while applying the excitation signal to another pair of the at least two pairs of connection terminals.

The circuit comprises a switch unit for switching the electrical excitation signal generated by the excitation signal generator from being supplied to a first pair of the at least two pairs of connection terminals to another pair of the at least two pairs of connection terminals and for switching the detection circuit from being connected to a second pair of the at least two pairs of connection terminals to another pair of the at least two pairs of connection terminals.

The circuit furthermore comprises a controller for controlling the switch unit to switch the first pair from being connected to the excitation signal generator at an instant in time when the excitation signal generator generates the excitation signal in a predetermined signal range, in which range the excitation signal value is substantially equal to a zero reference value.

In a circuit in accordance with embodiments of the present invention, the controller may comprise a comparator and/or an analog to digital converter to detect when the excitation signal is in the predetermined signal range.

In a circuit in accordance with embodiments of the present invention, the controller may be adapted for predicting a time at which the excitation signal value is in the predetermined signal range.

In a circuit in accordance with embodiments of the present invention, the non-constant periodic continuous function may be differentiable, continuously differentiable and/or smooth. References to ‘differentiable,’ ‘continuously differentiable’ and ‘smooth’ may be interpreted in the mathematical sense of such terms.

In a circuit in accordance with embodiments of the present invention, the excitation signal generator may be adapted for generating a sinusoidal electrical current waveform and/or a sinusoidal voltage waveform, and/or a combination of at least two sinusoidal electrical current or voltage waveforms, e.g. A1·cos(ω1*t+c1)+A1·cos(ω2*t+c2), where A1and A2are amplitudes, ω1and ω2are angular frequencies and c1and c2are phase constants.

In a circuit in accordance with embodiments of the present invention, the excitation signal generator may be adapted for generating the electrical excitation signal in the form of two complementary voltages, e.g. such that the sum of these complementary voltages is substantially constant, e.g. is constant. The controller may be adapted for detecting each zero-crossing of a differential voltage signal, at which zero-crossing the absolute value of the difference between the two complementary voltages is zero or, alternatively, less than a predetermined tolerance threshold, e.g. such that the absolute value of the differential voltage signal is less than the predetermined tolerance threshold at the zero-crossing.

In a circuit in accordance with embodiments of the present invention, the switch unit and the controller may be adapted for applying the two complementary voltages to a selected pair of the at least two pairs of connection terminals, where the selected pair for applying the excitation signal is changed at a predetermined zero-crossing of the detected zero-crossings, e.g. by the switch unit as controlled by the controller, thus iterating over a plurality of spinning phases delimited in time by the detected zero-crossings or a subset of the detected zero-crossings. For example, each detected zero-crossing may mark a next spinning phase, or each spinning phase may comprise the same number of consecutive detected zero-crossings. The spinning phases may be repeatedly iterated over in the same order.

In a circuit in accordance with embodiments of the present invention, the at least two pairs of connection terminals may consist of two pairs of connection terminals for connecting respectively to two node pairs of the bridge sensor structure, in which the switch unit and the controller are adapted for exchanging connections of the pairs of connection terminals to the excitation signal generator and the detection circuit such that the excitation signal is alternatingly applied to each node pair while sensing the sensor signal via the other pair.

In a circuit in accordance with embodiments of the present invention, the detection circuit may be adapted for obtaining the sensor signal in the form of a Hall plate readout signal from the bridge sensor structure in the form of a Hall element, in which the instant in time when the controller controls the switching of the switch unit corresponds to an instant in time when a voltage over the first pair of the at least two pairs of connection terminals is substantially equal to zero.

In a circuit in accordance with embodiments of the present invention, the electrical current supplied to the first pair of connection terminals as excitation signal may be substantially equal to a zero current in the predetermined signal range.

In a circuit in accordance with embodiments of the present invention, the detection circuit may comprise a low noise amplifier for amplifying the sensor signal.

