Abstract

　　A 2.88nJ/Conversion low energy biopotential acquisition system is designed for portable healthcare device. Two dry copper contact electrodes with 1.2-cm diameter are used to easily interface between skin and healthcare device. Chopping technique is adopted at readout front end to obtain thermal noise floor of 1.3uVrms over 0.5~200Hz and CMRR over 100dB to mitigate common-mode body potential induced from AC power line. A 4-stage gain control and band selection blocks are integrated to digitally calibrate for different types of biomedical signal and an 8-bit synchronous successive approximation register （SAR） A/D is used to digitize sensed biopotentials. A test chip is implemented in 0.18um， 1.8V supply CMOS technology and successively verified by readout ECG signal with two electrodes contact at chest of body with separating 6cm.

　　I. INTRODUCTION

　　ECG， EMG and EEG are common biomedical signals that give continuous information for patients undergoing surgery or those admitted to intensive care units （ICUs）。 Conventionally， patients are measured by bulky and mainpowered instrument with several wired leads connected on body which can greatly reduce patient’s mobility and white coat effect in hospital also makes an erroneous diagnosis. Common portable health care approaches which use three Ag/AgCl electrodes with conductive solution provide good signal quality in Skin-Electrode interface ［1］。 However， it has the problem of inconvenient patient attachment; also it may cause inflammation of the human skin and limits its reusability and continuous health monitoring. Furthermore， the third reference electrode forms a low impedance node to ground. Hence， patients suffer from hazard of leakage current. Consequently， a low energy， small form factor， two dry electrodes biopotential acquisition system is attractive for ambulatory healthcare application. Previously， several types of commercial metals were investigated as a wearable electrode ［2］。 The impedance fluctuation of the different metal electrodes is less than one decade in the frequency range less than 1kHz compared with Ag/AgCl. This indicates that a copper on PCB as an interface between skin and portable instrument can replace the Ag/AgCl electrode without any electrically conductive solution. However， its main drawback is that power line interference problem is severe in twoelectrode recording system ［3］［4］。 Short distance between two electrodes can reduce electromagnetic interference （EMI） in biopotential measurements since this approach will cause power line 60Hz noise common to readout front end. However， it also suppresses differential signal strength because of the short distance between the electrodes. Moreover， due to the lack of the conductive solution， the skin-electrode interface impedance of dry electrode is much larger than conventional Ag/AgCl electrode which could also degrade signal potential as shown in Fig.1. Therefore， in order to extract biopotenials under such circumstances， the noise and CMRR requirements of analog readout front end are more stringent， while consuming low power. According to ［4］， only a micro volts noise level is allowed to maintain recorded signal quality at the front end and over 100dB in band CMRR and PSRR of readout front end are also required for robust operation regards to power line interference in such circumstances. Simultaneously， the biopotential acquisition system need to have a programmability for its gain and bandwidth ［5］ to meet different signal potential and frequency characteristics.