Circuit Design Techniques For Integrated Bio--Signal Acquisition Front--Ends

With the rapid development of microelectronic,communication and signal process-ing technologies,wearable physiological signal acquisition system becomes possible and gradually grows into a research hotspot.On the one hand,real-time acquisition of physio-logical signals such as electrocardiography(ECG),electroencephalography(EEG),elec-tromyography(EMG),etc.,realizes long-term monitoring of patients with disease as car-diovascular and epilepsy,achieves timely treatment and prevention,shortens the time and space of physician-patient interaction,reduces medical costs,and releases scarce medical resources.On the other hand,wearable physiological signal acquisition is potentially used in entertainment,health,education and other emerging industries.Brain-computer inter-face(BCI)builds control and communication channels between electronic devices and human brain,thus deriving infinite possibilities.For example,the combination of virtual reality(VR),EMG acquisition and other technologies has accelerated the development of immersive entertainment games.The combination with mechanical electronics and ex-oskeleton has raised human physical limits,and even made the super human in science fiction movies possible.This dissertation mainly studies the front-end circuit design of physiological electri-cal signal acquisition under non-invasive conditions.The main difficulty of non-invasive physiological signal acquisition is to acquire small signals under large interference.Large interference includes electromagnetic interference from the environment,especially the50/60 Hz interference of the power line,as well as various artifacts interference.And because of the relatively weak signal,the front end need to have strong anti-interference capability at the system level,which requires the front end to have high common-mode(CM)rejection ratio(CMRR),high input impedance and low noise.Low power circuit design is also preferred to enhance the equipment endurance.This dissertation firstly describes the background and significance of physiological electrical signal acquisition,and introduces the basis of physiological electrical signal acquisition,including the generation mechanism,classification and characteristics of bio-electrical signal,the working principle and classification of bio-electrode,and the non-ideal factors in the signal acquisition process.Then,the main circuit structures of the front-end are studied,and the techniques of power optimization,impedance enhancement,CMRR enhancement,pseudo resistor design,electrode offset suppression,output ripple suppression and input CM range enhancement are introduced.Capacitively-coupled in-strumentation amplifiers(CCIA)are widely used in physiological signal acquisition front-end due to its simple structure and low power consumption.However,the anti-interference capability of CCIA is weak,where the CMRR is usually limited to 60-90 dB.Based on this structure,the following technologies are proposed to improve the CM rejection ability:1)Virtual ground chopping including the pseudo resistors:By placing the pseudo resistors used for CM biasing and high-pass corner frequency generation inside the chopping loop,the low frequency CMRR degradation is mitigated.2)Successive-approximation based capacitor mismatch trimming:With the proposed virtual ground chopping,the trimming loop further reduces the mismatch of capacitors and increases the CMRR of CCIA from traditional 60-90 dB to about 110 dB.3)Common-mode replication:By replicating the input CM to output,the CM current flow in differential signal branch is greatly impeded,thus increasing the CMRR.The measured prototypes achieve>130 dB CMRR at 50 Hz,which is 20 dB higher than the state-of-the art.4)Self-regulating bias:With CM replication and self-regulating bias,all nodes of the transistors in the dif-ferential branch follow the input common mode.This ensures that the voltage difference between all terminals of the transistor remains stable,thereby increasing the input CM range of the amplifier.Measurement results show that the amplifier achieves greater than110 dB CMRR with input CM up to 900 mV.5)Common-mode current neutralization:With CM replication,the CM input impedance of the chip is only limited by the par-asitic of the ESD cells.In this dissertation,a CM current neutralization loop is proposed,which can be equivalent to a negative CM capacitor,thus canceling the CM parasitic of ESD cells.Test result shows that the chip achieves 50 G?(@50 Hz)CM input impedance,which leads to >102 dB total CMRR under 1 M? || 10 nF electrode impedance mismatch.The first two points are the main innovation of the first work,and the last two points refers to the second work during the doctoral period,which will be detailed in Chapter ? and ? respectively.Compared with similar designs,the performance of the two IAs in this dissertation achieve the leading level.