In a circuit in accordance with embodiments of the present invention, the detection circuit may comprise a continuous-time integrator or averager for integrating or averaging the sensor signal, in which the continuous time integrator or averager may be adapted for providing an output signal representative of a continuous-time integral or average of a momentary differential voltage signal over the pair of connection terminals connected to the detection circuit.

In a circuit in accordance with embodiments of the present invention, the circuit may comprise an excitation integrator and/or excitation averager for determining an average bias current and/or a total charge flowing through the pair of connection terminals connected to the excitation signal generator during a predetermined time interval, in which the excitation integrator and/or excitation averager may comprise a capacitor for receiving, during said predetermined time interval, a current which is representative for the biasing current corresponding with the excitation signal.

In a circuit in accordance with embodiments of the present invention, the controller may be adapted for determining an amount of charge stored in the capacitor in that predetermined time interval.

In a circuit in accordance with embodiments of the present invention, the controller may be adapted for determining a sensor readout value based on the integrated or averaged readout signal and on a value indicative of a change of the amount of charge stored in the capacitor during the time interval.

In a circuit in accordance with embodiments of the present invention, the controller may be adapted for applying a zero-banding technique, e.g. a guard banding technique, to exclude residual transient effects in the observed sensor signal.

In a circuit in accordance with embodiments of the present invention, the controller may be adapted for controlling the detection circuit to obtain the sensor signal from the bridge sensor structure only in a further predetermined time interval during each spinning phase and for ignoring or zeroing the sensor signal during the spinning phase when outside this time interval, in which a spinning phase refers to the time in between two consecutive switching actions of the switch unit.

In a circuit in accordance with embodiments of the present invention, the further predetermined time interval may exclude a time frame immediately after switching to the spinning phase. The further predetermined time interval may optionally also exclude a time frame immediately before switching to a next spinning phase.

In a circuit in accordance with embodiments of the present invention, the switch unit and the controller may be adapted for reversing a polarity of the sensed signal.

In a circuit in accordance with embodiments of the present invention, the controller may be adapted for controlling a conventional sensing mode of operation during a first time interval in each spinning phase and controlling a charge-integrating mode of operation during a second time interval in each of spinning phase.

In a circuit in accordance with embodiments of the present invention, the further time interval may be symmetrical in time with respect to a maximum of the applied excitation signal.

In a circuit in accordance with embodiments of the present invention, the further time interval may comprise a central part of the time corresponding to the spinning phase and may exclude start and/or end parts of the duration of the spinning phase.

A circuit in accordance with embodiments of the present invention may be adapted for switching between a floating-plate biasing mode of operation and a charge integrating mode of operation.

In a second aspect, the present invention also relates to a sensor device comprising a bridge sensor structure, e.g. a resistive bridge sensor structure, and a circuit in accordance with embodiments of the first aspect of the present invention, in which each pair of connection terminals of the circuit is connected to corresponding terminals of the bridge sensor structure.

In a further aspect, the present invention also relates to a method for biasing of a bridge sensor structure, e.g. a resistive bridge sensor structure, and reading out a signal therefrom. The bridge sensor structure comprises at least two pairs of nodes, e.g. of complementary terminals. The method comprises generating an electrical excitation signal for biasing the resistive bridge sensor structure, in which the excitation signal is provided as a non-constant periodic continuous function of time. The method also comprises biasing a first pair of nodes of the resistive bridge sensor structure by applying the electrical excitation signal, while obtaining a sensor signal from a second pair of nodes of the resistive bridge sensor structure, e.g. from another pair of nodes that are different from the nodes of the first pair. The method further comprises, e.g. substantially simultaneously, switching the electrical excitation signal from being applied to the first pair of nodes to being applied to another pair of nodes of the bridge sensor structure and switching a sensor signal from being obtained from a second pair of nodes to being obtained from another pair of nodes. For example, the method may comprise switching the first pair of nodes from being connected to receive the electrical excitation signal to being connected to obtaining the sensor signal and switching another pair of nodes, such as the second pair of nodes, from being connected to receive the sensor signal to being connected to receive the electrical excitation signal. This switching is performed at an instant in time when the excitation signal is generated in a predetermined signal range, in which range the excitation signal value is substantially equal to a zero reference value. Thus, in a method in accordance with embodiments, the steps of applying the electrical excitation signal and the step of obtaining the sensor signal may be repeated for different pairs of nodes selected by repeated switching at instants in time when the excitation signal is generated in the predetermined signal range. Each time interval between consecutive switching actions in this repeated switching may be referred to as a spinning phase.

