High dynamic capacitance type digital microphone system based on electrostatic force feedback

Abstract

The application discloses a high dynamic capacitance type digital microphone system based on electrostatic force feedback, comprising a MEMS microphone and an interface circuit chip thereof, wherein the interface circuit chip comprises a charge pump, an analog front end, an analog-digital conversion circuit, an amplitude detection module, an amplitude adjustment control module and a pulse density modulation module. The application utilizes the characteristic that the capacitance type sensor can adjust sensitivity by changing a bias voltage, adopts an output voltage adjustable charge pump to provide electrostatic force feedback, further improves the adjustable range of the front end gain, and relaxes the requirement on the noise performance of a readout circuit. Therefore, the system has the characteristics of high dynamic range and high precision quantization.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of CMOS integrated circuits, and particularly relates to a high dynamic capacitance type digital microphone system based on electrostatic force feedback. BACKGROUND

[0002] A digital microphone and an interface circuit chip thereof are electronic devices capable of converting an analog audio signal input from the outside into a digital signal through filtering, amplification and other processing. The digital microphone interface circuit converts an analog signal into a digital signal, is not disturbed and affected by other signal sources from the outside, has strong anti-interference ability, and is more easily processed in a computer system with high efficiency compared with an analog signal, and thus is widely applied in many fields such as language communication, medical diagnosis, environmental monitoring and the like. The digital microphone and the interface circuit chip thereof are usually composed of a micro-electro-mechanical system (MEMS) for converting a sound signal into an electric signal, an analog front end (AFE) for analog signal processing, and an analog-to-digital converter (ADC) for converting an analog signal into a digital signal.

[0003] There are some problems in the process of capturing and digitizing an audio signal:

[0004] ① Quantization error: when the change of an analog signal is smaller than the quantization step of an ADC, a quantization error will be generated.

[0005] ② Signal saturation: when an input analog signal is too large, information will be lost after being amplified by an analog front end; when an input analog signal is too small, it will be submerged in noise and cannot be measured.

[0006] ③ Noise amplification: noise will be amplified together with a signal.

[0007] The influence of quantization error and noise amplification can be greatly reduced by improving the accuracy of an ADC and reducing the noise of an analog front end. The problem of signal saturation is closely related to the dynamic range of a digital microphone interface circuit. The dynamic range is the range between the system background noise and the maximum output voltage thereof, and the dynamic range of a digital microphone interface circuit is the range of signal amplitude that can be distinguished by the interface circuit. The range of change of an audio signal is large, and if the dynamic range is insufficient, large signals will appear amplitude distortion, and small signals will be difficult to identify, resulting in a decrease in signal-to-noise ratio, and thus a high dynamic range is crucial.

[0008] Capacitive digital microphones are increasingly used in battery-powered devices. Due to the limitations of battery power, capacitive digital microphones tend to be low-voltage and low-power. However, the reduction in voltage severely limits the dynamic range of the signal chain. In order to achieve a high dynamic range under low supply voltage, previous research [E. Bach et al., "9.5A1.8V true-differential 140dB SPL full-scale standard CMOS MEMS digital microphone exhibiting 67dB SNR," 2017 IEEE International Solid-State Circuits Conference (ISSCC), San Francisco, CA, USA, 2017, pp. 166-167] uses a voltage multiplication module to increase the supply voltage of the analog front end, thereby expanding the dynamic range of the analog front end, and uses a double-backplate microphone to achieve a signal-to-noise ratio of 67dB, a maximum acoustic overload point of 136dB SPL However, the power efficiency of the voltage multiplication module is limited, and the overall power consumption of the system is high. Another solution is to use an extremely low-noise and gain-adjustable analog front end to expand the dynamic range, as described in [L. Sant et al., "A 130dB SPL 72dB SNR MEMS Microphone Using a Sealed-Dual Membrane Transducer and a Power-Scaling Read-Out ASIC," in IEEE Sensors Journal, vol. 22, no. 8, pp. 7825-7833, 15 April 15, 2022]. However, due to the strict trade-off between noise performance and power current, in order to suppress the front-end noise to a low level, the current of the system is limited, and the overall power consumption is high. SUMMARY

[0009] In view of the above, the present application provides a high dynamic capacitive digital microphone system based on electrostatic force feedback, which can improve the dynamic range of the system while having the characteristics of high dynamic range, high precision quantization and adjustable gain.

