Circuit for sensing an analog signal, corresponding electronic system and method

Abstract

A circuit configured to sense an input analog signal generated by a sensor at a first frequency and to generate an output digital signal indicative of the sensed input analog signal. The circuit includes a conditioning circuit, an ADC, a feedback circuit, and a low-pass filter. The conditioning circuit is configured to receive the input analog signal and to generate a conditioned analog signal. The ADC is configured to provide a converted digital signal based on the conditioned analog signal. The feedback circuit includes a band-pass filter configured to selectively detect a periodic signal at a second frequency higher than the first frequency and to act on the conditioning circuit to counter variations of the periodic signal at the second frequency. The low-pass filter is configured to filter out the periodic signal from the converted digital signal to generate the output digital signal.

Claims

1. A sensing circuit comprising: a conditioning circuit configured to receive, at an input port, an input analog signal generated by a sensor at a first frequency, and to generate, at an output port, a conditioned analog signal; an analog-to-digital converter (ADC) coupled to the output port of the conditioning circuit, the ADC configured to receive the conditioned analog signal and provide, at a converter output node, a converted digital signal based on the conditioned analog signal, the converted digital signal being indicative of the input analog signal; a feedback circuit coupled between the output port of the conditioning circuit and a control input of the conditioning circuit, the feedback circuit comprising a band-pass filter configured to selectively detect a periodic signal at a second frequency, the second frequency being higher than the first frequency, wherein the feedback circuit is configured to act on the conditioning circuit to counter variations of the periodic signal at the second frequency; and a signal generator circuit configured to superimpose the periodic signal at the second frequency to the input analog signal received at the conditioning circuit.

2. The sensing circuit of claim 1, wherein the feedback circuit is coupled to the output port of the conditioning circuit via the ADC.

3. The sensing circuit of claim 1, further comprising a low-pass filter coupled to the converter output node of the ADC and having a low-pass cut-off frequency higher than the first frequency, the low-pass filter configured to filter out the periodic signal from the converted digital signal to generate an output digital signal indicative of the input analog signal.

4. The sensing circuit of claim 3, wherein the low-pass cut-off frequency is lower than the second frequency.

5. The sensing circuit of claim 3, wherein the low-pass cut-off frequency is higher than the second frequency.

6. The sensing circuit of claim 1, wherein the periodic signal is a square wave signal or a sinusoidal signal.

7. The sensing circuit of claim 1, wherein the periodic signal is a current signal.

8. The sensing circuit of claim 1, wherein the feedback circuit is configured to detect an amplitude or phase of the periodic signal at the second frequency and act on the conditioning circuit based on the detected amplitude or phase to counter variations of the periodic signal at the second frequency.

9. The sensing circuit of claim 1, wherein the signal generator circuit comprises a voltage oscillator and a voltage-to-current converter circuit configured to generate the periodic signal at the second frequency in the form of a square wave current signal.

10. The sensing circuit of claim 1, wherein the signal generator circuit is configured to superimpose the periodic signal at the second frequency to the input analog signal received at the conditioning circuit by injecting the periodic signal into a supply voltage of the sensor.

11. The sensing circuit of claim 1, wherein the conditioning circuit comprises an operational amplifier, and wherein the signal generator circuit is configured to superimpose the periodic signal at the second frequency to the input analog signal received at the conditioning circuit by injecting the periodic signal directly into an input of the operational amplifier.

12. The sensing circuit of claim 11, wherein the input analog signal is a differential signal, and wherein the periodic signal is a differential signal.

13. The sensing circuit of claim 1, wherein the signal generator circuit is configured to generate the periodic signal by applying a temperature-independent reference voltage to a resistor.

14. A micro-electro-mechanical (MEM) system comprising: a MEM actuator; a piezoresistive sensor configured to sense mechanical information of the MEM actuator and produce an input analog signal based on the mechanical information of the MEM actuator; and a sensing circuit comprising: a conditioning circuit configured to receive, at an input port, the input analog signal, and to generate, at an output port, a conditioned analog signal; an analog-to-digital converter (ADC) coupled to the output port of the conditioning circuit, the ADC configured to receive the conditioned analog signal and provide, at a converter output node, a converted digital signal based on the conditioned analog signal, the converted digital signal being indicative of the input analog signal; a signal generator circuit configured to superimpose a periodic signal to the input analog signal; and a feedback circuit coupled between the output port of the conditioning circuit and a control input of the conditioning circuit, wherein the feedback circuit is configured to act on the conditioning circuit to counter variations of the periodic signal.

15. The MEM system of claim 14, wherein the signal generator circuit is configured to inject the periodic signal to a supply node of the piezoresistive sensor.

