The Study Of The Key Technology On Multiplex Preamplifier Based On AFE Application

With the introduction of semiconductor technology in the biomedical field,the new cross-disciplinary “biomedical microelectronics technology” is also developing rapidly.The bioelectrical signal acquisition system uses the relevant technology of the discipline to combine microelectronics and modern medical care to realize real-time monitoring of patients at home,which not only alleviates the lack of medical resources,but also greatly improves the quality of patients' life.The analog front-end circuit,as the first stage of processing the bioelectric signal,plays an important role in the bioelectric signal acquisition system.Based on the weak and low frequency characteristics of bioelectrical signals,a multiplexed analog front-end circuit for bioelectrical signals is studied in this paper.The background and status of the research content was introduced first in this paper,and the biological electrical signal and biological electrode was introduced in detail,then we analyzes the noise and interference that will be encountered during the collection process.At last the detailed design of all modules in the circuit is carried out.Among them,the structure of the preamplifier is current feedback instrument amplifier(CBIA),and eliminate the low-frequency noise with a low-pass filter connected in the form of negative feedback,common-mode feedback(CMFB)was introduced to stabilize the common-mode output.For the purpose of chip area saving,the design of the gain stage consists of a fixed gain amplifier and a variable gain amplifier,a structure of symmetric OTA was adopted for the operational amplifier.The different gain and bandwidth can be obtained by changing the value of the feedback capacitance.The multiplexer module uses a four-data selector to complete the function of selective output;the band gap reference source provides a stable reference voltage and current for the analog front end.In the auxiliary module,the right leg drive circuit is used to eliminate the human frequency interference and improve the common mode rejection ratio,the connection of the electrode can be monitored in real time by the lead monitoring circuit.Finally,the input impedance of the circuit was be optimized.The technology of the circuit was SMIC 0.18?m CMOS,the power supply voltage is 1.8V,and made the simulation under Cadence.The structure of the preamplifier is the full differential structure of current feedback instrument amplifier,the gain is 20 d B,the high pass cut-off frequency is 0.5Hz.The programmable gain amplifier consists of a fixed gain amplifier and a variable gain amplifier.There are 5 kinds of gain selectable: 20 d B,50.8d B,54.7d B,56.5d B,58.6d B.The bandwidth is 1360 Hz,348Hz,231 Hz,189Hz,152 Hz respectively.The multiplexer's simulation results show that the output can be correctly selected based on the different control signals.The bandgap reference circuit provides the reference voltage and current for the circuit.The temperature drift coefficient of the voltage and current is 9.87ppm/°C and 12ppm/°C respectively.The convolution noise in 0-100 Hz is 32 n V,and the noise performance is excellent.Finally,the whole analog front end was simulated.The gain can be adjustable to 40.5d B,75.8d B,77.7d B and 78.7d B,The corresponding bandwidth is 171 Hz,234Hz,347 Hz,and 1210 Hz,the high pass frequency is 0.5Hz,the equivalent input reference noise is 1.75?V,and the CMRR of the circuit under the Monte Carlo simulation is 91 d B.The input impedance is optimized by adopting the structure of capacitive positive feedback and increased to 2.6G?,the signal accuracy is effectively improved.In summary,the analog front-end circuit designed for bioelectrical signal acquisition meets the design requirements.