Low-power differential photoplethysmographic pulse transit time detector for ambulatory cardiovascular monitoring

This paper presents a low voltage and low power differential photoplethysmographic (PPG) pulse transient time (PTT) detector for ambulatory cardiovascular monitoring. PTT is defined as the time required for the heart beat pressure wave to travel between two points in the body along the arteries and is used as an indirect way of measuring blood pressure (BP). The system´s input dynamic rage is optimized by early cancellation of the inherent DC component of the PPG signal at the sensor stage. Common mode light signal rejection is implemented using two crossed zero-biased photodiodes per PPG sensor. Also, robustness of PTT measurement to non-matched input signal magnitudes between the different PPG sensors is achieved by means of an amplitude and rate of change ratio comparison algorithm. The circuit operates with a maximum power supply of 1.25 V and consumes 2.5 muW of power.