16. The MEM system of claim 14, wherein the conditioning circuit comprises an operational amplifier configured to receive the analog input signal, and wherein the signal generator circuit is configured to inject the periodic signal directly to an input of the operational amplifier.

17. The MEM system of claim 14, wherein the MEM actuator is a micro-mirror and wherein the mechanical information comprises deformation or movement of the micro-mirror.

18. A method comprising: generating, with a sensor, an input analog signal at a first frequency; superimposing a periodic signal at a second frequency to the input analog signal to generate a combined analog signal, wherein the second frequency is higher than the first frequency; generating, using a conditioning circuit, a conditioned analog signal based on the combined analog signal; selectively detecting the periodic signal from the conditioned analog signal; and acting on the conditioning circuit to counter variations of the periodic signal detected from the conditioned analog signal.

19. The method of claim 18, further comprising: converting the conditioned analog signal to a converted digital signal; and filtering out the periodic signal from the converted digital signal to generate an output digital signal, wherein selectively detecting the periodic signal from the conditioned analog signal comprises selectively detecting the periodic signal from the converted digital signal.

20. The method of claim 18, wherein superimposing the periodic signal to the input analog signal to generate the combined analog signal comprises injecting the periodic signal to a supply terminal of the sensor.

21. The method of claim 18, wherein superimposing the periodic signal to the input analog signal to generate the combined analog signal comprises injecting the periodic signal to an operational amplifier of the conditioning circuit.

22. The method of claim 18, further comprising generating the periodic signal by applying a temperature independent reference voltage to a resistor.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) One or more embodiments will now be described, by way of example only, with reference to the annexed figures, wherein:

(2) FIG. 1 shows an exemplary piezoresistive sensor and sensing circuit;

(3) FIG. 2A shows an example of a possible simplified transfer function of the AFE of FIG. 1.

(4) FIG. 2B shows an example of possible waveforms of the sensing circuit of FIG. 1;

(5) FIG. 3 illustrates the presence of parasitic capacitances in the circuit of FIG. 1;

(6) FIG. 4 is a circuit diagram exemplary of a possible architecture of an AFE circuit;

(7) FIG. 5 is a circuit diagram exemplary of embodiments;

(8) FIG. 6 is exemplary of implementation details of the embodiments of FIG. 5;

(9) FIG. 7 is a circuit diagram exemplary of further embodiments; and

(10) FIG. 8 is exemplary of implementation details of the embodiments of FIG. 7.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

(11) In the ensuing description, one or more specific details are illustrated, aimed at providing an in-depth understanding of examples of embodiments of this description. The embodiments may be obtained without one or more of the specific details, or with other methods, components, materials, etc. In other cases, known structures, materials, or operations are not illustrated or described in detail so that certain aspects of embodiments will not be obscured.

(12) Reference to “an embodiment” or “one embodiment” in the framework of the present description is intended to indicate that a particular configuration, structure, or characteristic described in relation to the embodiment is comprised in at least one embodiment. Hence, phrases such as “in an embodiment” or “in one embodiment” that may be present in one or more points of the present description do not necessarily refer to one and the same embodiment. Moreover, particular conformations, structures, or characteristics may be combined in any adequate way in one or more embodiments.

(13) Throughout the figures annexed herein, like parts or elements are indicated with like references/numerals and a corresponding description will not be repeated for brevity.

(14) The references used herein are provided merely for convenience and hence do not define the extent of protection or the scope of the embodiments.

(15) By way of general introduction to the detailed description of exemplary embodiments, reference may be first had to FIG. 4.

(16) FIG. 4 is a circuit diagram exemplary of a possible architecture of an AFE circuit 121, having input nodes PZR_P and PZR_N for coupling to the output of a sensor 10 (e.g., a piezoresistive sensor cooperating with a micro-mirror) and output nodes AFE_P and AFE_N for providing a conditioned replica V.sub.S,C of the input signal V.sub.S received from the sensor 10.

(17) In particular, an AFE circuit 121 comprises analog amplifiers such as operational amplifiers which implement a single-stage or multi-stage amplifier circuit. Various stages of the AFE circuit 121 are coupled by using RC networks, optionally tunable (e.g., by using variable resistors, as exemplified in FIG. 4, and/or variable capacitors). The input stage of the AFE circuit 121 may comprise a pair of operational amplifiers, having respective first input nodes coupled to the nodes PZR_P and PZR_N for receiving the input signal V.sub.S.

