A 1mW Vitals Monitoring System for Asthmatic Patients based on Photoplethysmography

The number of patients suffering from asthma, especially in air-polluted regions, has dramatically risen. Thus, the need for a wearable system that is able to monitor the patient's condition is essential. Continuous heart rate (HR) and blood oxygen concentration (SpO2) measurements are crucial for asthmatic patients. This paper presents a low power, portable/wearable system to measure the HR, heart rate variability (HRV) and SpO2 based on Photoplethysmography. The analog front end (AFE) consists of a switched capacitor trans-impedance amplifier and switched capacitor low pass filter to realize a high gain (120dB) and low cut off frequency (10Hz) on-chip. The proposed AFE enables a photodiode input DC current up to 20µA with input-referred current noise of 7.3pA/√Hz. The system is implemented in 180nm CMOS with a die area of 2.6mm2 while consuming 1mW/2.3µW from a 0.5V supply for LED drivers and AFE, respectively. The HR/HRV/SpO2 extraction processor is implemented on FPGA. The maximum absolute error percentage in HR/SpO2 measurements from an experiment involving 21 subjects comes out to be <1%.