Sub-threshold Operation and Cross-Hierarchy Design for Ultra Low Power Wearable Sensors

This paper examines the requirements of wearable sensing applications and their implications for designing the next generation of body area sensors. We define key metrics for wearable sensors and discuss how body area sensors differ from generic wireless sensors. To explore the system level issues with a wearable node, we show measurements from a wearable electrocardiogram (ECG) sensor prototype. Using heart rate monitoring as an example, we show how ultra low power (ULP) circuit design must be applied to support the stringent energy and/or power demands of long-life wearable sensors. Specifically, sub-threshold operation of digital circuits creates opportunities for re-thinking the entire system. We conclude that we can only reach the lower limits of power consumption through cross-hierarchy design of the entire sensor node that leverages ULP digital circuits.