(18) Generally, the frequency behavior of an AFE circuit 121 as exemplified in FIG. 4 may be approximated, in the frequency range of interest for micro-mirror applications, with a single-pole transfer function as already discussed with reference to FIG. 2A. Generally, the frequency f.sub.o of such single pole may be a decade higher than the frequency f.sub.S of the input signal V.sub.S received at nodes PZR_P and PZR_N. For example, f.sub.o may have a value in the range from a few kHz (e.g., 3 kHz) to hundreds of kHz (e.g., 700 kHz).

(19) The transfer function of an AFE circuit 121 may comprise further poles and/or zeroes, which are however typically located at frequencies which are not of interest in the context of the present description.

(20) According to some embodiments, e.g., and as exemplified in FIG. 5, a circuit 12 for sensing an analog signal V.sub.S generated by a sensor 10 comprises: an AFE circuit 121 configured for conditioning an analog input signal V.sub.S sensed between nodes PZR_P and PZR_N, thereby generating an analog conditioned signal V.sub.S,C; an ADC 122 configured for providing a digital output signal ADC_out resulting from conversion to digital of the analog conditioned signal V.sub.S,C provided by the AFE circuit 121; a feedback circuit 125 coupled between the output node of the ADC 122 and a control input of the AFE circuit 121, the feedback circuit 125 being configured to detect, at the output of the ADC 122, a reference periodic signal at a frequency f.sub.H higher than (or comparable to) the cut-off frequency f.sub.o of the AFE circuit 121 (thus, also higher than the frequency f.sub.S: f.sub.H>f.sub.o>f.sub.S) propagated through the AFE circuit 121 and the ADC 122, and act on the AFE circuit 121 to counter amplitude and/or phase variations of the reference periodic signal detected; and a digital low-pass filter 126 coupled at the output of the ADC 122, the digital low-pass filter 126 having a cut-off frequency f.sub.126 lower than the cut-off frequency f.sub.o of the AFE circuit 121 and lower than the frequency f.sub.H of the reference periodic signal.

(21) Therefore, the digital low-pass filter 126 is configured to filter the digital output signal ADC_out to provide a further digital output signal ADC_out′ at frequency f.sub.S indicative of the analog signal V.sub.S generated by the sensor 10, and not disturbed by the propagation of a signal at frequency f.sub.H through the AFE circuit 121 and the ADC 122.

(22) The frequency f.sub.H is preferably higher than f.sub.o (e.g., f.sub.H>2f.sub.o), e.g., to facilitate filtering out the reference periodic signal at the low-pass filter 126.

(23) However, in some embodiments, operation of a circuit as disclosed herein may be effective provided that the frequency f.sub.H is “comparable” to the cut-off frequency f.sub.o, i.e., provided that propagation of a signal at frequency f.sub.H through the AFE circuit 121 is affected (in terms of amplitude and/or phase) by variations (even slight variations) of the cut-off frequency f.sub.o. To this regard, frequency f.sub.H may also be lower than the cut-off frequency f.sub.o.

(24) In one or more embodiments, the reference periodic signal at frequency f.sub.H is superimposed to the input signal V.sub.S received at nodes PZR_P and PZR_N of the AFE circuit 121.

(25) In a circuit arrangement as described above and illustrated in FIG. 6, the reference periodic signal at frequency f.sub.H (e.g., a square wave signal) propagates through the AFE circuit 121 to the output nodes AFE_P and AFE_N, and thus through the ADC circuit 122, in a way analogous to that of the input analog signal V.sub.S received at nodes PZR_P and PZR_N from the sensor 10 (also referred to as the “effective input signal” in the present description).

(26) As a result of the frequency f.sub.H being higher than the frequency f.sub.S and higher than (or comparable to) the cut-off frequency f.sub.o, propagation (in terms of amplitude and/or phase) of the reference periodic signal at frequency f.sub.H is affected (also) by slight variations of the cut-off frequency f.sub.o (e.g., due to temperature variations).

(27) For instance, as a result of the cut-off frequency f.sub.o (slightly) decreasing, the amplitude of the reference periodic signal propagated at the output of the ADC circuit 122 may decrease (i.e., attenuation may take place) and the phase may negatively shift.

(28) Conversely, as a result of the cut-off frequency f.sub.o (slightly) increasing, the amplitude of the reference periodic signal propagated may increase (i.e., amplification may take place) and the phase may positively shift.

(29) Therefore, sensing amplitude and/or phase variations of the reference periodic signal at frequency f.sub.H may facilitate acting on the AFE circuit 121 in order to compensate (even slight) variations of the cut-off frequency f.sub.o, with the aim of letting the effective input signal V.sub.S at frequency f.sub.S propagate unaffected.

