The present invention relates generally to adaptive digital non-linearity compensation on a silicon microphone and a corresponding system.
Generally, silicon microphones (also referred to as “digital microphones”) include an analog-to-digital converter (ADC) for converting an analog signal from a micro-electro-mechanical system (MEMS) device into a digital signal. The digital signal also includes noise generated by the ADC, which affects the signal-to-noise ratio (SNR) of the digital microphone. The digital signal also includes nonlinearities caused by both the ADC and the MEMS device, which affects the distortion of the digital microphone.
Market trends regarding digital microphones compel higher SNRs and lower distortion levels. In the design of traditional microphone systems, solutions for improving either of these two specifications are usually inversely correlated. This leads to a trade-off between improving SNR and improving distortion. Thus, improving SNR of the microphone will generally result in increased distortion levels, whereas improving linearity of the microphone will generally result in a lower SNR.
According to an embodiment, an apparatus includes a nonlinear system configured for receiving an input signal; a digital nonlinear compensation component having an input coupled to an output of the nonlinear system, and having an output for generating an output signal; a low pass filter having an input coupled to the output of the digital nonlinear compensation component; a first summer having a first input configured for receiving a digital reference value and a second input coupled to an output of the low pass filter; and an error minimization component having an input coupled to an output of the first summer, and an output coupled to the digital nonlinear compensation component.
According to an embodiment, an apparatus includes a nonlinear system configured for receiving an input signal; a first low pass filter having an input coupled to an output of the nonlinear system; a first summer having a first input coupled to an output of the first low pass filter, and having a second input coupled to the output of the nonlinear system; a digital nonlinear compensation component having an input coupled to an output of the first summer, and having an output for generating an output signal; a second low pass filter having an input coupled to the output of the digital nonlinear compensation component; a second summer having a first input coupled to of the first low pass filter, and having a second input coupled to an output of the second low pass filter; and an error minimization component having an input coupled to an output of the second summer, and an output coupled to the digital nonlinear compensation component.
According to an embodiment, a method includes converting an analog signal into a digital signal, wherein the analog signal includes nonlinearities; compensating the digital signal using a nonlinear transfer function fitted to the nonlinearities in the analog signal to provide a linearized digital signal; generating an error voltage from the linearized digital signal; reducing the error voltage to generate a reduced error voltage; and updating the nonlinear transfer function with the reduced error voltage.
For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which:
The making and using of the presently preferred embodiments are discussed in detail below. It should be appreciated, however, that the present invention provides many applicable inventive concepts that can be embodied in a wide variety of specific contexts. The specific embodiments discussed are merely illustrative of specific ways to make and use the invention, and do not limit the scope of the invention.
In the following detailed description, reference is made to the accompanying drawings, which form a part hereof and in which are shown by way of illustrations specific embodiments in which the invention may be practiced. It is to be understood that other embodiments may be utilized and structural or logical changes may be made without departing from the scope of the present invention. For example, features illustrated or described for one embodiment can be used on or in conjunction with other embodiments to yield yet a further embodiment. It is intended that the present invention includes such modifications and variations. The examples are described using specific language, which should not be construed as limiting the scope of the appending claims. The drawings are not scaled and are for illustrative purposes only. For clarity, the same or similar elements have been designated by corresponding references in the different drawings if not stated otherwise.
According to embodiments, an apparatus and method for digital systems such as a digital microphone allows lowering distortion without impacting the SNR of the system. Improvements in the system SNR can thus be made independently from distortion specifications and leads to an overall enhancement of system performance. The non-linearity generated by the system from both the MEMS device and the readout circuit is compensated in the digital signal processing path through a nonlinear compensation component that is described below. Various embodiments of the nonlinear compensation component are described in further detail in co-pending U.S. patent application Ser. No. 17/675,801, entitled “Digital Non-Linearity Compensation in a Silicon Microphone” that is hereby incorporated by reference in its entirety. The nonlinear compensation component can include open loop embodiments and closed loop embodiments. For example, in an open loop embodiment, a non-linear correction function, such as a polynomial function can be applied to the digitized output of the MEMS device and readout signal in order to linearize the signal. In closed loop embodiments, linearity correction may be achieved by using a non-linear model of the system in a feedback path of a control loop.
The non-linearity of a system, such as a digital microphone, can be modelled through accurate simulations that model the response of the MEMS device and readout circuit at different input sound wave pressures. Knowing the non-ideality of the transfer function of the digital system, it is possible to apply a correction in the digital domain with a nonlinear compensation component to obtain an output signal with an improved linearity with respect to an uncorrected digital system.
