This paper describes a novel photoplethysmography-based blood pressure estimation circuit developed as a non-invasive and continuous alternative to current clinical blood pressure monitoring devices. The system integrates the functionalities of analogue front-end and analogue-to-digital converter elements in conventional photoplethysmography sensors and extracts features related to blood pressure on chip. The topology operates in analogue domain, current mode and ultra-low-power, thus suppressing digitisation and current-to-voltage conversion errors and reducing overall power consumption. The circuit has been designed in CMOS 65nm technology. It employs a 1.5V supply, consumes $\\sim$345nW (five times less power than state-of-the-art) and generates feature voltages with a 50mV max. deviation. A linear regressor trained with the extracted features achieves 0.7$\\pm$0.9 mmHg accuracy (grade A performance). The operation of the topology is described and results depicting its performance are presented.