(30) The feedback circuit 125 is configured to filter the output digital signal ADC_out sensed at the output of the ADC 122 with a band-pass filter 1251 centered around the frequency f.sub.H of the reference periodic signal (i.e., f.sub.1251≈f.sub.H).

(31) The feedback circuit 125 may thus be configured to measure the amplitude of such filtered signal at the frequency f.sub.H, e.g., measuring the amplitude of the 1st harmonic, and to drive a current-output digital-to-analog converter (DAC) 1253 in order to vary the bias current of at least one operational amplifier in the AFE circuit 121 so as to counter (any) variation of the amplitude of the filtered signal, with this operation also resulting in countering (any) variation (or shift) of the frequency f.sub.o of the main pole of the AFE circuit 121.

(32) Alternatively or additionally, in some embodiments, the feedback circuit 125 may be configured to measure the phase of such filtered signal at the frequency f.sub.H and to drive the current-output DAC converter 1253 in order to vary the bias current of at least one operational amplifier in the AFE circuit 121 so as to counter (any) variation of the phase of the filtered signal, with this operation also resulting in countering any variation (or shift) of the frequency f.sub.o of the main pole of the AFE circuit 121.

(33) Alternatively or additionally, the feedback circuit 125 may be configured to operate on parameter(s) of at least one operational amplifier in the AFE circuit 121 other than the bias current, in order to counter (any) variation (be it an amplitude variation or a phase variation) of the filtered signal at the frequency f.sub.H.

(34) Alternatively or additionally, in some embodiments, the feedback circuit 125 may be configured to operate on other parameters of the AFE circuit 121, even not related to the operational amplifiers included therein (e.g., values of resistors and/or capacitors).

(35) Therefore, the feedback circuit 125 implements a sort of control loop operating in parallel with the normal behavior of the AFE circuit 121, in order to (indirectly) measure the sign and magnitude of the phase shift of the AFE circuit 121 (e.g., due to temperature variations) and consequently adjust the biasing and/or other parameters of the AFE circuit 121 to compensate such phase shift.

(36) As described, such measuring the sign and magnitude of the phase shift of the AFE circuit 121 may be performed by: directly measuring the phase shift of the reference periodic signal propagated through the AFE circuit 121 and the ADC circuit 122, and/or measuring the amplitude variation of the reference periodic signal propagated through the AFE circuit 121 and the ADC circuit 122, and correlating such amplitude variation to a corresponding phase shift.

(37) In the above exemplified embodiments, the feedback circuit 125 takes as digital input the signal ADC_out and operates through discrete steps in order to keep constant the amplitude and/or the phase of the propagated reference periodic signal, thereby facilitating keeping constant the cut-off frequency f.sub.o of the main pole of the transfer function of the AFE circuit 121.

(38) The width of such discrete steps should be lower than the maximum phase shift that can be tolerated in the specific application, and the number of available steps should be related to the range of the phase shift for which compensation is sought.

(39) Optionally, in some embodiments, additional digital filtering may be implemented in the feedback circuit 125 by a digital filter circuit 1252 between the band-pass filter 1251 and the current-output DAC converter 1253, e.g., to filter out noise and provide a low-frequency feedback behavior of the feedback circuit 125. The amplitude of the signal at the frequency f.sub.H filtered by the band-pass filter 1251 may be quite low and possibly may have a low signal-to-noise ratio (SNR), so that low-pass digital filtering at the filter circuit 1252 may facilitate detecting slow variations of the feedback signal (e.g., over a time scale comparable with the typical time scale of temperature variations) and thus provide an improved dynamic range of the measured feedback signal.

(40) Therefore, in some embodiments, additional digital filtering performed at the digital filter circuit 1252 may be advantageous in providing a low-frequency compensation signal (e.g., a current compensation signal with frequency lower than f.sub.S) to improve stability of the system.

(41) In one or more embodiments, the circuit 12 may comprise a signal generator circuit 124, configured to generate the reference periodic signal at the frequency f.sub.H and providing such reference periodic signal to the AFE circuit 121 so to be superimposed to the analog input signal V.sub.S.

(42) In particular, as exemplified in FIG. 6, the signal generator circuit 124 is configured for generating a reference periodic signal such as a square wave signal, a sinusoidal signal, or others.

(43) For instance, the signal generator circuit 124 may comprise a voltage oscillator and a voltage-to-current (V2I) circuit in order to generate a square-wave reference current signal. In some embodiments, generating the reference periodic signal(s) by using a voltage oscillator and a voltage-to-current circuit may facilitate reducing the dependency on the gain resistance of the AFE circuit 121.

