Detection circuit using a differential capacitive sensor with input-common-mode control in a sense interface

A detection circuit is provided with a differential capacitive sensor and with an interface circuit having a first sense input and a second sense input, electrically connected to the differential capacitive sensor. Provided in the interface circuit are: a sense amplifier connected at input to the first sense input and to the second sense input and supplying an output signal related to a capacitive unbalancing of the differential capacitive sensor; and a common-mode control circuit, connected to the first sense input and to the second sense input and configured to control a common-mode electrical quantity present on the first sense input and on the second sense input. The common-mode control circuit is of a totally passive type and is provided with a capacitive circuit, which is substantially identical to an equivalent electrical circuit of the differential capacitive sensor and is driven with a driving signal in phase opposition with respect to a read signal supplied to the differential capacitive sensor.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a detection circuit using a differential capacitive sensor with input-common-mode control in a sense interface, in particular a sense interface of the fully differential switched-capacitor type, to which the following description will make reference, without this implying any loss of generality.

2. Description of the Related Art

The last few years have witnessed a widespread use of detection circuits that employ differential capacitive sensors (for example, inertial sensors, accelerometers, pressure or force sensors) in applications that envisage low supply voltages and low power consumption, such as for example in battery-supplied portable devices (PDAs, digital audio players, cell phones, digital camcorders and the like). As is known, capacitive differential sensors base their operation on a capacitive unbalancing, which occurs as a function of a quantity to be detected (an acceleration, a pressure, a force, etc.). In particular, there is a widespread use of micro-electromechanical-system (MEMS) sensors, obtained with techniques of microfabrication of semiconductor materials. In a known way, these sensors comprise a fixed body (“stator”) and a mobile mass (designated by the term “rotor”), both of which are generally made of appropriately doped semiconductor material and which are connected to one another by means of elastic elements (springs) and constrained so that the rotor has, with respect to the stator, pre-set translational and/or rotational degrees of freedom. The stator has a plurality of fixed arms, and the rotor has a plurality of mobile arms, said arms facing one another so as to form pairs of capacitors having a capacitance that varies as a function of the relative position of the arms, i.e., as a function of the relative position of the rotor with respect to the stator. Accordingly, when the sensor is affected by the quantity to be determined, the rotor shifts and a capacitive unbalancing of the pairs of capacitors occurs, from which it is possible to determine the desired quantity. According to the type of structure and the type of relative movement between rotor and stator, it is possible to provide MEMS sensors of a linear or rotational type, with variation of the gap (i.e., the distance between the mobile arms and the respective fixed arms) and/or with variation of a degree of facing (i.e., variation of the area of mutual facing between the mobile arms and the respective fixed arms).

Purely by way of example,FIG. 1aschematically illustrates a differential capacitive sensor1, of a linear MEMS type. The following exposition is in any case to be understood as valid for MEMS sensors having different configurations. In detail, the differential capacitive sensor1comprises a stator, of which just the first fixed arms2aand second fixed arms2bare illustrated, and a rotor, constituted by a mobile mass3and by mobile arms4fixed to the mobile mass3. Each mobile arm4is set between a respective first fixed arm2aand a second fixed arm2b. The mobile mass3is suspended via springs5to anchoring elements6, and is mobile along an axis x that constitutes the preferential axis of detection of the differential capacitive sensor1. The first fixed arms2aand the second fixed arms2bare electrically connected to a first stator terminal7aand to a second stator terminal7b, respectively, whilst the mobile arms4are electrically connected to a rotor terminal8.

As illustrated inFIG. 1b, the differential capacitive sensor1has an equivalent electrical circuit comprising a first sense capacitor9aand a second sense capacitor9b, with plane and parallel faces, arranged in “half-bridge” configuration, i.e., connected in series between the first stator terminal7aand the second stator terminal7b, and having in common the rotor terminal8. The capacitances of the first sense capacitor9aand of the second sense capacitor9bare variable as a function of the distance between the mobile arms4and the fixed arms2a,2b, and thus as a function of the displacement of the rotor with respect to the stator. In particular, the first sense capacitor9ais the parallel of the capacitances formed between the first fixed arms2aand the mobile arms4, whilst the second sense capacitor9bis the parallel of the capacitances formed between the second fixed arms2band the mobile arms4. When the differential capacitive sensor1is subjected to an acceleration along the axis x, the mobile mass3moves along this axis, and consequently a capacitive variation of the first sense capacitor9aand a capacitive variation of the second sense capacitor9bare produced, said variations being equal in absolute value and opposite in sign with respect to one another. In particular, given a common sense capacitance Csat rest for the first sense capacitor9aand the second sense capacitor9b(assuming the differential capacitive sensor1as being symmetrical at rest), due to unbalancing, the first sense capacitor9aassumes a value of capacitance equal to Cs1=Cs+ΔCs, and the second sense capacitor9bassumes a value of capacitance equal to Cs2=Cs−ΔCs.

