Ultralow-power developments target next-gen wireless sensors

Maarten Lont, Pieter Harpe, Cui Zhou, and Tobias Gemmeke -December 05, 2012

The ultrasmall sensors of the future will monitor our health parameters, vehicles, machines and processes, buildings and smart constructions, and the environment. They will operate autonomously for long periods on a small battery, and they will communicate wirelessly. A key factor for their success, therefore, is their low power consumption, which will define the range of applications and functionalities for which they can be used.

At the 38th European Solid-State Circuits Conference in September, Imec and Holst Centre (Eindhoven, Netherlands) presented four ultralow-power developments to drive next-generation sensors and sensor networks: a frequency-shift-keying receiver for body-area networks, a flexible successive-approximation-register A/D converter for wireless sensor nodes, fast start-up techniques for duty-cycled impulse radio receivers, and a design approach targeting subthreshold operation.

ULP receiver for body-area network applications
Imec and Holst have developed a power-efficient receiver for ULP BAN (ultralow-power body-area network) applications. Whereas most transceivers exploit OOK (on-off keying) modulation, the new receiver uses FSK (frequency-shift keying) modulation and is hence less sensitive to interference. The complete receiver, fabricated in 40-nm CMOS technology, consumes 382.5 μW. The sensitivity measured at a bit error rate of 10−3 is –81 dBm for a 12.5-kbit/sec bit rate. The bit rate is scalable up to 625 kbits/sec, enabling a trade-off between sensitivity and bit rate. Taking advantage of the short-range nature of BAN applications, a mixer-first architecture is proposed, leading to a good dynamic range.

Flexible SAR ADC for ULP wireless sensor nodes
Wireless sensor nodes for electroencephalography, electrocardiography, and temperature and pressure monitoring require ULP ADCs for both the sensor-readout interface and the wireless-communication front end. Each of these applications, however, has its own requirements for accuracy and bandwidth. Imec and Holst Centre have realized a flexible, power-efficient SAR (successive approximation register) ADC that designers can use for a variety of applications. The device supports resolutions from 7 to 10 bits and sample rates from dc to 2M samples/sec; the flexibility is achieved by implementing a reconfigurable comparator and a reconfigurable DAC. The chip, in a 90-nm process, occupies 0.047 mm2, and achieves power efficiencies of 2.8- to 6.6-fJ/conversion step at 2M samples/sec and with a 0.7V supply. 

Die photo of the ADC in 90-nm CMOS.

Fast start-up techniques for duty-cycled impulse radio receivers
For medium- to high-data-rate communications or for accurate localization, IR-UWB (impulse-radio ultrawideband) radios are the preferred solution. In these radios, the transmitter sends out short pulses at regular intervals, thereby using a large bandwidth, and switches itself off in between to save power. The receiver does likewise, at exactly the same time as the transmitter. In aggressively duty-cycled systems, however, the start-up time for the IR receiver is a critical factor in power consumption.

Imec and Holst Centre used three fast-start-up techniques and dc-coupled interconnections to develop a 10-ns-start-up IR receiver, achieving the time the fastest start-up time reported to date for duty-cycled IR receivers and enabling ULP consumption in this receiver class. The measured power consumption scales proportionally to the duty-cycling rate, from 46 mW in continuous mode down to 58 µW for a duty-cycle rate of <0.01%.

Chip photo shows the impulse-radio receiver.

Design approach targets subthreshold operation
Finally, Imec and Holst Centre have developed new design points for standard cells that operate around the energy-optimal point at ultralow supply voltages.

Subthreshold operation is an attractive technique for reducing energy consumption in energy-constrained applications, but the large increase in energy efficiency typically comes at the cost of severe speed degradation and increased susceptibility to process variations. The new design points are therefore based on a detailed analysis of variability, inverse-narrow-width, and reverse-channel effects. The team combined the results of that analysis with layout techniques to reduce systematic variability in nanometer technologies. The resulting cell library outperforms a standard library by more than 3×. Imec and Holst Centre confirmed the benefit of the cells in a FIR (finite impulse response) filter design combining higher energy efficiency with the 3× speedup.

Maarten Lont started as a full-time researcher at Imec and Holst Centre in August. His research interests are low-power circuit design and communications theory. He received a master of science degree in electrical engineering at Eindhoven University of Technology (Netherlands) and is pursuing his doctoral degree at the same university, focusing on the design of ultralow-power wake-up receivers.

Pieter Harpe has been a research at Imec and Holst Centre since 2008, working on ultralow-power wireless transceivers, with a main focus on ADC research and design. He received his master of science and doctoral degrees from Eindhoven University of Technology, where he has been an assistant professor since 2011 focusing on low-power mixed-signal circuits.

Cui Zhou joined Imec and Holst Centre as a researcher in 2008 and focuses on baseband amplifiers, IWB receivers and PLLs. She received master of science degrees in microelectronics from Delft University of Technology (Netherlands) and FuDan University (Shanghai, China).

Tobias Gemmeke is senior researcher at Imec and Holst Centre, focusing on energy-efficient digital design for next-generation biomedical systems. He focuses on the energy-optimal design of active sensors, considering all steps from the system level down to the underlying technologies. He received his master’s and doctoral degrees from RWTH Aachen University (Germany).

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