Research And Design Of 24-bit Sigma-delta Modulator For Medical Electronics

The analog-to-digital converter(ADC)is an important interface for connecting analog and digital circuits.With the widespread use of mobile consumer electronics products,such as mobile phones,bracelets,and tablet computers,and the emergence of emerging wearable devices,high-performance,low-power ADCs have become the focus and goals of researchers in the field of analog and digital-analog mixed circuits.With the rapid development of medical technology,the ability of medical equipments to detect and analyze physiological signals is essential.Whether it is a thermometer,a sphygmomanometer,or drawing an electrocardiogram or an electroencephalogram,the detection of analog signals depends on sensors,and the core unit for analyzing analog signals is ADC.Therefore,the performance of the analog-to-digital converter directly determines the performance of medical equipments.Sigma-delta ADC uses oversampling,noise shaping,and down-sampling digital decimation filtering techniques to process low-precision signals in the analog field through less demanding analog circuits,reducing the difficulty of analog circuit design,and performing high-precision signals in the digital domain processing fully utilizes the powerful signal processing capabilities of digital circuits.The extremely high effective number of bits is obtained with low power consumption and simple circuit.It is used in high-precision audio circuits,high-precision measuring instruments,and physiological signal measurement.The existing SOC also widely uses mature ADC IP.Sigma-delta ADC has been more and more widely used and gradually researched because of its high resolution,high linearity and low cost.This article first analyzes the macro working principle of analog-to-digital converters,based on a systematic understanding of most types of ADCs,and then compares the advantages and disadvantages of various converters and applicable occasions,combined with the input signal characteristics of the target application field,selected Sigma-delta ADC as the design plan.Next,we will conduct an in-depth analysis of the working principle of the Sigma-delta modulator,by comparing the pros and cons of various architectural methods and the requirements for use.After summarizing the relationship between the performance of the Sigma-delta modulator and the order,oversampling rate,and quantizer of the modulator,the SD Toolbox is used in Simulink to model and simulate the Sigma-delta modulator.The second-order single-loop quantized CIFB structure is selected as the target,and the rough parameter requirements corresponding to each circuit module are obtained.In the specific circuit design process,on the one hand,the use of more advanced engineering circuit structures(such as high-performance low-power switched-capacitor operational amplifiers,switched-capacitor common mode feedback,etc.),on the other hand,introduce some that can significantly optimize the modulator circuit technical means of performance(such as chopper technology,etc.).Finally,under the SMIC 0.18 μm CMOS process condition,using 3.3V analog voltage and 1.8V digital voltage power supply,the circuit of the second-order Sigma-delta modulator of the 24 bits output is completed in cadence.The design goal is reaching 16-bit effective number of bits.The simulation results show that when the input signal amplitude is 1.08 V and the frequency is 2.5k Hz,the output signal obtains 98.3d B in signal to noise ratio.That is to say,the effective number of bits is 16.03 bits.The power consumption is 21.6m W,and the whole circuit has achieved the expected design goal.