In a method in accordance with embodiments of the present invention, the non-constant periodic continuous function may be differentiable, continuously differentiable and/or smooth.

In a method in accordance with embodiments of the present invention, the excitation signal may be generated as a sinusoidal electrical current waveform and/or a sinusoidal voltage waveform, and/or a combination of at least two sinusoidal electrical current or voltage waveforms, e.g. A1·cos(ω1*t+c1)+A2·cos(ω2*t+c2), where A1and A2are amplitudes, ω1and ω2are angular frequencies and c1and c2are phase constants.

In a method in accordance with embodiments of the present invention, the electrical excitation signal may be generated in the form of two complementary voltages, e.g. complementary sinusoidal voltages. The method may further comprise detecting each zero-crossing of a differential voltage signal, at which zero-crossing the absolute value of the difference between the two complementary voltages is zero or, alternatively, less than a predetermined tolerance threshold.

In a method in accordance with embodiments of the present invention, the step of biasing the first pair of nodes of the resistive bridge sensor structure by applying the electrical excitation signal may comprise applying the larger of the two sinusoidal voltages to the first pair of nodes.

In a method in accordance with embodiments of the present invention, the pair of nodes to which the excitation signal is applied may alternate at each detected zero-crossing, thus iterating over a plurality of spinning phases delimited in time or by the detected zero-crossings or a subset of the detected zero-crossings.

In a method in accordance with embodiments of the present invention, the resistive bridge sensor structure may be a Hall element, the step of detecting the sensor signal from the second pair of nodes may comprise obtaining the sensor signal in the form of a Hall plate readout signal, and the instant in time when the switching is performed may correspond to an instant in time when a voltage over the first pair of nodes is substantially equal to zero.

In a method in accordance with embodiments of the present invention, an electrical current supplied to the first pair of nodes may be substantially equal to a zero current, e.g. may be a zero current, e.g. may have a current magnitude below a predetermined threshold, in the predetermined signal range.

A method in accordance with embodiments of the present invention may comprise using a low noise amplifier for amplifying the sensor signal.

A method in accordance with embodiments of the present invention may comprise integrating or averaging the sensor signal.

In a method in accordance with embodiments of the present invention, the step of integrating or averaging the sensor signal may comprise providing an output signal representative of a continuous-time integral or average of a momentary differential voltage signal over the second pair of nodes.

A method in accordance with embodiments of the present invention may comprise collecting, e.g. integrating over time or averaging in time, a total charge flowing through the first pair of nodes during a predetermined time interval.

A method in accordance with embodiments of the present invention may comprise determining a sensor readout value based on the integrated or averaged readout signal and on a value indicative of the amount of charge stored in the capacitor.

In a method in accordance with embodiments of the present invention, the step of obtaining the sensor signal may comprise obtaining the sensor signal only in a further predetermined time interval during each spinning phase.

In a method in accordance with embodiments of the present invention, the step of obtaining the sensor signal may comprise ignoring or zeroing the sensor signal during the spinning phase when outside the further predetermined time interval.

In a method in accordance with embodiments of the present invention, the further predetermined time interval may exclude a time frame immediately after switching to the spinning phase, e.g. the present spinning phase.

Particular and preferred aspects of the invention are set out in the accompanying independent and dependent claims. Features from the dependent claims may be combined with features of the independent claims and with features of other dependent claims as appropriate and not merely as explicitly set out in the claims.

These and other aspects of the invention will be apparent from and elucidated with reference to the embodiment(s) described hereinafter.