[0010] A high dynamic capacitive digital microphone system based on electrostatic force feedback, comprising a MEMS microphone and an interface circuit chip thereof, the MEMS microphone being configured to convert an externally input variable sound pressure into an audio analog signal, and the interface circuit chip comprising:

[0011] a charge pump with adjustable output voltage, configured to provide a bias voltage for the MEMS microphone;

[0012] Low noise gain adjustable analog front end for filtering and amplifying audio analog signal and outputting differential signal;

[0013] Analog-digital conversion circuit for filtering, sampling, quantizing and outputting digital code word of the signal outputted by the analog front end;

[0014] Amplitude detection module for reproducing high-speed low-bit digital code word into low-speed high-bit signal, obtaining the difference between the maximum value and the minimum value of the signal, and comparing the difference with the set upper and lower threshold values;

[0015] Amplitude adjustment control module for generating voltage control code word and gain control code word according to the comparison result outputted by the amplitude detection module, so as to adjust the output of the charge pump and the analog front end;

[0016] Pulse density modulation module for modulating the digital code word and outputting pulse density modulation wave matching the system input amplitude;

[0017] Power module for providing working voltage and current for each functional circuit module in the chip.

[0018] Further, the sound pickup capacitor of the MEMS microphone is composed of a highly compliant diaphragm, a perforated rigid back plate and a cavity, the rigid back plate has dense perforations, which helps the sound wave to be transmitted to the diaphragm, reduces air damping and noise; the diaphragm has a vent hole, which quickly releases pressure when the diaphragm vibrates, making it easier to vibrate; the cavity is formed by etching a plurality of different additional layers deposited on the silicon wafer, which is used to improve the sensitivity; the sensitivity of the MEMS microphone represents the sound-electric conversion efficiency of the microphone, which is directly related to the bias voltage provided to the MEMS microphone, the higher the bias voltage, the higher the sensitivity of the microphone.

[0019] Further, the charge pump is used to provide electrostatic force feedback, and the feedback depth is adjusted by the voltage control code word provided by the amplitude adjustment control module; the charge pump includes a boost circuit, a sequence reference voltage generation circuit and a logic control circuit, wherein the boost circuit adopts a two-stage Dickson charge pump structure, the sequence reference voltage generation circuit selects a reference voltage through the voltage control code word, the reference voltage enters the logic control circuit and is compared with a feedback voltage (a parameter related to the output voltage of the charge pump), and the comparison result is used to control whether the boost circuit works or not, thereby realizing the function of adjustable output voltage.

[0020] Further, the amplification degree of the analog front end to the audio analog signal is adjusted by the gain control code word provided by the amplitude adjustment control module, and the analog front end has multiple gain positions, and by adjusting the gain when the input signal size is different, the dynamic range of the overall signal chain can be expanded.

[0021] Further, the analog front end is composed of a signal common-mode biasing network, an amplification circuit and a feedback network, the signal common-mode biasing network is located at two input ends of the analog front end and is composed of two pseudo resistors, one end of each pseudo resistor is connected with a common-mode voltage and the other end is connected with an input end of the analog front end, an input signal entering the biasing network is biased by the pseudo resistors to realize high-pass filtering and filter out signals below an audio frequency acceptable by human ears; the amplification circuit is mainly composed of two operational amplifiers, the inverting input ends of the two operational amplifiers are connected through a capacitor, and the non-inverting input ends of the two operational amplifiers correspond to the two input ends of the analog front end; the feedback network is composed of a pseudo resistor, a capacitor array and a control switch of the capacitor array, the capacitor array and the control switch are connected across the two operational amplifiers in a negative feedback form, and the pseudo resistor is connected between a feedback node and an output end of the operational amplifier to provide a voltage for the feedback node, and the gain is determined by the number of capacitors in the capacitor array connected in the feedback loop.

[0022] Further, the analog-to-digital conversion circuit includes an anti-aliasing filter and an analog-to-digital converter, the anti-aliasing filter is a low-pass filter, used to perform low-pass filtering on the signal output by the analog front end and suppress out-of-band signals to prevent noise from aliasing during sampling, and the signal after filtering is output to the analog-to-digital converter, which is used to sample, quantize and output digital code words.

[0023] Further, the amplitude detection module includes a sampling filter for signal reproduction and a digital module for comparing signal amplitudes, the sampling filter converts the high-speed low-bit digital code words output by the analog-to-digital conversion circuit into low-speed high-bit signals, and the sampling conversion process includes low-pass filtering and downsampling, the low-pass filtering uses a sinc filter to filter out out-of-band signals and prevent them from aliasing into the band; the downsampling is realized by an integrator, an accumulation dump extractor and a differentiator, after signal reproduction is completed through downsampling, the digital module takes the lower limit of the bandwidth as the detection frequency, obtains the maximum and minimum values of the reproduced signal in the detection period and calculates the difference, and compares the difference with the set upper and lower threshold values.