For clarity, a generalized digital system transfer function is shown in
For an embodiment nonlinear compensation component, a third order polynomial can be described by the equation: VOUT=VIN+k1*VIN2+k2*VIN3, wherein the coefficients k1 and k2 are determined by measuring the output total harmonic distortion THD0, wherein THD0 is the uncompensated total harmonic distortion THD measured at the output of the digital system. Once the characteristics of THD0 are measured, the coefficients k1 and k2 can be adjusted such that the transfer function of the digital system is linear and the THD is improved with respect to THD0. In an embodiment, the THD0 measurements and adjustment of the coefficients k1 and k2 can be performed on a product including the digital system during system test and before the product is shipped to the customer.
For another embodiment nonlinear compensation component, a second order polynomial can be described by the equation: VOUT=VIN+k1*VIN2, wherein the coefficient k2 is similarly determined by measuring THD0, wherein THD0 is the uncompensated THD measured at the output of the digital system. Once the characteristics of THD0 are measured, the coefficient k1 can be adjusted such that the transfer function of the digital system is linear and the THD is improved with respect to THD0. In an embodiment, the THD0 measurements and adjustment of the coefficient k1 can be performed on a product including the digital system during fabrication and before the product is shipped to the customer.
The digital nonlinear compensation component thus associates at each input voltage value a corresponding corrected output tracking the ideal linear desired behavior of the digital system. The digital correction function is obtained with a fitting polynomial that can be second order or third order, and is made as low of an order as possible to in order to reduce system complexity. Higher order polynomials can also be used if desired in some embodiments.
As the non-linearity of the digital system is strongly process dependent it is desirable to adjust or optimize the polynomial to cover the process variations. Different coefficients and different order polynomials can be used for different digital systems. The choice of the proper correction function is performed in a calibration of the digital system, such as a digital microphone, and is based on the measurement of the system THD0 without compensation applied. A very accurate modelling of the system is desired when building the correction functions, as the method relies on the prediction of the distortion introduced by the digital specific system. In embodiments, the measured effect on an existing digital system product can result in a THD reduction on the order of 20 dB.
While the above compensation embodiments provide significant benefits when compared to uncompensated digital microphones and nonlinear systems, the nonlinear compensation component coefficients are determined during an initial calibration phase that may include a training signal. The training signal is a signal that scans the appropriate frequency range in a specific sequence so that the coefficients can be properly determined. The initial calibration phase may occur after fabrication of the nonlinear system, but before the nonlinear system is placed in a normal operating mode. Thus, the choice of the proper correction function of the nonlinear compensation component is performed in the calibration phase of the digital microphone or nonlinear system, and is based on the measurement of the system total harmonic distortion (THD) without the compensation being applied. A very accurate modelling of the nonlinear system is thus needed when building the correction function, as the above method relies on the accurate prediction of the distortion introduced by a specific nonlinear system. As the non-linearity of the system is often strongly process dependent, and may even change over time and in response to environmental effects, a more flexible compensation method may be desirable in some applications that addresses these process variations, aging, and environmental effects.
According to embodiments, an adaptive calibration apparatus, system and method for a nonlinear compensation component is described in detail below. The embodiment calibration method simplifies the calibration process and also enables periodic or continuous calibration during a normal operational mode. The non-linearity generated by a nonlinear system comprising a MEMS device and readout circuitry is compensated in the digital signal processing path. For the adaptation/calibration of the optimal parameters (coefficients), no specific training signal and no specific calibration phase is needed.
The transfer function of the nonlinear compensation component 406 is a second order polynomial transfer function described by the following equation:
x
lin=1+c1[k]*ynl2.
The coefficient c1[k] of the second order term is continually updated by the action of the error minimization component 420 that is in communication with the digital nonlinear compensation component 406. Error minimization component 420 receives an error signal and generates the adapted c1[k] coefficient based on the error signal. The error minimization function of the error minimization component 420 will be explained in further detail below. A second order polynomial transfer function is used because the squaring function will always provide a non-zero positive error signal (which can also be considered an “offset”) no matter what type of input signal is presented to the linearized system.
The transfer function of the nonlinear compensation component 506 is a second order polynomial transfer function described by the following equation:
x
lin=1+c2[k]*ynl2.
The coefficient c2[k] of the second order term is continually updated by the action of the error minimization component 520 that is in communication with the digital nonlinear compensation component 506. Error minimization component 420 receives an error signal e[k] and generates the adapted c2[k] coefficient based on the error signal e[k]. The error minimization function of the error minimization component 520 will be explained in further detail below.