As is known, in the aforesaid detection circuits, an appropriate sense circuit is coupled to the differential capacitive sensor, and usually comprises a charge-integrator interface stage (or charge-amplifier, operating as charge-to-voltage converter), and appropriate stages of amplification, filtering and noise canceling, cascaded to the interface stage. The sense circuit applies a read pulse (having a voltage in the region of a few volts) to the rotor terminal, reads the resultant capacitive unbalancing ΔCs, and generates, from said capacitive unbalancing, an output electrical signal correlated to the quantity that is to be detected. In applications with low supply voltage and low consumption, the performance required to the sense circuit in terms of resolution and of thermal and long-term (aging) stability are particularly stringent, and call for the development of read techniques that are as immune as possible from error, such as noise (thermal noise and low-frequency noise) and offset. For this reason, recently-see for example the article “A Three-Axis Micromachined Accelerometer with a CMOS Position-Sense Interface and Digital Offset-Trim Electronics” by M. Lemkin, B. E. Boser, IEEE Journal of Solid-State Circuits, Vol. 34, No. 4, April 1999, pp. 456-468 which is considered included herein in its entirety—the use of fully differential sense circuits of the switched-capacitor type (operating in discrete-time) has been proposed, which make it possible to operate at low supply voltages and which intrinsically meet the need of current consumption reduction. In particular, the use in the detection circuit of differential sensors coupled to fully differential sense circuits enables numerous advantages to be obtained, amongst which: the increase in the rejection of noise coming from the supply (and/or from the substrate in case of integrated technologies); the reduction of errors, such as charge injection or the so-called “dock feedthrough” (the latter being intrinsically due to the use of switches); and the increase in the dynamics of the output signals by a factor of two. However, a problem linked to the use of fully differential circuits regards the need to eliminate or at least limit the effects due to the common-mode signal at their input. In particular, the read pulse applied to the rotor terminal (which is chosen as wide as possible compatibly with the design requirements, for the purpose of increasing the signal-to-noise ratio at the output of the sense stage) produces a common-mode signal at the inputs of the charge integrator of the interface stage. Said common-mode signal is caused by a common-mode amount of charge (i.e., an amount of charge that is the same on both of the inputs of the charge integrator) injected by the first sense capacitor9aand by the second sense capacitor9bfollowing application of the read pulse. The common-mode signal must be cancelled in order to reduce the read errors that can derive therefrom, in particular a gain error and offset error depending on a mismatch of parasitic capacitances (in particular, the “pad” capacitance and substrate capacitance) at the input terminals of the charge integrator. To solve this problem, in the aforesaid article it is proposed to implement an input-common-mode control circuit of an active type, which uses a feedback loop (so-called ICMFB-Input-Common-Mode Feedback).

The above solution is now described briefly with reference toFIG. 2, which shows a detection circuit comprising an interface circuit10, of the fully differential switched-capacitor type, coupled to the differential capacitive sensor1, represented schematically, in accordance with what has been described previously, with the first sense capacitor9aand the second sense capacitor9bhaving sense capacitance at rest Cs, and having first terminals connected together and to the rotor terminal8, and second terminals connected, respectively, to the first stator terminal7aand to the second stator terminal7b. The interface circuit10is connected at input to the first stator terminal7aand to the second stator terminal7band comprises a charge integrator12and a feedback stage14implementing the ICMFB active circuit for input-common-mode control. The parasitic capacitances are represented schematically as a first parasitic capacitor15and a second parasitic capacitor16, connected, respectively, between the first stator terminal7aand the second stator terminal7band a reference-potential line18(coinciding, in particular, with the signal ground), and having parasitic capacitance Cp.

In detail, the charge integrator12comprises a sense operational amplifier20, in charge-integrator configuration (which carries out a conversion of an input charge into an output voltage), which has an inverting input connected to the first stator terminal7aand a non-inverting input connected to the second stator terminal7b, and two outputs, between which an output voltage Vois present. The charge integrator12further comprises a first integration capacitor22and a second integration capacitor23, having the same integration capacitance Ciand connected the first between the inverting input and an output, and the second between the non-inverting input and the other output of the sense operational amplifier20.

