Design Of Vibrational Energy Harvesting Powered ECG Measurement System

Portable electronics have fueled the rich emergence of new applications including multi-media handsets,ubiquitous smart sensors and actuators,and wearable or implantable biomedical devices.Compared with professional medical equipment,although the performance of wearable health equipment is less superior,it is able to provide health care treatment anytime,anywhere and has great advantage in preventing and alleviating disease.In recent decades,the energy efficiency of electronic devices is significantly improved and biomedical electronic devices such as pacemakers and defibrillators as well.Still,these devices require external batteries to be activated.However,the external batteries have comparably large volume than devices and regular exchange is required due to lifetime issue.Furthermore,some batteries may have a safety issue in certain circumstances.Energy Harvesting Wireless Sensor Network is a distributed sensing system.It consists of a number of low cost sensor nodes which have the ability of energy data acquisition,detection,data aggregation and data transmission.By introducing energy harvesting technology,the energy in the surrounding environment can be transformed into electricity and stored in batteries.The electric power can be used to supply load circuits,such as the wearable biomedical equipments mentioned above.So,energy harvesting has gained more and more attention from both academia and industry.The issue to current self-powered wearable heart monitoring equipments is as follows:low efficiency in energy harvesting interface circuit;low power,low frequency of the ECG signal,the detection process is easily affected by environment and circuit noise.Based on energy harvesting powered ECG analog front end system,this paper presents the following two aspects:the rectifier and low voltage DC converter for vibration energy harvesting and the analog front-end circuit for ECG.In the vibration energy harvesting system,based on SMIC 0.18μm standard CMOS technology,an input-powered cross-coupled vibration energy harvesting rectifier is proposed.It can be applied in energy harvesting devices for the extremely low voltage and high conversion rate.The simulation results show that the minimum input voltage could be as low as 0.15V.Correspondingly,the voltage conversion efficiency can reach up to 80%.And the power conversion efficiency is also 80%when voltage equals to 0.25V.The proposed interface circuit can automatically shut down if the input voltage amplitude is enough low,thereby avoiding unnecessary energy loss.A low-input step-up DC/DC converter and power manager is also presented in this paper.It consists of a resonant boost oscillator,a reference voltage module,a charge control module,a power control module,a power switch module,a LDO circuit,a program resistor module and a power detection module.The proposed converter can work with the input voltage as low as 20mV.The extremely low input voltage makes it suitable for energy harvesting and power management.Four logic controlled outputs provide the best voltage for various applications to accommodate low-power design requirements.A low current LDO is utilized to provide a regulated 2.2V output for powering low power processors or other low power ICs.Reserve energy on the storage capacitor CSTORE provides power when the input voltage source is unavailable,thus prolongs the life of the system and expands the application range.Extremely low quiescent current（6μA）and high efficiency design（64%@300μA load current）ensure the fastest possible charge times of the output reservoir capacitor.Biomedical signals,which closely related to human health,have found their frequency distribution form a few Hz to several kHz,amplitude from dozens ofμV to dozens of mV.The acquisition of these signals are extremely vulnerable to medical equipment,detection platform and the surrounding environment.A high performance active dry electrode system for ECG signal monitoring in SMIC 0.18μm CMOS process is presented in this paper.AC-coupled core amplifier,input impedance boost loop and Back-End CMFB/DRL circuit are applied in this analog front-end circuit.The programmable gain is set from 20dB to 40dB.With the input impedance boost loop,a constant 2.4G?input impedance within the frequency range0.1Hz to 150Hz achieved.With the CMFB/DRL circuit,the CMRR reaches 85dB with the configuration of ac gain 40dB.In this design,amplifier-based active dry electrode is utilized,which achieves better noise performance than buffer-based active electrode,the equivalent input noise is 4.511μV_rms（0.5-100Hz）.The current consumption of the DRL-based and the Back-End-CMFB-based ECG front end circuit is 32μA and 23μA,separately.The circuit performance is good enough for biomedical signal acquisition.And the ECG front end circuit is compatible to the load requirement of the energy harvesting system proposed in this paper.