In some embodiments ASIC 304 can comprise a single integrated circuit, two or more integrated circuits, individual digital and analog components, processors, or a combination thereof. In some embodiments MEMS device 202 can comprise a capacitive MEMS device fabricated out of silicon, and having one or more flexible membranes, and one or more fixed membranes.
Example embodiments of the present invention are summarized here. Other embodiments can also be understood from the entirety of the specification and the claims filed herein.
Example 1. According to an embodiment, an apparatus includes a nonlinear system configured for receiving an input signal; a digital nonlinear compensation component having an input coupled to an output of the nonlinear system, and having an output for generating an output signal; a low pass filter having an input coupled to the output of the digital nonlinear compensation component; a first summer having a first input configured for receiving a digital reference value and a second input coupled to an output of the low pass filter; and an error minimization component having an input coupled to an output of the first summer, and an output coupled to the digital nonlinear compensation component.
Example 2. The apparatus of Example 1, wherein the digital nonlinear compensation component includes a second order transfer function.
Example 3. The apparatus of any of the above examples, wherein the second order transfer function includes an adapted second order coefficient.
Example 4. The apparatus of any of the above examples, wherein the digital reference value includes a logic zero value.
Example 5. The apparatus of any of the above examples, wherein the error minimization component includes a step size generator having an input configured for receiving an error signal from the first summer; a second summer having a first input coupled to an output of the step size generator; and an integrator having an input coupled to an output of the second summer and an output coupled to a second input of the second summer, wherein the output of the integrator is configured for generating an adapted coefficient.
Example 6. The apparatus of any of the above examples, wherein the step size generator is configured for generating a constant step size.
Example 7. The apparatus of any of the above examples, wherein the step size generator is configured for generating a step size including a function of the error signal.
Example 8. The apparatus of any of the above examples, wherein the nonlinear system includes a digital microphone.
Example 9. According to an embodiment, an apparatus includes a nonlinear system configured for receiving an input signal; a first low pass filter having an input coupled to an output of the nonlinear system; a first summer having a first input coupled to an output of the first low pass filter, and having a second input coupled to the output of the nonlinear system; a digital nonlinear compensation component having an input coupled to an output of the first summer, and having an output for generating an output signal; a second low pass filter having an input coupled to the output of the digital nonlinear compensation component; a second summer having a first input coupled to of the first low pass filter, and having a second input coupled to an output of the second low pass filter; and an error minimization component having an input coupled to an output of the second summer, and an output coupled to the digital nonlinear compensation component.
Example 10. The apparatus of Example 9, wherein the digital nonlinear compensation component includes a second order transfer function.
Example 11. The apparatus of any of the above examples, wherein the second order transfer function includes an adapted second order coefficient.
Example 12. The apparatus of any of the above examples, wherein the error minimization component includes a step size generator having an input configured for receiving an error signal from the first summer; a third summer having a first input coupled to an output of the step size generator; and an integrator having an input coupled to an output of the third summer, and having an output coupled to a second input of the third summer, wherein the output of the integrator is configured for generating an adapted coefficient.
Example 13. The apparatus of any of the above examples, wherein the step size generator is configured for generating a constant step size.
Example 14. The apparatus of any of the above examples, wherein the step size generator is configured for generating a step size including a function of the error signal.
Example 15. The apparatus of any of the above examples, wherein the nonlinear system includes a digital microphone.
Example 16. According to an embodiment, a method includes converting an analog signal into a digital signal, wherein the analog signal includes nonlinearities; compensating the digital signal using a nonlinear transfer function fitted to the nonlinearities in the analog signal to provide a linearized digital signal; generating an error voltage from the linearized digital signal; reducing the error voltage to generate a reduced error voltage; and updating the nonlinear transfer function with the reduced error voltage.
Example 17. The method of any of the above examples, further including iteratively reducing the error voltage.
Example 18. The method of any of the above examples, wherein the error voltage is reduced until a predetermined minimum error voltage is attained.
Example 19. The method of any of the above examples, wherein reducing the error voltage includes iteratively reducing the error voltage by a fixed amount, or iteratively reducing the error voltage by an amount that is a function of the error voltage.
Example 20. The method of claim 16, wherein the nonlinear transfer function includes a second order transfer function.
While this invention has been described with reference to illustrative embodiments, this description is not intended to be construed in a limiting sense. Various modifications and combinations of the illustrative embodiments, as well as other embodiments of the invention, will be apparent to persons skilled in the art upon reference to the description. It is therefore intended that the appended claims encompass any such modifications or embodiments.