The feedback stage14comprises an amplifier circuit25, and a first feedback capacitor26and a second feedback capacitor27having the same feedback capacitance Cfb. The amplifier circuit25, the structure and operation of which are described in detail in the article referred to above, is a switched-capacitor circuit having an output25a, a first differential input25band a second differential input25c, which are connected, respectively, to the inverting input and to the non-inverting input of the sense operational amplifier20, and a reference input25d, connected to the reference-potential line18. The first feedback capacitor26and the second feedback capacitor27have first terminals connected to one another and to the output25aof the amplifier circuit25, and second terminals connected to the first stator terminal7aand to the second stator terminal7b, respectively. In use, the amplifier circuit25detects the voltage between the first differential input25band the second differential input25c, determines its mean value, and generates at the output25aa feedback voltage Vfbproportional to the difference between said mean value and the reference voltage of the reference-potential line18.

Reading of the differential capacitive sensor1is obtained by supplying to the rotor terminal8(and to the mobile mass3) a step read signal Vr(which has a voltage variation having a value, for example, equal to the supply voltage of the interface circuit10, or else equal to a fraction of said supply voltage). The charge integrator12integrates the differential amount of charge supplied by the first sense capacitor9aand by the second sense capacitor9b(i.e., caused by the capacitive unbalancing ΔCsof the two capacitors), and consequently generates the output voltage Vo. In particular, the following relation of proportionality is valid for the output voltage Vo:

Vo⁢αVr⁢Δ⁢⁢CsCi
where, as mentioned previously, ΔCsis the capacitive unbalancing of the differential capacitive sensor1, i.e., the equal and opposite variation of capacitance of the first sense capacitor9aand of the second sense capacitor9b, which occurs due to displacements of the mobile mass4with respect to the stator. The feedback stage14, through the feedback voltage Vfb, keeps the first stator terminal7aand the second stator terminal7bat a constant common-mode voltage with respect to the reference voltage. Furthermore, since the sense operational amplifier20keeps the voltage between its inputs substantially at zero, the first stator terminal7aand the second stator terminal7bare practically virtual-ground points. In this way, the influence of the parasitic capacitors15,16on the sense circuit is eliminated, in so far as they are kept at a constant voltage and consequently do not absorb electric charge.

However, even though the feedback stage14is advantageous in so far as it enables elimination of the common-mode problems, it should be designed taking into account the requirements of low supply voltage and of low power consumption. In particular, the output dynamics of the amplifier circuit25should be lower than the supply voltage of the interface circuit10; for example, it may be equal to one third of said supply voltage. It follows that the feedback capacitance Cfbof the feedback capacitors26,27should be greater than the sense capacitance at rest Csof the differential capacitive sensor1; for example, it may be equal to three times said value, in the case where the voltage variation of the read signal Vris equal to the supply voltage. As described in the article referred to, however, the fluctuations of voltage at the input of the sense operational amplifier20due to the noise cause a flow of charge in the integration capacitors22,23, which comes both from the sense capacitors9a,9band from the parasitic capacitors15,16, causing the noise to be amplified by a factor equal to:

Vo2_Vo⁢⁢p2_=(1+Cs+Cp+Cf⁢⁢bCi)2
where Vopis the value of an equivalent input-noise generator. Consequently, it is evident from said relation that the noise at output from the charge integrator12increases quadratically as the value of the feedback capacitance Cfbincreases. Consequently, said value should be as contained as possible in order to reduce the output noise (or equivalently the current consumption given the same noise). The introduction of the ICMFB circuit, which itself involves a non-negligible current consumption (on account of the presence of amplifier components), although solving the problem linked to the common mode, because of the high value of the feedback capacitance Cfbrisks worsening the noise performance (or further increasing the current consumption, given the same noise) of the sense interface. The current consumption may even be excessive for portable applications.

BRIEF SUMMARY OF THE INVENTION

One embodiment of the present invention provides an interface circuit for a differential capacitive sensor that enables the aforesaid disadvantages and problems to be overcome, and in particular that will have an input-common-mode control circuit that does not jeopardize the performance in terms of current consumption and noise.

