CMOS Readout Interfaces for MEMS Capacitive Microphones (2009)

This dissertation demonstrates the feasibility of three novel low-power and low-noise schemes for the readout interfaces of MEMS Capacitive Microphones (MCM) by presenting their detailed design descriptions and measurement results as application-specific ICs (ASIC) in CMOS technology developed to exploit their application scope in consumer electronics and hearing aids. MCMs are a new generation of acoustic sensors, which offer a significant scope to improve miniaturization, integration and cost of the acoustic systems by leveraging the MEMS technology. Electret-Condenser-Microphones (ECM) are the current market solution for acoustic applications; however, MCMs are being considered as the future microphone-of-choice for mobile phones in consumer electronics and for hearing aids in medical applications. The readout interface of MCM in an acoustic system converts the output of the MEMS sensor into an appropriate electrical representation (analog or digital). The output of a MCM is in the form of capacitive-variations in femto-Farad range, which necessitates a low-noise signal-translation employed by the readout interface together with a low-power profile for its portable applications. The main focus of this dissertation is to develop novel readout schemes that are low-noise, low-power, low-cost and batch-producible, targeting the domains of consumer electronics and hearing-aids. The presented readout interfaces in this dissertation consist of a front-end, which is a preamplifier, and a backend which converts the output of the preamplifier into a digital representation. \n\nThe first interface presents a bootstrapped preamplifier and a third-order sigma-delta modulator (SDM) for analog-to-digital conversion. The preamplifier is bootstrapped to the MCM by tying its output to the sensorâs substrate. This bootstrapping technique boosts the MCM signal by ~17dB and also makes the readout insensitive to the parasitic capacitors in MCM electro-mechanical structure, achieving 55dBA/Pa of SNDR. The third-order low-power SDM converts output of the PAMP into an over-sampled digital bitstream demonstrating a dynamic-range (DR) of 80dBA. This ASIC operates at 1.8V single-supply and 460uA of total current consumption; thus, highlighting the feasibility of low-power integrated MCM readout interface. This ASIC is also acoustically characterized with a MCM, bonded together in a single package, demonstrating a reasonable agreement with the expected performance.\n\nThe second interface presents a readout scheme with force-feedback (FFB) for the MCM. The force-feedback is used to enhance the linearity of the MCM and minimize the impact of drift in sensor mechanical parameters. Due to the unavailability of the sensor, the effect of FFB could not be measured with an MCM; however, the presented results point out a significant performance improvement through FFB. The preamplifier in this ASIC utilizes a high-gain OTA in a capacitive-feedback configuration to achieve parasitic insensitive readout in an area and power-efficient way, achieving 40dBA/Pa of SNDR. The digital output of the third-order SDM achieved 76dBA of DR and was also used to apply the electrostatic FFB by modulating the bias voltage of the MCM. A dummy-branch with dynamic matching converted the single-ended MCM into a pseudo-differential sensor to make it compatible with force-feedback. This interface operates at 3.3V supply and consumes total current of 300uA.\n\nThe third interface presents a chopper-stabilized multi-function preamplifier for MCM. Unlike typical MCM preamplifiers, this preamplifier employs chopper-stabilization to mitigate low-frequency noise and offset and it also embeds extra functionalities in the preamplifier core such as controllable gain, controllable offset and controllable high-pass filtering. This preamplifier consists of two stages; the first stage is a source-follower buffering the MCM output into a voltage signal and the second-stage is a chopper-stabilized controllable capacitive gain-stage. This preamplifier employs MÎ© bias resistors to achieve consistent readout sensitivity over the audio band by utilizing the miller effect, avoiding the conditionally-linear GÎ© bias resistors. The offset control functionality of this preamplifier can be used to modulate idle tones in the subsequent sigma-delta modulator out of the audio-band. The high-pass filtering functionality can be used to filter-out low-frequency noises such as wind-hum. This preamplifier operates at 1.8V and consumes total current of 50u with SNDR of 44dB/PA, demonstrating the feasibility of a low-power low-noise multifunction preamplifier for the MCM sensor.