Welcome to the homepage of Team 02: HBO2. This homepage contains information about our project: A Photoplethysmogram (PPG) based Fully-Differential Heart and Breathing Rate Monitor, completed as a part of the requirements for the course EE6350: VLSI Design Lab, Spring 2024.

Over the course of two semesters, we designed, simulated, taped out and successfully demonstrated the functionality of our chip. The Spring semester focused on the Chip Level design - system characterization, building different circuit blocks from scratch, designing their layouts, chip level floorplanning and simulations. The Fall semester focused on the measurements - initial prototyping, measurements characterization, PCB Design, testing and demonstration. Please find the video summary of our work over the Spring and Fall 2024 semesters below.

PPG is a popular technique used to measure vitals as heart-rate, breathing rate and blood oxygenation in a non-invasive manner. There are two types of PPG: Reflective and Transmittive. This project adapted the reflective PPG method in which the LED and photodiode (PD) are on the same plane as opposed to opposite sides of the finger. This was chosen to simplify the testing process (finger placement over a pad vs. clip contraption that would have to fit a variety of finger widths).

PPG captures variation cardiovascular data by assessing changes in blood volume. During each cardiac cycle, the blood pumping through the body distends through capillaries and subcutaneous tissues. Light is shone on a compressed region of the skin and reflects from the tissues. Since the volume inside the body part, usually the finger, varies during the cardiac cycle with the blood flow, the reflected light manifests as a pulsatile waveform, reflecting the cardiovascular vitals. This reflected light wave is shone on a Photodiode, which generates a current proportional to the light incident on it. This current signal can then be electronically conditioned to monitor the vitals. An image of reflective PPG is shown below. (Image source: Lambert Cause, J.; Sole Morillo, A.; Garcia-Naranjo, J.C.; Stiens, J.; da Silva, B. The Impact of Contact Force on Signal Quality Indices in Photoplethysmography Measurements. Appl. Sci. 2024, 14, 5704. )

The classical three LED colors (red, IR, and green) were all considered when determining which LED would be used for which signal chain measurement (heart rate and breathing rate). Both signal chains can be measured with green light which has the lowest wavelength and the least penetrative power, which means that it is just enough to reach the capillaries containing the pulsatile blood near the skin without accumulating noise picked up by non pulsatile (and therefore nonimportant) elements deeper in the finger like from the venous return, tissues, etc. Therefore this LED was used, benefitting breathing rate in particular, since the signal amplitude for breathing rate is typically much lower and hence more susceptible to being drowned out by noise.

The primary signal conditioning blocks (amplification, filtering and Analog to Digital Conversion) are accomplished through the chip. External sensors. active components and microcontrollers have been used to capture inputs and create system interfaces for measurements. The snapshot of the die of the chip is shown below.