DETAILED DESCRIPTION OF THE INVENTION

One embodiment of the present invention envisages using, for the input-common-mode control in a sense interface of a differential capacitive sensor, a circuit of a purely passive type (accordingly defined Input-Common-Mode Passive Control-ICMPC), configured to generate a common-mode amount of charge equal in absolute value and opposite in sign to the common-mode amount of charge generated by the sensor due to application of a read pulse. In this way, a balance of common-mode charges and the value of a common-mode voltage on the input terminals of the sense interface are kept constant.

FIG. 3, where parts that are similar are identified with the same reference numbers used previously and are not described again in detail, illustrates a detection circuit comprising an interface circuit30and a differential capacitive sensor1. In detail, the differential capacitive sensor1is again represented schematically with a first sense capacitor9aand a second sense capacitor9b, connected between a rotor terminal8and, respectively, a first stator terminal7aand a second stator terminal7b. The interface circuit30is connected at input to the first stator terminal7aand to the second stator terminal7band comprises a charge integrator12and a first common-mode control circuit32, of a purely passive (ICMPC) type. Again, parasitic capacitances at the input of the charge integrator12are represented schematically as a first parasitic capacitor15and a second parasitic capacitor16, which are connected between the first stator terminal7aand second stator terminal7b, respectively, and a reference-potential line18, and have parasitic capacitance Cp. The charge integrator12comprises: a sense operational amplifier20, in charge-integrator configuration, connected to the first stator terminal7aand to the second stator terminal7band supplying an output voltage Vo; and a first integration capacitor22and a second integration capacitor23. The first common-mode control circuit32has a first output terminal32aand a second output terminal32b, connected to the first stator terminal7aand second stator terminal7b, respectively, and a driving terminal32c.

According to one embodiment of the present invention, the first common-mode control circuit32is a capacitive circuit having a circuit configuration substantially identical to the equivalent electrical circuit of the differential capacitive sensor1, and consequently comprises a first control capacitor34and a second control capacitor35having first terminals connected to one another and to the driving terminal32cand second terminals connected, respectively, to the first output terminal32aand to the second output terminal32b, and having the same value of a control capacitance Cpa, in particular substantially equal to the sense capacitance Cs. In use, a read signal Vris supplied to the rotor terminal8of the differential capacitive sensor1, via a first signal generator36; the read signal Vrbeing in particular a step pulse having a first voltage variation ΔVr. A driving signalVris supplied to the driving terminal32c, via a second signal generator37; the driving signalVralso being a step pulse having a second voltage variation −ΔVr, equal and opposite (and substantially simultaneous) to the first variation ΔVr. In particular, in the case of the read signal Vrhaving a periodic pattern (for example, a pulse train), the driving signalVris 180° out of phase (i.e., it is in phase opposition) with respect to the read signal Vr.

The first control capacitor34and the second control capacitor35consequently generate on the input terminals of the charge integrator12a common-mode amount of charge equal and opposite to the common-mode amount of charge generated by the first sense capacitor9aand by the second sense capacitor9b(in other words, the amount of charge injected/extracted by the control capacitors is equivalent to an amount of charge extracted/injected by the sense capacitors). The value of the common-mode voltage on said terminals is thus kept constant, and the contribution of the parasitic capacitances15,16, which, being at a constant voltage, do not absorb electrical charges, is inhibited.

Since the circuit described is based on an open-loop control, it is able to control the input common mode as long as the control capacitance Cpaof the control capacitors34,35is effectively equal to the sense capacitance Csof the differential capacitive sensor1. Consequently, in the case where, for reasons of implementation (for example, because different technologies are used), the control capacitance Cpais different from the sense capacitance Cs, or in any case undergoes different effects of aging and of drift over time, a further embodiment of the present invention envisages using in combination with the first common-mode control circuit32, a second common-mode control circuit14, of an ICMFB active type, which implements a closed feedback loop.

In particular (seeFIG. 4), the detection circuit in accordance with a second embodiment of the present invention also comprises the second common-mode control circuit14, which is substantially similar to the ICMFB circuit described with reference to the prior art, and consequently has an amplifier circuit25and a first feedback capacitor26and a second feedback capacitor27, which have feedback capacitance Cfband are connected between an output25aof the amplifier circuit25and the first stator terminal7a, and second stator terminal7b, respectively. However, in this case, the value of the feedback capacitance Cfbcan be much smaller than the corresponding value of the prior art, and in particular smaller than the sense capacitance Cs. In fact, thanks to the presence in combination of the first common-mode control circuit32, the second common-mode control circuit14balances possible differences between the sense capacitance Csand the control capacitance Cpadue to possible technological differences of fabrication, and generates small amounts of charge proportional to said differences (once again via the feedback voltage Vfb). Consequently, the current consumption and the contribution of noise of the sense interface is also in this case contained and compatible with that of portable applications.

