IEEE JOURNAL OF SOLID-STATE CIRCUITS, VOL. 37, NO. 3,MARCH 2002 279 A 1.8-V Modulator Interface for an Electret Microphone With On-Chip Reference Ovidiu Bajdechi, Student Member, IEEE, and Johan H. Huijsing, Fellow, IEEE Abstract—The design of a delta–sigma ( ) analog-to-digital converter (ADC) for direct voltage readout of an electret micro- phone is presented. The ADC is integrated on the same chip with a bandgap voltage reference and is designed to be packaged to- gether with an electret microphone. Having a power consumption of 1.7 mW from a supply voltage of 1.8 V, the circuit is well suited for use in mobile applications. The single-loop, single-bit, fourth- order ADC operates at 64 times oversampling for a signal bandwidth of 11 kHz. The measured dynamic range is 80 dB and the peak signal-to-(noise distortion) ratio is 62 dB. The harmonic distortion is minimized by using an integrator with an instrumen- tation amplifier-like input which directly integrates the 125-mV peak single-ended voltage generated by the microphone. A com- bined continuous-time/switched-capacitor design is used to mini- mize power consumption. Index Terms—Analog-to-digital conversion, CMOS analog IC, continuous time, delta–sigma modulator, electret micro- phone, high input impedance, low-voltage bandgap reference, single-ended input, switched capacitor. I. INTRODUCTION H IGH-PERFORMANCE CMOS processes are currently evolving toward increased switching speeds and transistor densities. Even if this evolution is driven by the needs of digital circuits, the performance versus cost ratio of analog circuits is also increasing [1], [2], though at a slower rate. In systems-on- chip, a large digital section can perform high-complexity data processing at high speeds while the analog circuits provide high- performance data conversion, in many cases without the need for pre-/post-conditioning of analog signals. Integration of the analog-to-digital converters (ADCs) and digital-to-analog converters (DACs) on the same chip with the digital signal processor is usually desired to reduce the overall system cost. However, there are cases where distributing the ADCs outside the main chip set can improve performance and reduce system costs. This is generally the case with specially packaged sensors [3]. The audio input device most used in telephony these days, the electret microphone, is such a specially packaged sensor. Fig. 1(a) shows the structure of an electret microphone. It con- sists of a variable capacitor, with one fixed plate and one flex- ible plate which bends under the pressure of sound. The fixed plate is covered with an electret layer which has a built-in charge resulting in an electric field of 200–300 V. Thus the micro- phone works as a high-impedance voltage generator, as shown Manuscript received July 12, 2001; revised October 8, 2001. The authors are with Delft University of Technology, 2628 CD Delft, The Netherlands (e-mail: o.bajdechi@its.tudelft.nl). Publisher Item Identifier S 0018-9200(02)01692-X. (a) (b) Fig. 1. Electret microphone model. in Fig. 1(b) by its electrical model. The microphone’s output voltage depends on its geometry and the materials used [4]. The silicon chip IC placed inside the microphone package contains an FET which is used as a high input-impedance am- plifier. A schematic of the A/D conversion chain presently used in mobile phones is shown in Fig. 2(a). The JFET packaged along with the microphone is used as a voltage-to-current con- verter. The signal is ac coupled to the Codec chip by a large- valued capacitor . The cost of the system can be reduced by integrating along with the JFET and designing new Codec chips that can be connected directly to the JFET drain [5]. How- ever, two other disadvantages of this system remain. At approx- imately 60-dB dynamic range and 1% total harmonic distortion, the performance is limited by the JFET’s noise and nonlinearity, even when this is biased at a relatively high drain current. The analog signal at the JFET drain is also sensitive to externally generated electrical noise. In this paper, a different approach to analog-to-digital con- version of electret microphone signals is presented. The concept is illustrated in Fig. 2(b). The JFET is replaced by a chip con- taining an ADC and a voltage reference. The ADC is connected directly to the microphone, without making use of an amplifier or filter for signal conditioning. The voltage reference is placed on the same chip, so no sensitive analog connections are taken outside the metal package. The only needed external connec- tions, besides ground and supply, are the digital Clock input and 1-bit Data output. The next section gives a high-level description of the designed one-chip system. Section III covers the design aspects of the modulator, both at the behavioral level and at circuit level. In Section IV, the design of the on-chip low-voltage bandgap reference is explained. Conversion nonlinearity is analyzed in Section V. Section VI presents experimental results and the con- clusions are drawn in Section VII. 0018–9200/02$17.00 © 2002 IEEE