Abstract
Microelectromechanical systems (MEMS) microphone sensors have significantly improved in the past years, while the readout electronic is mainly implemented using switched-capacitor technology. The development of new battery powered “always-on” applications increasingly requires a low power consumption. In this paper, we show a new readout circuit approach which is based on a mostly digital Sigma Delta () analog-to-digital converter (ADC). The operating principle of the readout circuit consists of coupling the MEMS sensor to an impedance converter that modulates the frequency of a stacked-ring oscillator—a new voltage-controlled oscillator (VCO) circuit featuring a good trade-off between phase noise and power consumption. The frequency coded signal is then sampled and converted into a noise-shaped digital sequence by a time-to-digital converter (TDC). A time-efficient design methodology has been used to optimize the sensitivity of the oscillator combined with the phase noise induced by and thermal noise. The circuit has been prototyped in a 130 nm CMOS process and directly bonded to a standard MEMS microphone. The proposed VCO-based analog-to-digital converter (VCO-ADC) has been characterized electrically and acoustically. The peak signal-to-noise and distortion ratio (SNDR) obtained from measurements is 77.9 dB-A and the dynamic range (DR) is 100 dB-A. The current consumption is 750 A at 1.8 V and the effective area is 0.12 mm. This new readout circuit may represent an enabling advance for low-cost digital MEMS microphones.
Highlights
The fast growth of the Internet of Things (IoT) and the upward trend of the mobile market are increasing the use of voice communication and speech recognition applications [1,2,3]
The proposed voltage-controlled oscillator (VCO)-analog-to-digital converter (ADC) has been fabricated in a 130 nm standard CMOS process
To evaluate the performance of the proposed VCO-based analog-to-digital converter (VCO-ADC) architecture, the implemented ASIC has been packaged together with a silicon capacitor array directly bonded to the ADC input, in order to emulate the real capacitance of a DBP Microelectromechanical systems (MEMS) microphone
Summary
The fast growth of the Internet of Things (IoT) and the upward trend of the mobile market are increasing the use of voice communication and speech recognition applications [1,2,3]. An alternative option to replace Σ∆ modulators are the oscillator-based analog-to-digital converters (ADCs) They are noise-shaped architectures wherein the information is encoded into the frequency of a digital signal; in contrast with the classical voltage encoding, the dynamic range (DR) is not limited by the power supply. Both approaches behave like a CT-Σ∆ modulator, showing the first-order noise-shaping property at its output spectrum.
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