[0024] Further, when the comparison result is that the difference is greater than the upper threshold value, the amplitude adjustment control module reduces the gain of the analog front end by outputting a gain control code word and reduces the bias voltage output by the charge pump by outputting a voltage control code word, thereby reducing the sensitivity of the MEMS microphone and expanding the range of detectable signals upward; when the comparison result is that the difference is less than the lower threshold value, the amplitude adjustment control module increases the gain of the analog front end by outputting a gain control code word and increases the bias voltage output by the charge pump by outputting a voltage control code word, thereby increasing the sensitivity of the MEMS microphone and expanding the range of detectable signals downward; when the comparison result is that the difference is between the upper and lower threshold values, the amplitude adjustment control module maintains the gain control code word and the voltage control code word output in the last period.

[0025] Further, when the gain of the analog front end has been adjusted to the minimum, and the comparison result is still that the difference is greater than the upper threshold, at this time the amplitude adjustment control module outputs the voltage control code word to the charge pump for electrostatic force feedback adjustment, to reduce the bias voltage provided by the charge pump to the MEMS microphone, thereby reducing the sensitivity of the MEMS microphone to identify larger sound signals; when the gain of the analog front end has been adjusted to the maximum, and the comparison result is still that the difference is less than the lower threshold, at this time the amplitude adjustment control module outputs the voltage control code word to the charge pump for electrostatic force feedback adjustment, to increase the bias voltage provided by the charge pump to the MEMS microphone, thereby increasing the sensitivity of the MEMS microphone to identify smaller sound signals. According to the above process, the analog front end can be adjusted to a gain matching the input, and electrostatic force feedback is performed when a larger dynamic range is required, to adjust the appropriate output voltage provided by the charge pump, thereby expanding the dynamic range of the signal chain, ensuring sufficient signal-to-noise ratio, and also avoiding amplitude clipping distortion caused by excessively large signal input.

[0026] Further, the pulse density modulation module adopts a digital Delta Sigma modulator, which modulates the digital code word output by the analog-to-digital conversion circuit according to the control code word (voltage control code word and gain control code word), to generate a pulse density modulation wave matching the system input amplitude to meet the format output required by the audio sensor.

[0027] The high dynamic capacitance type digital microphone system of the present application provides a system-level solution for improving the dynamic range and high-precision quantization, which utilizes the characteristic that the capacitance type sensor can adjust the sensitivity by changing the bias voltage, adopts an output voltage adjustable charge pump to provide electrostatic force feedback, further improves the adjustable range of the front end gain, and relaxes the requirement for the noise performance of the readout circuit. Meanwhile, the present application performs signal reproduction and amplitude detection through the amplitude detection module, dynamically monitors the input signal amplitude, and outputs the gain control code word by the amplitude adjustment control module, to timely feedback to the analog front end to adjust the gain of the analog front end, to increase the gain to expand the dynamic range when the signal is small, and to reduce the gain to avoid amplitude clipping distortion when the signal is large; when the gain is adjusted to the limit, the amplitude adjustment control module outputs the voltage control code word to the charge pump; when the signal is small, the bias voltage provided to the MEMS microphone is increased to improve the sensitivity of the MEMS microphone and improve the signal-to-noise ratio of the signal input to the analog front end, and when the signal is large, the bias voltage is reduced to avoid subsequent amplitude clipping distortion. Therefore, the system of the present application has the characteristics of high dynamic range and high-precision quantization. BRIEF DESCRIPTION OF DRAWINGS

[0028] Figure 1 The system structure block diagram of the high dynamic capacitance type digital microphone system of the present application and the conventional digital microphone and its interface circuit chip.

[0029] Figure 2Circuit structure schematic diagram of analog front end of the present application.

[0030] Figure 3 Circuit structure schematic diagram of charge pump of the present application.

[0031] Figure 4 Structure and timing schematic diagram of analog-digital signal conversion circuit of the present application.

[0032] Figure 5 Signal judgment principle schematic diagram of amplitude detection module and amplitude adjustment control module of the present application.

[0033] Figure 6 Control code word composition and meaning schematic diagram in the embodiment of the present application.

[0034] Figure 7 Simulation frequency spectrum diagram of digital signal output by digital microphone system of the present application.

[0035] Figure 8 System signal-to-noise ratio and acoustic overload point change with electrostatic force feedback depth schematic diagram of the present application. DETAILED DESCRIPTION

[0036] In order to describe the present application more specifically, the technical solutions of the present application are described in detail below in combination with the drawings and specific embodiments.

