| With the cross and integration of information science,microelectronics,neurophysiology and medical electronic technology,a new research field has been formed on bioelectrical signal acquisition technology for medical monitoring.In the process of surgical treatment of epilepsy and other neurological diseases,intracranial electrode EEG recording plays a very important role in the accurate location of epileptic lesions.It is of great significance for the clinical work to transmit the intracranial EEG signal wirelessly,realize the long-time EEG recording without wound and reduce the risk of postoperative infection.At the same time,capturing the high-frequency nerve discharge pulses that cause epileptic seizure "origin" can improve the accuracy of lesion location and greatly reduce the brain tissue damage caused by surgical treatment.Therefore,the development of implantable multi-channel EEG recording system has a very urgent need in the diagnosis and treatment of epilepsy.The amplitude of EEG is small and the source impedance is high.To capture high-frequency neural discharge pulses,higher requirements are put forward for the performance of EEG recording analog front-end circuit,such as low-noise and broadband design with high input impedance range.In addition,the interference of DC offset and power frequency introduced by recording electrode will seriously affect the recording quality of EEG signal,which brings great challenges to the signal conditioning ability of analog front-end circuit.This thesis introduces the generation mechanism and electrical characteristics of electroencephalogram(EEG)from the perspective of medical diagnosis,and analyzes the electrode sensing model of EEG signal and the interference factors affecting the integrity of EEG signal.Focus on the research of low noise,low power consumption,high input impedance and interference suppression technology of EEG signal conditioning analog front-end circuit,so as to effectively improve the overall performance of EEG signal recording analog front-end circuit.In order to deeply understand the relationship between the parameters of each functional module and the parameters of the analog front-end chopper amplifier,and to guide the parameter design range and adjustment direction of the circuit design process,the transmission characteristics of the two-stage closed-loop chopper amplifier are analyzed by using the harmonic transfer matrix(HTM)analysis method.The frequency compensation method of two-stage closed-loop chopper amplifier is obtained through the analysis,and the compensation characteristics are quantitatively analyzed based on the experience of parameter design of two-stage closed-loop chopper amplifier.A new fully integrated front-end circuit with low noise,low power consumption and high input impedance is proposed,which is composed of low-noise high input impedance chopper instrumentation amplifier,low-pass filter,programmable gain amplifier and channel selection switch.In order to meet the wide band application requirements of high input impedance range,a three-OTA two-stage closed-loop chopper amplifier structure is proposed,which makes the front-end amplifier have high original input impedance.On this basis,the impedance boosting circuit with negative impedance compensation has been introduced,which makes the input impedance of the amplifier reach a higher level,and has the effect of high input impedance frequency broadband effect.According to the high-performance requirements of common mode interference suppression in EEG acquisition process,a common mode feedback(CMFB)loop is designed to make the amplifier have high CMRR and input common mode swing tolerance.A DC servo loop(DSL)is designed to suppress the DC offset caused by the polarization of the recording electrode.A novel 4-stage switched capacitor integrator is designed in the DC servo loop.The integrator uses small on-chip capacitors to achieve a larger integral time constant,so that the high pass cut-off frequency of the chopper instrumentation amplifier is extended below 1 Hz.Integral OTA adopts fully differential chopper amplifier structure to suppress the flicker noise effectively.Based on 0.18 μ m CMOS technology,the analog front-end circuit designed in this thesis has been taped twice.The first is a 24-channel analog front-end using the three-OTA two-stage closed-loop chopper instrumentation amplifier.In order to further improve the input impedance of the analog front-end circuit,the negative impedance compensation impedance boosting technology is introduced on the basis of the first one.The chip test results show that the chopper amplifier itself has a high input impedance of 280 MΩ before negative impedance compensation is introduced.The input impedance is increased to 5.7 GΩ after compensation.It can still reach 4.6 GΩ at 100 Hz.The signal bandwidth of 1 GΩ input impedance is 300 Hz.The test results show that the input impedance of the front-end amplifier reaches or exceeds the highest level reported in the literature.The CMRR and PSRR are 98 d B and 83 d B at 50 Hz AC power frequency.The maximum input common mode voltage tolerance is more than 320 m Vpp,and the maximum input DC offset tolerance is greater than ± 150 m V.The analog front-end has a-3d B bandwidth of 0.6 Hz to 5.4 k Hz and a programmable gain of 39.8 to 52.9 d B.The equivalent input noise spectral density is 125 nv/rt Hz.The equivalent input noise in the 0.5 Hz to 1 k Hz integral bandwidth is 4.1 μVrms.The single channel power consumption of the analog front-end for EEG recording is 1.8 μW,the noise efficiency factor is 5.3.All the parameters of the circuit meet the design requirements.The programmable amplifier gain configuration can meet the application requirements of different electrodes and implant position.In order to verify the performance of the designed chip in bioelectrical signal acquisition,ECG amplification test is carried out based on the three-electrode lead method,without using the right leg drive circuits.Clear heartbeat pulse rhythm can be seen through the oscilloscope with band limit setting.It is further proved that the analog front-end circuit designed in this thesis has excellent performance in the input impedance and common mode rejection characteristics,which lays a good practical foundation for the later implantation test. |
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