A Bufferless Interface for Single-Ended ECM Sensors

This paper presents a bufferless interface that can be directly connected to a single-ended capacitive sensor such as an electret condenser microphone. A high-input impedance interface is developed using only a continuous-time loop filter, whereas conventional interface circuits are composed of a buffer, a preamplifier, an antialiasing filter, and a high-order switched-capacitor loop filter. Equipped with an active gm-C integrator, this interface chip is designed to implement all the required functions, i.e., high-input impedance buffering, preamplification, antialiasing filtering, and analog-to-digital conversion. The inherent antialiasing filtering function also provides a significant advantage in terms of silicon areas and power consumption. The complete interface channel achieves a 75-dB dynamic range and a 60.3-dB signal-to-noise-plus-distortion ratio over a 25-kHz signal band, which satisfies the requirements for electret microphones. The power consumption of the interface channel is 600 μW, and the chip dissipates a total of 860 μW from the 3.3-V supply.