[0037] The application discloses a high dynamic capacitance type digital microphone system based on electrostatic force feedback, which comprises a MEMS microphone and an interface circuit chip of the MEMS microphone.

[0038] The bias voltage of the MEMS microphone is provided by the output voltage adjustable charge pump, and the bias voltage is increased, so that the sensitivity of the microphone is improved, and the signal-to-noise ratio of the microphone is improved; the bias voltage is reduced, so that the sensitivity of the microphone is reduced, and the amplitude distortion of a large sound signal is prevented.

[0039] The output voltage adjustable charge pump is used for providing the bias voltage for the MEMS microphone, and comprises a boost circuit, a sequence reference voltage generating circuit and a logic control circuit, wherein the boost circuit is a two-stage Dickson charge pump structure, the first-stage Dickson charge pump supplies power for the second-stage Dickson charge pump, and can output a voltage of 6-13V per 1V; the sequence reference voltage generating circuit selects a reference voltage through a voltage control code output by the amplitude adjustment control module, changes the reference voltage, changes the output voltage of the charge pump, and thus realizes voltage adjustment; and the logic control circuit controls the working state of the boost circuit by comparing the reference voltage and a feedback voltage.

[0040] The low-noise gain adjustable analog front end comprises a signal common-mode bias network, an amplification circuit and a feedback network, wherein:

[0041] The signal common-mode bias network is located at two input ends of the analog front end and is composed of two pseudo resistors, one end of each pseudo resistor is connected with a common-mode voltage Vcm = V dd / 2, the other end is connected with the input end; the target audio signal frequency is between 20-20 kHz, and the input signal is biased by the pseudo-resistance when entering the common-mode bias network to realize high-pass filtering, so as to filter out signals below the frequency of the audio signal acceptable by human ears.

[0042] The main body of the amplification circuit is two operational amplifiers, the inverting input ends of the two operational amplifiers are connected through capacitors, and the non-inverting input ends are respectively the non-inverting input end and the inverting input end of the analog front end.

[0043] The feedback network is composed of a capacitor array, a switch and a pseudo-resistance, and the switch in series with the capacitor is controlled by the gain control code word output by the amplitude control module to determine whether the capacitor is connected to the feedback loop, and the gain is determined by the size of the capacitor connected to the feedback loop:

[0044]

[0045]

[0046] It is concluded that:

[0047]

[0048] Wherein: R FB1 , R FB2 is a pseudo-resistance connected across the amplifier (output end and inverting input end) and providing voltage for the feedback node, C1 and C2 form a feedback capacitor array connected to the feedback loop of the two amplifiers, and C is a capacitor connected between the inverting input ends of the two amplifiers; V outn , V outp are the inverting output and non-inverting output of the analog front end, respectively, V in , V ip are the inverting input and non-inverting input of the analog front end, respectively.

[0049] It can be known from the gain formula that the gain coefficient of the amplification circuit of this structure can be controlled by adjusting the proportion of the two capacitors; the size of the capacitor between the inverting input ends of the two amplifiers is not easy to adjust, and is related to the noise performance of the analog front end, so the feedback capacitor is designed as a capacitor array to realize gain adjustment.

[0050] The analog-digital signal conversion circuit includes an anti-aliasing filter and a high-precision analog-digital converter, which is used to filter, sample and quantize the analog signal output by the analog front end and output digital code words, wherein:

[0051] The anti-aliasing filter is a low-pass filter, which suppresses out-of-band signals and prevents noise from aliasing during sampling. The filtered signal is sent to the high-precision analog-digital converter.

[0052] The high-precision analog-to-digital converter comprises a bootstrap switch, an integrator, a feedforward summing accumulator, a comparator and a clock generating circuit.

[0053] The amplitude detection module is an undersampling amplitude detection module, which has two stages of signal reproduction and amplitude detection.

[0054] The signal reproduction process is completed by an extraction filter: first, low-pass filtering is performed to filter out out-of-band signals, and then down-sampling is performed; the low-pass filtering adopts a low-order sine filter, which is composed of an integrator, an accumulation dump extractor and a differentiator, the integrator suppresses shaped noise, the accumulation dump extractor is realized by a counter, the output result is a down-sampled signal, and the differentiator is used to offset the low-frequency gain of the integrator to ensure that the amplitude of each frequency point is consistent with the input, and finally the result is saved in a register every certain clock period.

