A pulse oximeter is an optical device that monitors tissue oxygenation levels. Traditionally, these devices estimate the oxygenation level by measuring the intensity of the transmitted light through the tissue and are embedded into everyday devices such as smartphones and smartwatches. However, these sensors require prior information and are susceptible to unwanted changes in the intensity, including ambient light, skin tone, and motion artefacts. Previous experiments have shown the potential of Time-of-Flight (ToF) techniques in measurements of tissue hemodynamics. Our proposed technology uses histograms of photon flight paths within the tissue to obtain tissue oxygenation, regardless of the changes in the intensity of the source. Our device is based on a 45ps time-to-digital converter (TDC) which is implemented in a Xilinx Zynq UltraScale+ field programmable gate array (FPGA), a CMOS Single Photon Avalanche Diode (SPAD) detector, and a low-cost compact laser source. All these components including the SPAD detector are manufactured using the latest commercially available technology, which leads to increased linearity, accuracy, and stability for ToF measurements. This proof-of-concept system is approximately 10cm×8cm×5cm in size, with a high potential for shrinkage through further system development and component integration. We demonstrate preliminary results of ToF pulse measurements and report the engineering details, trade-offs, and challenges of this design. We discuss the potential for mass adoption of ToF based pulse oximeters in everyday devices such as smartphones and wearables.
KEYWORDS: Field programmable gate arrays, Time metrology, Tomography, Measurement devices, Logic, LIDAR, Imaging devices, Data transmission, Computer programming, Time of flight imaging
Time-to-digital converter (TDC) is a major component for the measurement of time intervals. Recent developments in field-programmable gate array (FPGA) have enabled the opportunity to implement high performance TDC which previously was only possible in dedicated hardware. We propose a TDC with cascaded carry chains, a customized encoder, a highly efficient histogram generator. It’s implemented on a 16nm FPGA while utilizing a Gigabit Ethernet for data transmission. Comparisons with previous works show the proposed TDC has lower resource utilization whilst achieving a better raw linearity which enables the path to high performance multichannel TDCs in demanding time-of-flight (ToF) imaging application.
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