Study And Design Of Sigma-Delta Modulator For Biomedical Applications

With the development of science and technology,people put forward higher requirements for the monitoring accuracy and power consumption of portable wearable equipment.Analog signals generated by nature need to be converted into digital signals for processing,so analog-to-digital converter(ADC)has become a research hotspot.Biomedical signals are characterized by low frequency,low amplitude and high noise interference,so high precision analog-to-digital converters are needed to restore the collected signals to a greater extent.Sigma-Delta ADC is the first choice of high-precision ADC,and the performance of Sigma-Delta modulator directly determines the performance of the whole system.In this paper,a 14 bits three-order CIFF one-bit quantization structure Sigma-Delta modulator is analyzed and designed,and the ΣΔ modulator is modeled and simulated using MATLAB to provide theoretical guidance for the whole system;in order to avoid introducing non-linear errors,one-bit quantization is adopted;in order to improve the stability of the system and reduce power consumption,a high-order single-loop structure is adopted;in order to improve the accuracy of the modulator and further reduce power.Full feed forward CIFF structure is adopted.Through the validation of MATLAB SIMULINK modeling,the accuracy of 16.65 bits and 102 dB SNDR can be achieved by using OSR=128,third-order single-loop and full feed-forward CIFF structure.The modulator coefficients are mapped by MATLAB and simulated in SIMULINK to get the preliminary data of transistor-level circuit.Based on the simulation results of MATLAB,the transistor-level circuit design and Simulation of Sigma-Delta modulator system are carried out by using TSMC 0.18 um CMOS process.Because the high precision reference voltage source can improve the accuracy of ADC,a high precision low power sub-threshold full CMOS reference voltage source is proposed in this paper.In order to improve the accuracy,current addition technology is introduced;in order to reduce the area,passive resistance is replaced by 1.8V and 3.3V MOS transistors;in order to reduce power consumption,MOS transistors working in sub-threshold region are designed.The simulation results show that under 1.8V supply voltage,the PSRR of the reference voltage source is-81dB@100Hz,with a temperature drift coefficient of 8.2ppm/C in the temperature range of-35-150 and its power consumption is 185 nW,with a voltage adjustment rate of 0.21% in the voltage range of 1.3V-3.3V supply.The whole Sigma-Delta modulator system is simulated by Cadence Spectre.The simulation results show that the effective bit(ENOB)of the modulator is 14.3 bits,the signal-to-noise distortion ratio(SNDR)is 87.8 dB and the power consumption of the ΣΔ modulator is 0.7 mW at 1.8 V supply voltage,OSR=128 and input signal frequency of 1 KHz.