[0055] After the signal reproduction is completed, the amplitude detection stage is entered: taking the lower limit value of the bandwidth as the detection frequency (to ensure that at least one complete cycle of the signal can be obtained for low-frequency signals), the maximum value and the minimum value of the reproduced signal are obtained in the detection period and are compared with the preset upper threshold and lower threshold.

[0056] The amplitude adjustment control module outputs a control code word according to the judgment result of the amplitude detection module: when the difference value is greater than the upper threshold, it means that the signal amplitude is too large, so the analog front-end gain is reduced by one step, if the analog front-end gain is already the minimum multiple at this time, the output voltage of the charge pump is reduced; when the difference value is between the upper and lower thresholds, it means that the signal amplitude is moderate, and the current control code word is maintained; when the difference value is less than the lower threshold, it means that the signal amplitude is too small, so the analog front-end gain is increased by one step, if the analog front-end gain is already the maximum multiple at this time, the output voltage of the charge pump is increased.

[0057] According to the above process, the analog front-end can be adjusted to the gain matched with the input signal, and the output voltage of the charge pump can be adjusted to the size matched with the input signal, so that the amplitude clipping distortion caused by too large signal input is avoided while ensuring sufficient signal-to-noise ratio.

[0058] The pulse density modulation module is used for modulating the digital code word output by the analog-to-digital conversion circuit to meet the format output requirement of the audio sensor, and the input of the pulse density modulation module is the control code word and the quantization result of the high-precision analog-to-digital converter, and the output of the pulse density modulation module is a pulse density modulation wave matched with the system input amplitude.

[0059] As Figure 1An embodiment of the high dynamic range capacitive digital microphone system is shown, which includes a MEMS microphone and its interface circuit chip. The interface circuit chip includes an output voltage adjustable charge pump, a low noise gain adjustable analog front end, an analog-digital signal conversion circuit, an amplitude detection module, an amplitude adjustment control module, and a pulse density modulation module. The overall signal path of the system is as follows: the MEMS microphone powered by the charge pump generates different electrical signals on a pair of differential capacitors with the change of the sound pressure of the external sound signal. The pair of differential analog signals are input to the digital microphone interface circuit, and the power supply of the interface circuit is derived from the chip power supply module (including a bandgap reference source and a low dropout linear regulator). After the signal enters the interface circuit, it is biased by a pseudo resistor to achieve high-pass filtering, filtering out signals below the frequency of the audio signal acceptable to the human ear (signals less than 20 Hz). The differential analog signal is amplified by the analog front end, and the output differential signal is input to the analog-digital converter through the anti-aliasing filter to convert it into a digital signal. The digital signal is on one hand down-sampled and reproduced by the amplitude detection module, and the amplitude detection module outputs a gain control code word to adjust the gain range of the analog front end and an output voltage control code word to adjust the output voltage of the charge pump. The control code word is involved in the subsequent pulse density modulation of the digital signal. On the other hand, the digital signal is input to the pulse density modulation module, which outputs a pulse density modulation wave matching the input amplitude of the interface circuit.

[0060] As shown in Figure 2 , the low noise gain adjustable analog front end is a differential input and differential output, which includes two amplifiers OTA1 and OTA2. The inverting input terminals of OTA1 and OTA2 are connected through a capacitor C. The non-inverting input terminal of OTA1 is the non-inverting input terminal of the analog front end, and the non-inverting input terminal of OTA2 is the inverting input terminal of the analog front end. The input terminal of the analog front end is coupled to the output terminal of the amplifier through pseudo resistors R Bias1 , Bias2 , which provide common mode bias for the incoming signal. The capacitor array C 0dB , 6dB , 12dB , 18dB and its control switch S 0dB , 6dB , 12dB are connected to the output terminal of the amplifier in a negative feedback form and to the inverting input terminal of the amplifier. The pseudo resistors R FB1 , FB2 connect the feedback node (inverting input terminal) and the output terminal to provide voltage for the feedback node. The gain is determined by the number of capacitors in the capacitor array connected to the feedback loop. The gain range of the low noise gain adjustable analog front end is 1x, 2x, 4x, and 8x, and the specific gain determination mode is as follows: switches S 0dB , 6dB , 12dB are open, and capacitors C18dB Connect the feedback loop, the gain is 8 times; switch S 0dB , 6dB Disconnect, S 12dB Close, capacitor C 12dB , 18dB Connect the feedback loop, the gain is 4 times; switch S 0dB Disconnect, S 6dB , 12dB Close, capacitor C 6dB , 12dB , 18dB Connect the feedback loop, the gain is 2 times; switch S 0dB , 6dB , 12dB Close, capacitor C 0dB , 6dB , 12dB , 18dB Connect the feedback loop, the gain is 1 times; by adjusting the gain when the input signal size, the overall signal chain dynamic range can be expanded 18dB.