(44) As exemplified in FIG. 6, the reference periodic signal(s) may be input as current signal(s) at the inverting inputs of the operational amplifiers in the first stage of the AFE circuit 121. Such current signals result in an equivalent input voltage having a value which depends on the current magnitude and on the gain resistance of the first stage of the AFE circuit 121, and which is further amplified in the following stages of the AFE circuit 121. The reference periodic current signals may be generated by applying a precise and temperature-independent reference voltage (e.g., a bandgap reference) to a resistance of the same technological type as the gain resistance and matched thereto, so that the magnitude of the equivalent input voltage may not depend on the value of the reference periodic current signal(s) or the value of the gain resistance. In some embodiments, the magnitude of the equivalent input voltage may depend (almost) only on the value of the reference voltage, so to be in turn almost temperature-independent.

(45) Optionally, in some embodiments, the signal generator circuit 124 may generate a pair of reference periodic signals, e.g., square-wave current signals, with the second signal in the pair being anti-phased with respect to the first signal in the pair.

(46) The first signal in the pair of reference periodic signals may be provided at a second input of the first operational amplifier of the input stage of the AFE circuit 121 (with the first input of the first operational amplifier configured for coupling to node PZR_P), and the second signal in the pair of reference periodic signals may be provided at a second input of the second operational amplifier of the input stage of the AFE circuit 121 (with the first input of the second operational amplifier configured for coupling to node PZR_N).

(47) It will be understood that providing a pair of anti-phased reference periodic signals as exemplified in FIG. 6 may be advantageous in the context of a fully-differential architecture as illustrated. However, in one or more embodiments a single reference periodic signal may be provided to the AFE circuit 121.

(48) According to one or more embodiments, e.g., and as exemplified in FIG. 7, the signal generator circuit 124 may be configured for providing the reference periodic signal at a frequency f.sub.H to the sensor 10 instead of the AFE circuit 121.

(49) In the embodiments of FIG. 7, the reference periodic signal is superimposed to the “effective input signal” V.sub.S generated by the sensor 10 directly at the sensor 10.

(50) For instance, as exemplified in FIG. 8, the reference periodic signal may be superimposed to the bias voltage of the piezoresistive sensor 10, being propagated to the output signal V.sub.S thereby.

(51) Therefore, in the circuit arrangement illustrated in FIG. 8, the reference periodic signal at frequency f.sub.H propagates not only through the AFE circuit 121, but also through the electrically conductive lines which couple the sensor 10 to the sensing circuit 12.

(52) As a result, propagation (in terms of amplitude and/or phase) of the reference periodic signal at frequency f.sub.H may be affected not only by variations of the cut-off frequency f.sub.o, but also by variations of the frequency of the low-pass filter pole due to the parasitic capacitances C.sub.P and C.sub.N combined with the piezoresistors R1, R2, R3, and R4 in the piezoresistive sensor 10.

(53) Embodiments according to the topology illustrated in FIGS. 7 and 8, therefore, advantageously provide means for compensating phase shifts of a whole “sensor plus circuit” system, by making the reference periodic signal propagate through the entire propagation path of the effective signal V.sub.S.

(54) It will be appreciated that various additional or optional features described with reference to the embodiments of FIGS. 5 and 6 may apply also to the embodiments of FIGS. 7 and 8.

(55) Embodiments of the present disclosure thus may facilitate reducing phase shift phenomena in AFE circuits, e.g., due to temperature variations, by using a feedback loop configured for sensing a reference periodic signal superimposed to the effective input signal and having a frequency higher than the frequency of the effective input signal, with improved performance over the prior art approaches.

(56) Additionally, embodiments of the present disclosure may provide the possibility of implementing AFE circuits, such as 121, with an “aggressive” low-pass filter functionality (i.e., with a dominant pole frequency f.sub.o closer to the signal frequency f.sub.S if compared to prior art solutions) which may advantageously provide improved noise rejection performance.

(57) One or more embodiments are suitable for use with continuous-time AFE circuits and rely on few additional circuits for implementing phase shift compensation, thereby resulting in a negligible increase of silicon area and power consumption if compared to non-compensated circuits.

(58) One or more embodiments may facilitate compensating the effect of non-negligible parasitic capacitances C.sub.P, C.sub.N and/or the effect of piezoresistors with PTC behavior without the need of NTC resistors being mounted on the PCB.

(59) One or more embodiments are effective in providing phase shift compensation not only with respect to temperature variations, but also with respect to power supply variations or other variations of operating conditions.

(60) Embodiments of the present description advantageously do not require an expensive temperature calibration process.

(61) Without prejudice to the underlying principles, the details and embodiments may vary, even significantly, with respect to what has been described byway of example only, without departing from the extent of protection.