As illustrated inFIG. 5, the interface circuit30described may be used to advantage in an electronic device40. The electronic device40is for example a portable device, such as a mobile phone, a digital audio player, a PDA, a digital camcorder or camera, or a portable computer (laptop), and is equipped with a detection circuit42(in particular provided as ASIC-Application Specific Integrated Circuit), configured to determine the value of a given quantity associated to the electronic device40(for example, an acceleration, a pressure, a force, etc.). The detection circuit42comprises: a differential capacitive sensor1(for example an accelerometer, a pressure sensor, a force sensor, etc.), configured to sense said quantity and generate a differential capacitive variation as a function of its value; and a sense stage44associated to the differential capacitive sensor1. In turn, the sense stage44comprises: the interface circuit30described previously and configured to convert the capacitive unbalancing ΔCsinto an output electrical signal Vo; and a gain and filtering circuit45, for example one that uses a correlated-double-sampling (CDS) technique, configured to amplify and filter said output electrical signal, and generate a sense signal that can be used within the electronic device40. For the purpose, the detection circuit42is connected at output to a microprocessor circuit46of the electronic device40, configured to activate given functions of the electronic device40as a function of the value of the sense signal. Advantageously, according to one embodiment of the present invention, the detection circuit42is made in a single silicon die, so that no differences of technology of fabrication are possible between the differential capacitive sensor1and the sense stage44.

The advantages of the described detection circuit are clear from the foregoing description.

In any case, it is emphasized that the circuit proposed enables a reduction in the current consumption necessary for reading of the differential capacitive sensor1. In particular, said circuit enables a reduction in a total capacitance used for input-common-mode control and hence the noise of the system, or, equivalently, a reduction in the current consumption given the same output noise.

The first embodiment described (which envisages the use of just the passive circuit-ICMPC) has a particularly simple configuration and has an extremely low current consumption, and enables control of the common mode in the case where the sense capacitances Csare (and remain) equal to the control capacitance Cpa. For this reason, it is particularly advantageous to manufacture the differential capacitive sensor1with the same technology used for the interface circuit30, in particular in one and the same silicon die.

The second embodiment is advantageous in the case where it is desired to compensate for possible differences between the aforesaid capacitances, due, for instance, to a non-uniform aging of the components. Anyway, even in the case where said embodiment is used, the value of the feedback capacitance Cfbof the feedback capacitors26,27is much smaller as compared to the prior art, with the advantage of enabling a smaller current consumption (or a lower output noise).

Finally, it is clear that modifications and variations may be made to what is described and illustrated herein, without thereby departing from the scope of the present invention, as defined in the annexed claims.

In particular, it is evident that in the detection circuit any type of differential capacitive sensor can be used, built with MEMS technology or even with different technologies. For example, the differential capacitive sensor can be a displacement sensor, a gyroscope, a linear or rotational acceleration sensor, a pressure sensor, or a force sensor.

The electronic device40may also be a storage device, for example a hard disk, and the detection circuit can contribute to the detection of a condition of free fall.

Furthermore, other types of waveforms can be used in reading, and the read signal Vrmay, for example, comprise a pulse train, or a staircase. In this regard, the first signal generator36and the second signal generator37for generation of said read signal Vr(and of the corresponding driving signal in phase opposition of the ICMPC control circuit) can be obtained with any known technique, and in particular, in a known way which is not described in detail herein, by means of switches connected to lines at reference potentials.

The value of the control capacitance Cpaand the amplitude of the driving signalVrcan moreover be different, provided that they enable generation of a common-mode amount of charge such as to balance the common-mode amount of charge generated by the differential capacitive sensor; for example, the value of the control capacitance Cpacould be twice the sense capacitance Cs(at the expense, however, of a certain increase in the output noise), and the variation of the driving signalVrcould have a value equal to one half of the corresponding variation of the read signal. In this case, the first common-mode ICMPC control circuit operates correctly as long as a ratio between the values of the control capacitance and the sense capacitance is kept at a design value.

Finally, as mentioned, the interface circuit is of the switched-capacitor type, and so switches (in a known way which is consequently not described herein) are envisaged and controlled in an appropriate way to enable biasing and reading operations.