[0061] As Figure 3 shown, the output voltage adjustable charge pump includes a boost circuit, a sequence reference voltage generation circuit and a logic control circuit, wherein the boost circuit adopts two-stage Dickson charge pump structure, through the output of the first stage Dickson charge pump for the second stage Dickson charge pump power supply, can be 1V for a 6-13V voltage output, the highest can be stable output 16V bias voltage; sequence reference voltage generation circuit consists of eight resistors, seven switches and decoder, the voltage control code output by the amplitude adjustment control module enters the sequence reference voltage generation circuit through the decoder to control the opening and closing of the seven switches, output reference voltage V REF ; the logic control circuit compares the reference voltage V REF and feedback voltage V FB (1 / 16 of the charge pump output voltage or 1 / 8 of the output voltage) when V FB is greater than V REF , the boost circuit stops working; when V FB is less than V REF , the boost circuit works; when reaching steady state, V FB fluctuates around V REF .

[0062] As Figure 4 shown, the analog-digital signal conversion circuit includes an anti-aliasing filter and a high-precision analog-to-digital converter, wherein the high-precision analog-to-digital converter adopts Delta Sigma ADC architecture, having seven sub-modules, including:

[0063] Operational amplifier, the core unit of the switched-capacitor integrator;

[0064] Chopped integrator, the first stage integrator, including chopped modulation circuit and operational amplifier;

[0065] Switched-capacitor integrator, the second to fifth stage integrators, operational amplifier is the main part, sampling capacitor and switched capacitor across the operational amplifier input and output;

[0066] Gate voltage bootstrap switch, input signal sampling switch;

[0067] Feedforward sum accumulator, the weighted sum of the output of each stage of integrator, output results to the comparator;

[0068] Comparator, single-bit comparator.

[0069] As shown in Figure 5 , the working process of the amplitude detection module is: the signal output by the analog-digital signal conversion circuit passes through a 2-order sine filter, and is converted into a multi-bit signal by a single-bit, and the shaping order of the out-of-band noise is reduced; the filtered signal SIG IN is reduced by four times through downsampling, and the out-of-band noise is overlapped; then, the signal SIG SF is output as a 5-bit digital signal SIG DEC by twice 1-order downsampling; and the 5-bit digital signal SIG TH is compared with the set threshold (High V TH , Low V TH ), and the comparison result is output to the amplitude adjustment control module. Through the low-order sine filtering and the step-by-step downsampling scheme, the number of operation bits is reduced as much as possible on the basis of ensuring sufficient signal-to-noise ratio and bandwidth for amplitude detection, and the amplitude detection module uses at most 6 bits.

[0070] The working process of the amplitude adjustment control module is: when the output result of the amplitude detection module is that the input signal is higher than High V TH , the amplitude adjustment control module outputs a control code to reduce the analog front-end gain or to lower the bias voltage provided by the charge pump for the MEMS; when the output result of the amplitude detection module is that the input signal is lower than Low V TH , the amplitude adjustment control module outputs a control code to increase the analog front-end gain or to increase the bias voltage provided by the charge pump for the MEMS; when the output result of the amplitude detection module is that the amplitude of the input signal is between the high and low thresholds, the amplitude adjustment control module keeps the control code of the last period, and the circuit does not adjust.

[0071] The workflow for analog front-end gain adjustment and charge pump electrostatic feedback depth adjustment is as follows: The analog front-end gain levels are 1x, 2x, 4x, and 8x, with an initial level of 8x; the charge pump output voltage is set to eight levels, from 1V to 13V, with an initial level of 13V. The control code consists of five bits, such as... Figure 6 As shown, the first three digits represent electrostatic force adjustment: 000 = 6V, 001 = 7V, 010 = 8V, 011 = 9V, 100 = 10V, 101 = 11V, 110 = 12V, and 111 = 13V. The last two digits represent gain adjustment: 00 = 1x, 01 = 2x, 10 = 4x, and 11 = 8x. The initial value of the control codeword is 00000, corresponding to a charge pump output voltage of 6V and an analog front-end gain of 1x. When the difference result is higher than High V... TH This indicates that the current signal amplitude is too large, exceeding the upper limit of the signal amplitude that the system can detect; when the difference result is lower than Low V TH This indicates that the current signal amplitude is too small. The amplitude adjustment control module will adjust the output control code to 00001, corresponding to a charge pump output voltage of 6V and an analog front-end gain of 2. If the difference result in the next cycle is still lower than LowV... TH If the control code word is adjusted to 00010, corresponding to a charge pump output voltage of 6V and an analog front-end gain of 4x, then the control code word remains at 00001. If the difference result is between the high and low thresholds, the control code word is adjusted to 00000. If the difference result is higher than the upper threshold, the control code word is adjusted to 00000. If the analog front-end gain is 8x and the difference result is still lower than Low V, then the control code word remains at 00000. TH At this point, electrostatic force feedback adjustment will be activated until the control code word is adjusted to 11111, at which point the upper limit of the system's sensitivity will be reached.

