Publications

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This paper reports on a near-zero power inertial wakeup sensor system supporting digital weighting of inputs and with protection against false positives due to mechanical shocks. This improves upon existing work by combining the selectivity and sensitivity (Q-amplification) of resonant MEMS sensors with the flexibility of digital signal processing while consuming below 10 nW. The target application is unattended sensors for perimeter sensing and machinery health monitoring where extended battery life afforded by the low power consumption eliminates the need for power cables. For machinery health monitoring, the signals of interest are stationary but may contain spurious mechanical shocks.

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Single inverter gate CMOS oscillator designs have been used for decades based on the CMOS technology of the time. While a single inverter gate can deliver very good performance dependent on the application, it presents design limitations due to parameter trade-offs of transconductance, output impedance, and bias current. This paper introduces a novel CMOS sustaining amplifier design that significantly increases the design flexibility beyond what a single inverter can provide. It uses a three-stage inverter with the center inverter incorporating negative feedback to allow for a wide range of transconductance with wide operational bandwidth. High transconductance can provide operation for high-resistance resonators and or resonators that have significant activity dips [1]. This design is resistant to parasitic oscillations seen with high transconductance sustaining amplifiers. An equation and model describing the circuit transconductance is derived and accurately determines this circuit gain using a small number of circuit parameters. Given a desired transconductance, this new amplifier operates with lower power and higher output impedance than an equivalent single inverter. Engineers at Sandia National Laboratories have fabricated and implemented this type of design with approximately 20 mS of transconductance at frequencies of 50 MHz and has been applied to frequencies up to 100 MHz.

Fingerprint sensing is pervasive in the cellular telecommunications market. Current commercial fingerprint sensors utilize capacitive scanning. This work focuses on the design, fabrication and characterization of post-complementary-metal-oxide-semiconductor (CMOS) compatible piezoelectric micro-machined ultrasonic transducers for use as ultrasonic pixels to improve robustness to contamination and allow for sub-epidermis scans. Ultrasonic pixels are demonstrated at frequencies ranging from 100 kHz to 800 kHz with several electrode coverages and styles to identify trends.

The Defense Advanced Research Project Agency has identified a need for low-standby-power systems which react to physical environmental signals in the form of an electrical wakeup signal. To address this need, we design piezoelectric aluminum nitride based microelectromechanical resonant accelerometers that couple with a near-zero power, complementary metal-oxide-semiconductor application specific integrated circuit. The piezoelectric accelerometer operates near resonance to form a passive mechanical filter of the vibration spectrum that targets a specific frequency signature. Resonant vibration sensitivities as large as 490 V/g (in air) are obtained at frequencies as low as 43 Hz. The integrated circuit operates in the subthreshold regime employing current starvation to minimize power consumption. Two accelerometers are coupled with the circuit to form the wakeup system which requires only 5.25 nW before wakeup and 6.75 nW after wakeup. The system is shown to wake up to a generator signal and reject confusers in the form of other vehicles and background noise.

The defense community desires low-power sensors deployed around critical assets for intrusion detection. A piezoelectric microelectromechanical accelerometer is coupled with a complementary metal-oxide-semiconductor comparator to create a near-zero power wakeup system. The accelerometer is designed to operate at resonance and employs aluminum nitride for piezoelectric transduction. At a target frequency of 160 Hz, the accelerometer achieves sensitivities as large as 26 V/g. The system is shown to require only 5.4 nW of power before and after latching. The combined system is shown to wake up to a target frequency signature of a generator while rejecting background noise as well as non-target frequency signatures.