A Programmable Sustaining Amplifier for Flexible Multimode MEMS-Referenced Oscillators

This paper presents a programmable single-chip sustaining amplifier for MEMS-referenced oscillators. The chip integrates a low-noise amplifier, variable-gain amplifiers (VGAs), band-pass filters (BPFs), all-pass filters (APFs), automatic level control, and output buffers. It also contains a second signal path for background cancellation, i.e., eliminating the effect of electrical feedthrough capacitance of the MEMS resonator. Important parameters of each circuit block (e.g., VGA gain, BPF center frequency and quality factor (Q), APF phase shift, and so on.) can be set using bias currents, thus enabling finegrained control over the overall transfer function. The amplifier was designed and fabricated in 0.5 μm CMOS. The chip was integrated with board-level current DACs (programmable via an I²C bus) and tested inside a vacuum chamber with various polysilicon (poly-Si) and single crystal (SC-Si) silicon comb-drive MEMS devices with resonant frequencies in the 10-90 kHz range. Oscillators referenced to a high-Q (~13000) SC-Si device had 20 dBc/Hz lower close-in phase noise (at 10 Hz) than those referenced to the low-Q (~320) poly-Si device. Experimental results highlight advantages of the design, including automatic mode selection, background cancellation, control of feedback phase to optimize phase noise, and frequency locking to an external reference.