[0072] Figure 7 This demonstrates the spectral characteristics under normal operating conditions in this embodiment. The sampling frequency was set to 2.5MHz, the number of sampling points to 4096, the input signal frequency to 7324Hz, and the input signal amplitude to 60mV. The resulting output digital signal is shown below. Figure 6 As shown. Observing the spectrum and calculating the performance indicators, its signal-to-noise ratio is approximately 84.5 dB-A, the effective number of bits is 13.8 bits, and the input equivalent noise is -117.5 dBFS (with 2.4 Vpp as 0 dBFS) obtained by subtracting the signal-to-noise ratio from the peak amplitude.

[0073] Figure 8 This embodiment demonstrates the changing trends of system signal-to-noise ratio and acoustic overload point with varying electrostatic feedback depth. The system reaches its maximum acoustic overload point of 133 dB at a MEMS bias voltage of 8V. SPLCompared with the fixed voltage bias, the electrostatic force feedback makes the acoustic overload point of the system increase by 11.4 dB; when the MEMS bias voltage is 10.5 V, the system reaches the maximum signal-to-noise ratio of 68.2 dB-A, and compared with the fixed voltage bias, the electrostatic force feedback makes the signal-to-noise ratio of the system increase by 6.2 dB.

[0074] The above description of the embodiments is to facilitate the understanding and application of the present application by those skilled in the art, and those skilled in the art can easily make various modifications to the above embodiments, and apply the general principles re-explained to other embodiments without the need for creative labor. Therefore, the present application is not limited to the above embodiments, and improvements and modifications made to the present application by those skilled in the art according to the disclosure of the present application should be within the scope of protection of the present application.

Claims

1. A high-dynamic-range capacitive digital microphone system based on electrostatic feedback, comprising a MEMS microphone and its interface circuit chip, wherein the MEMS microphone is used to convert externally input variable sound pressure levels into analog audio signals, characterized in that, The interface circuit chip includes: An output voltage adjustable charge pump is used to provide bias voltage for MEMS microphones; A low-noise, gain-adjustable analog front-end is used to filter and amplify analog audio signals before differential output. Analog-to-digital converter circuits are used to filter, sample, quantize, and output digital codewords from the signals output by the analog front end. The amplitude detection module is used to reproduce the high-speed low-bit digital codeword into a low-speed high-bit signal, obtain the difference between the maximum and minimum values ​​of the signal, and compare the difference with the set upper and lower limit thresholds. The amplitude adjustment control module generates voltage control codewords and gain control codewords based on the comparison results output by the amplitude detection module, which are used to adjust the output of the charge pump and the analog front end. The pulse density modulation module is used to modulate digital codewords and output a pulse density modulated wave that matches the system input amplitude. The power module is used to provide operating voltage and current to the various functional circuit modules within the chip. The charge pump is used to provide electrostatic force feedback, and its feedback depth is adjusted by the voltage control code provided by the amplitude adjustment control module. The charge pump includes a boost circuit, a sequence reference voltage generation circuit, and a logic control circuit. The boost circuit adopts a two-stage Dickson charge pump structure. The sequence reference voltage generation circuit selects a reference voltage through the voltage control code. The reference voltage enters the logic control circuit and is compared with the feedback voltage. The comparison result is used to control whether the boost circuit works, thereby realizing the function of adjustable output voltage. The analog front-end consists of three parts: a common-mode bias network, an amplifier circuit, and a feedback network. The common-mode bias network is located at the two input terminals of the analog front-end and consists of two pseudo-resistors. One end of the pseudo-resistor is connected to the common-mode voltage, and the other end is connected to the input terminal of the analog front-end. The input signal entering the bias network is biased by the pseudo-resistors to achieve high-pass filtering, filtering out signals below the audio frequency acceptable to the human ear. The main body of the amplifier circuit consists of two operational amplifiers. The inverting input terminals of the two operational amplifiers are connected through a capacitor, and the non-inverting input terminals of the two operational amplifiers correspond to the two input terminals of the analog front-end. The feedback network consists of pseudo-resistors, a capacitor array, and its control switch. The capacitor array and its control switch are connected across the two operational amplifiers in the form of negative feedback. The pseudo-resistors connect the feedback node to the output terminal of the operational amplifier, providing voltage to the feedback node. The gain is determined by the number of capacitors connected to the feedback loop in the capacitor array.

