Research And Design Of High Performance Sigma Delta ADC For Medical Ultrasound Imaging

With the continuous development of digital technology,new requirements for analog to digital converter(ADC)are put forward.Sigma Delta ADC adopts two key technologies:oversampling and noise shaping,which can achieve higher resolution even with low precision modulator.It is the best method to achieve high precision A / D conversion in CMOS technology.In recent years,with the rapid development of biomedical,communication and lossless audio,high-precision Sigma Delta ADC has been widely used.Sigma Delta ADC can effectively improve the signal-to-noise ratio in the signal bandwidth by oversampling technology,which can exchange time for precision.At the same time,oversampling technology reduces the requirements of traditional Nyquist rate ADC on the matching precision of components and alleviates the difficulty of analog circuit design.It is the mainstream technology of low speed and high precision ADC.How to further expand the bandwidth application range of Sigma Delta ADC while ensuring the accuracy is a hot topic in the research field of Sigma Delta ADC.In the low frequency band,the research and practical application of Sigma Delta ADC has been very rich.This paper focuses on the research of high-precision Sigma Delta ADC used in ultrasonic medical Megahertz band.For the input signal below 1MHz,high oversampling rate can effectively improve the quantization noise.The Sigma Delta ADC in ultrasonic field means that the sampling frequency can not be too high,and the modulation focus is on the noise shaping of multi-stage cascade and multi-bit quantizer.The important advantage of Sigma Delta ADC is that its matching requirements for analog modules are far lower than other Nyquist ADCs,so it has very high robustness.Generally speaking,Sigma Delta ADC is divided into analog modulator and digital decimation filter.This paper starts from the system architecture and compares the advantages and disadvantages of various ADCs in principle.After clearly adopting Sigma Delta ADC architecture,the sub architectures of Sigma Delta ADC are compared horizontally,and the system level design of MASH2-2 based on CIFF structure is completed.In the process of Matlab modeling,the coefficients of each integrator are defined.Then,on the basis of theoretical guidance and Simulink modeling and simulation,the overall structure of the modulator and the performance index of each circuit module are determined.Then,the paper focuses on the working principle and design process of each circuit module of the modulator,such as switched capacitor integrator,operational amplifier,non overlapping clock generation circuit,multi-bit quantizer,feedforward summation module and DAC.On this basis,the system design of the modulator is completed.Compared with other modulators,this paper uses multi-bit quantized Flash ADC as quantizer,which can still improve the bandwidth in the case of low oversampling rate,and uses DEM correction algorithm to reduce the harmonic distortion caused by the mismatch of multi-bit DAC capacitor array and improve the linearity.At the same time,in order to achieve the randomness of the correction algorithm,the pseudo-random number generator is built by using the simulation module to provide control bits for the correction algorithm.As for the digital circuit part,in Modelsim software,the Verilog code is used to realize the unique noise cancellation module of multi-stage cascade structure.Then the classic CIC filter,CIC compensation filter and half band filter are used to form a digital decimation filter,which can restore the oversampled signal to the Nyquist frequency and filter out the band modulated to the high frequency band Noise.TSMC 0.18 CMOS process is adopted in the whole circuit structure,and the power supply voltage is 1.8V.The bandwidth of the modulator is 1MHz and the sampling frequency is64 MHz.Under the conditions of 78.125 KHz input signal frequency,64 MHz sampling frequency and 32 times oversampling rate,the overall output SNDR is 86.3db,and the significant bit is 14.04,which meets the design requirements.