Design And Research On 24bit-2kHz High-precision Sigma-delta Modulator System

As people's living standards have generally reached a well-off stage,everyone's pursuit of health is increasing,stimulating the further development of medical equipment.The signal processed by the medical equipment is a digital signal,and the human body signal is an analog signal,so an analog-to-digital converter(ADC)is needed to convert the measured human bioelectric signal into a digital signal to facilitate processing.The bioelectric signal in the human body is a small signal with low frequency and low amplitude,which usually needs a high-precision ADC to process it.Among all ADCs,Sigma-Delta ADC performs best in terms of high accuracy.At the same time,due to the low frequency of the bioelectric signal,the Sigma-Delta ADC can take a high oversampling rate to achieve higher accuracy,so the Sigma-Delta ADC is usually used to process the bioelectric signal.In the Sigma-Delta ADC,the most critical part is the Sigma-Delta modulator,which directly determines the performance of the entire ADC.Sigma-Delta modulators are mainly divided into two structures: DT(Discrete Time)and CT(Continuous Time).Based on this,this paper adopts DT and CT structures to study and realize two high-precision Sigma-Delta modulators.This article first analyzes the basic structure and working principle of the Sigma-Delta modulator,focusing on oversampling technology and noise shaping technology.On this basis,the loop structure of the Sigma-Delta modulator is analyzed,and the advantages and disadvantages of different loops are discussed in depth,and then the difference between DT and CT structures is analyzed to lay a theoretical foundation for circuit design.Then,the DT Sigma-Delta modulator circuit is designed.As the basis of CT,DT structure has important research significance.In this paper,a system-level high-level model of the DT modulator is built,and the system function is verified by mathematical methods.Among them,the oversampling rate of the DT modulator is set to 256.This is because the frequency of the bioelectric signal is low,so a higher oversampling rate is allowed to improve the accuracy of the modulator.Because the DT modulator needs to sample the signal first,and use highprecision switches,this article uses a gate voltage bootstrap switch as the first-stage integrator sampling switch to ensure the accuracy of the sampled signal.Taking into account the high-precision application requirements,a multi-bit quantizer is introduced to quantize the signal.At the same time,due to the process deviation of the multi-bit quantizer,an inevitable mismatch error will be introduced in the feedback DAC.This design has been calibrated: on the basis of the DWA method,a pseudo-random DWA calibration method is proposed.Different from the traditional DWA,this method further randomizes the feedback DAC gating by periodically inserting pseudo-random numbers when the DWA rotates.The simulation results show that the pseudo-random DWA can calibrate the DAC mismatch well.Based on the above research on the DT structure,this paper proposes a new type of CT Sigma-Delta modulator structure,which has the following characteristics: In view of the fact that the CT Sigma-Delta modulator is less sensitive to the clock than the DT structure,this paper further improves oversampling The specific value is 2048,which effectively improves the modulation accuracy;considering that the noise transfer function generated by the SD toolkit is discrete time,and the loop transfer function of CT Simga-Delta is continuous time,this article uses Matlab to perform equivalent processing;The output of the modulator is delayed by a half cycle through the flip-flop,thereby compensating for the delay in the actual circuit of the quantizer,DWA and DAC.Finally,this paper has completed the physical realization and post-simulation verification of the two proposed modulator structures.Among them,the DT structure modulator circuit is implemented using SMIC 0.35?m process,its working voltage is 5V,the sampling frequency is 1.024 MHz,and the modulator output is analyzed by FFT.The result shows that the SNDR is 115.6d B,and the effective number of bits reaches 18.92bit;while the CT structure is realized by SMIC 0.18?m process,the working voltage is 1.8V,the sampling frequency is8.192 MHz,and the FFT output analysis shows that the SNDR is 125.7d B,which is effective The number of digits reaches 20.60 bit.The bandwidth of both modulators is 2k Hz.