A VCO-Based CMOS Readout Circuit for Capacitive MEMS Microphones

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...

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Bibliographic Details
Main Authors: Andres Quintero, Fernando Cardes, Carlos Perez, Cesare Buffa, Andreas Wiesbauer, Luis Hernandez
Format: Article
Language:English
Published: MDPI AG 2019-09-01
Series:Sensors
Subjects:
Online Access:https://www.mdpi.com/1424-8220/19/19/4126
Description
Summary: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 &#8220;always-on&#8221; 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 (<inline-formula> <math display="inline"> <semantics> <mrow> <mi mathvariant="sans-serif">&#931;</mi> <mo>&#916;</mo> </mrow> </semantics> </math> </inline-formula>) 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&#8212;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 <inline-formula> <math display="inline"> <semantics> <mrow> <mn>1</mn> <mo>/</mo> <mi>f</mi> </mrow> </semantics> </math> </inline-formula> 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 <inline-formula> <math display="inline"> <semantics> <mi mathvariant="sans-serif">&#956;</mi> </semantics> </math> </inline-formula>A at 1.8 V and the effective area is 0.12 mm<inline-formula> <math display="inline"> <semantics> <msup> <mrow></mrow> <mn>2</mn> </msup> </semantics> </math> </inline-formula>. This new readout circuit may represent an enabling advance for low-cost digital MEMS microphones.
ISSN:1424-8220