2. The high dynamic range condenser digital microphone system according to claim 1, characterized in that: The MEMS microphone's pickup capacitor consists of a highly flexible thin film, a perforated rigid backplate, and a cavity. The rigid backplate has dense perforations, which helps sound waves to be transmitted to the thin film, reducing air damping and noise. The thin film has ventilation holes, which allow for rapid pressure release when the film vibrates, making it easier to vibrate. The cavity is formed by etching multiple different additional layers deposited on a silicon wafer to improve sensitivity. The sensitivity of the MEMS microphone represents the microphone's sound-to-electrical conversion efficiency, which is directly related to the bias voltage provided to the MEMS microphone. The higher the bias voltage, the higher the microphone sensitivity.

3. The high dynamic range condenser digital microphone system according to claim 1, characterized in that: The amplification of the analog audio signal by the analog front end is adjusted by the gain control code provided by the amplitude adjustment control module. The analog front end has multiple gain levels. By adjusting the gain when the input signal magnitude is different, the dynamic range of the entire signal chain can be expanded.

4. The high dynamic range condenser digital microphone system according to claim 1, characterized in that: The analog-to-digital conversion circuit includes an anti-aliasing filter and an analog-to-digital converter. The anti-aliasing filter is a low-pass filter used to perform low-pass filtering on the signal output from the analog front end and suppress out-of-band signals to prevent noise from aliasing during sampling. After filtering, the signal is output to the analog-to-digital converter, which is used to sample, quantize and output digital codewords from the filtered signal.

5. The high dynamic range condenser digital microphone system according to claim 1, characterized in that: The amplitude detection module includes a sampling filter for signal reproduction and a digital module for comparing signal amplitudes. The sampling filter samples the high-speed, low-bit digital codewords output from the analog-to-digital converter circuit and converts them into a low-speed, high-bit signal. The sampling conversion process includes low-pass filtering and downsampling. The low-pass filtering uses a sinc filter to filter out out-of-band signals and prevent out-of-band signals from folding back into the in-band. Downsampling is implemented using an integrator, an accumulator-dump decimator, and a differential. After signal reproduction is completed through downsampling, the digital module uses the lower limit of the bandwidth as the detection frequency, acquires the maximum and minimum values ​​of the reproduced signal within the detection period, calculates the difference, and compares the difference with the set upper and lower limit thresholds.

6. The high dynamic range condenser digital microphone system according to claim 1, characterized in that: When the comparison result is that the difference is greater than the upper limit threshold, the amplitude adjustment control module will output the gain control codeword to reduce the gain of the analog front end and the voltage control codeword to reduce the bias voltage output by the charge pump, thereby reducing the sensitivity of the MEMS microphone and expanding the range of detectable signals. When the comparison result is that the difference is less than the lower threshold, the amplitude adjustment control module will output a gain control codeword to increase the gain of the analog front end and an output voltage control codeword to increase the bias voltage output by the charge pump, thereby improving the sensitivity of the MEMS microphone and extending the detectable signal range downward. When the comparison result is that the difference is between the upper and lower thresholds, the amplitude adjustment control module will maintain the gain control codeword and voltage control codeword output in the previous cycle.

7. The high dynamic range condenser digital microphone system according to claim 6, characterized in that: When the gain of the analog front end is adjusted to the minimum, but the comparison result is still that the difference is greater than the upper threshold, the amplitude adjustment control module will output the voltage control code to the charge pump for electrostatic feedback adjustment, reducing the bias voltage provided by the charge pump to the MEMS microphone, thereby reducing the sensitivity of the MEMS microphone to recognize larger sound signals; when the gain of the analog front end is adjusted to the maximum, but the comparison result is still that the difference is less than the lower threshold, the amplitude adjustment control module will output the voltage control code to the charge pump for electrostatic feedback adjustment, increasing the bias voltage provided by the charge pump to the MEMS microphone, thereby increasing the sensitivity of the MEMS microphone to recognize smaller sound signals.

8. The high dynamic range condenser digital microphone system according to claim 1, characterized in that: The pulse density modulation module employs a digital Delta Sigma modulator, which modulates the digital codeword output by the analog-to-digital converter circuit according to the control codeword, generating a pulse density modulation wave that matches the system input amplitude to meet the format output requirements of the audio sensor.

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