Low-voltage Low-power High-precision Sigma-delta Modulators

As the bridge between analog world and digital world,analog to digital converter(ADC)is an important part of integrated circuit design.With the rapid development of the Internet of things(IoT),the designing of the sensor and its readout circuit is becoming more and more important.Analog to digital converter,as the main circuit module of the sensor,often determines the performance of the sensor.In recent years,with the rapid scaling of integrated circuit technology to deep submicron,the integrated circuit supply voltage is getting lower and lower,which makes the voltage headroom of analog circuit design become smaller and smaller.In addition,the human implanted chips and some portable devices which are extremely sensitive to power consumption put forward further requirements on the analog circuit design.In this paper,the technology of low voltage,low power and high precision sigma delta ADC is studied.In the paper,the basic principle of sigma delta ADC is summarized,and two main features of the sigma delta modulator and different system structures have been discussed.The paper also analyzes the influence of some main non-ideal factors on the overall performance of the sigma delta modulator.Then,according to the design parameters and the comparison of different system structures,a three-order single-loop one-bit quantized feedforward sigma delta modulator system structure is selected to perform the system design simulating.Based on the Matlab/Simulink design tools,by analyzing some non-ideal factors which may exist in the actual circuit the simulation results can be obtained and the coefficient of the sigma delta modulator is adjusted by considering synthetically.At the same time,the key points and difficulties of the low voltage low power and high precision sigma delta ADC are determined,including the design of low voltage and low power amplifier,the suppression of 1/f noise and the realization of the three order feedback system.In order to solve these problems,this paper made a detailed research on subthreshold design technology and bulk-driven design technology,analyzed their advantages and shortcomings.On the basis of that this paper proposed to use the chopper technology to suppress 1/f noise and put forward the design of a low voltage low power amplifier.In addition,this paper also studied the use of auto-zero technology to compensate for the dynamic offset compensation,in order to eliminate the effect of 1/f noise and offset voltage,thereby improving the signal to noise ratio(SNR)of the circuit.Furthermore,based on the digital inverter,a low voltage and low power class AB operational amplifier is designed.Finally,a low voltage,low power and high precision three order sigma delta modulator has been designed based on the class AB operational amplifier.A three-order single-loop one-bit sigma delta modulator is designed using CSMC 0.5 um technology,using the signal feedforward and local feedback system structure,with a supply voltage of 1.8 V,signal bandwidth of 1000 Hz,sampling rate of 64,the sampling frequency of 128 kHz.The power consumption of the sigma delta modulator based on very low power consumption AB class op amp is 756 nW,the peak signal to noise ratio(SNR)is 90 dB,the peak signal to noise plus distortion ratio(SNDR)is 87.35 dB,and the precision is 14.3 bits,which fulfils the prospective target.