(A) single-chip simple-port-supply ADC for compact digital electret condenser microphones

Abstract

Microphones are widely used in various applications, from low-end toys to high performance multimedia equipment such as mobile phones, laptop computers, etc. Among various types of microphones, electret condenser microphones (ECMs) have been the most popular by virtue of strengths such as their structural compactness and the absence of need for a polarization power supply. The ECM can be simply modeled as a high output-impedance capacitive voltage source which needs a high input-impedance buffer for signal readout with minimum loss. This buffer is assembled in the ECM package. Traditional JFET-based analog ECMs, in which a JFET is used as the readout buffer, have long been popular owing to their compactness and low cost. Although JFET-based analog ECMs have been successfully applied in many commercial acoustic applications, such analog signal transfer is problematic in certain applications that have high power transmission or high speed digital switching. Under such conditions, the analog signal from the ECM is prone to corruption by the surrounding noise. Thus, amplifiers for such applications usually have relatively low output impedance, which results in low gain. This noise coupling problem can be alleviated by replacing the JFET amplifier with a CMOS amplifier with low output impedance, owing to overall feedback. Further improvement is also possible by completing the A/D conversion inside the ECM canister and providing a digital output. This advanced type of microphone is called a digital ECM. ADC development for digital ECMs faces several design challenges, such as a high impedance interface for the electret capacitor, small chip size to be embedded in a compact microphone canister, minimum number of I/O pads, no use of external components, switching noise decoupling, and so forth. In this dissertation, design techniques for compact high performance single-chip ADC for digital ECMs which features wide dynamic gain range, full audio-bandwidth, low power c...