Single-Chip Wireless Sensing Tag with Integrated Optical Energy Harvesting

As transistor dimensions continue to decrease, overall system size and cost can be reduced while maintaining the same functionality. In addition, more sensors are now able to become integrated onto a wireless sensor node, creating sensor rich and complex networks. However, the lifetime of these networks is limited by the node lifetime, which is limited by the battery capacity; this limitation is worsened by the shrinking system size since the maximum amount of energy stored in a battery is proportional to its physical dimensions. However, energy harvesting can be applied to extend the lifetime and address the limitations imposed by the battery and the system size. Ambient energy in the form of solar, wind, vibration, electromagnetic radiation, and thermal gradient is available for harvest in most environments. Although the harvested energy is typically miniscule, it can potentially extend the operational lifetime of an ultra-low power sensor node with low duty cycle and data-rates, from 1-5 years to 10+ years.;In this work, the signal processing datapath of an optical energy harvesting wireless sensor node is proposed and design details investigated. In particular, the analog and digital signal processing circuits, non-volatile memories, and low-power wireless transmitters are proposed and discussed in detail. One key feature of the analog and digital circuit blocks in the proposed system is their ability to scale performance with power, which is highly desirable in an energy harvesting system since their performance can be adapted to the available power, effectively increasing the operational lifetime.;Through measurement and simulation, the proposed optically-powered system can operate from wide range of illumination intensities. The scalable delta-sigma modulator that performs data conversion can produce a 4-8 bit output with 0.8-1.8 V supply voltage and 50-1600 kHz sampling rate. At 50 kHz sampling rate and 1.8 V supply voltage, the power consumption is approximately 2 microwatt and the data converter achieves 4 bits of resolution. The digital decimation filter for downsampling and filtering the delta-sigma modulator output, achieves 1 to 10 pJ of energy consumption per output word, depending on the datapath style (bit serial or parallel). At an OSR of 64, which is equivalent to a 128 kHz sampling rate, and 0.8 V supply voltage, the parallel digital filter that reuses two adders across 4 DSP operations can produce output words with maximum resolution of 13 bits at 2 kHz while dissipating 32 nW while the equivalent bit serial filter dissipates 93 nW. A passive backscatter transmitter dissipates approximately 0.08 microwatt of power when transmitting the 13-bit data at 200 kHz symbol rate. When no interrogator is present, the sampled and filtered data can be stored in a 500-bit non-volatile ferroelectric memory (FRAM) array, which dissipates approximately 1.5 microwatt of power on average, assuming 50% memory read and write probabilities. Although the energy efficiency of the FRAM array is 2-6X worse than volatile DRAM, using non-volatile FRAM eliminates the need for an area/volume-consuming energy storage component and large DRAM bitcell capacitance for achieving long data retention times, which is potentially large due to the small system duty cycle.;The proposed system dissipates 7.5 microwatt of total power while operating at 0.8 V supply voltage, 50 kHz ADC sampling rate, and 8 kHz symbol rate across the passive backscatter transmitter. Using integrated energy harvesting photodiodes capable of achieving 132 microwatt/mm2 power density at 7 kLux illumination, a system with 5.7 mm2 of total area can be realized. If an ultracapacitor with 26 J of energy and 0.054 mm3 of volume is used such that the system can operate in the dark, for 1 second every hour for 24 hours, the required total system volume is approximately 13.5 mm 3, which is equivalent to a cube with 2.4 mm for each of its edges. If the system duty cycle is increased 4 times, to operate at 15 minute intervals, the required total system volume is 148 mm3, equivalent to a 5x5x5 mm3 cube.