A Low-Power Heart Rate Sensor with Adaptive Heartbeat Locked Loop

Photoplethysmography (PPG) is one of the widely used noninvasive heart rate (HR) monitoring techniques in wearable devices. Lighting of the LED dominates the power consumption of a PPG sensor. Lowering the LED lighting duration is an effectively approach to save power. This paper proposes an adaptive heartbeat locked loop (AHBLL) technique, which can dynamically adjust the dividing ratio N according to the heart rate derivative (HRD). In this way, the LED pulse duty cycle can be significantly reduced to save power. To improve the robustness of the AHBLL system, the relationship between HRD and the dividing ratio N is theoretically analyzed, which provides an optimal design guideline. To verify the proposed technique, an HR sensing circuit including an HRD detector is designed and simulated. The results show that the LED power consumption is reduced by 2.2~3.3× compared with the state-of-the-art heartbeat locked loop (HBLL) technique.