Operation point stabilization for VCO-ADC using switched-capacitor frequency-to-current conversion

A circuit comprising:

• a micro-electromechanical system (MEMS) microphone configured to generate a first voltage signal at a first output terminal of the MEMS microphone in response to a sound signal;
• a first voltage-controlled-oscillator (VCO) coupled to the first output terminal of the MEMS microphone, wherein the first VCO is configured to output, at an output terminal of the first VCO, a first frequency modulated signal having a first frequency proportional to the first voltage signal;
• a first frequency-to-digital (FTD) converter coupled to the output terminal of the first VCO, wherein the first FTD converter is configured to convert the first frequency modulated signal into a first digital output signal proportional to the first frequency;
• a first frequency-to-voltage (FTV) conversion circuit coupled to the output terminal of the first VCO, wherein the first FTV conversion circuit is configured to generate a first bias voltage in accordance with the first frequency modulated signal, and is configured to output the first bias voltage at an output terminal of the first FTV conversion circuit;
• and
• a first resistor coupled between the first output terminal of the MEMS microphone and the output terminal of the first FTV conversion circuit.

A method of operating a micro-electromechanical system (MEMS) microphone system, the method comprising:

• generating, by a MEMS microphone, a differential output signal in response to a sound signal, wherein the differential output signal includes a first voltage signal at a first output terminal of the MEMS microphone and a second voltage signal at a second output terminal of the MEMS microphone;
• generating, by a first voltage-controlled-oscillator (VCO) coupled to the first output terminal of the MEMS microphone, a first frequency modulated signal having a first frequency proportional to the first voltage signal;
• generating, by a second VCO coupled to the second output terminal of the MEMS microphone, a second frequency modulated signal having a second frequency proportional to the second voltage signal;
• generating, by a Common-Mode Feedback (CMFB) circuit coupled to the first VCO and the second VCO, a bias voltage based on the first frequency modulated signal and the second frequency modulated signal, wherein the bias voltage is proportional to a rest frequency of the first VCO, wherein the rest frequency of the first VCO is the first frequency of the first frequency modulated signal when no sound signal is sent to the MEMS microphone;
• and
• setting a bias voltage of the MEMS microphone by sending the bias voltage to a first terminal of a first resistor and a first terminal of a second resistor, wherein a second terminal of the first resistor and a second terminal of the second resistor are coupled to the first output terminal of the MEMS microphone and the second output terminal of the MEMS microphone, respectively.