Design Of High Precision And Low Power SAR ADC For Audio System

Nowadays,the human-machine speech interaction technology is being gradually applied in electronic systems.And the analog-to-digital conversion of audio signals is essential in the speech interaction technology.Generally,the conversion of analog signal to digital signal is achieved by an analog-to-digital converter.With the advancement of CMOS technology,the number of audio ADCs using successive approximation register structure is increasing,since SAR ADCs don't contain operational amplifiers.A high precision and low power SAR ADC for audio systems was designed and implemented in this paper,according to the characteristics of audio signals,the precision requirement of the subsequent digital system and the power requirement of the whole system.To achieve high precision,a design method of multi-stage capacitor arrays for highresolution SAR ADC was proposed.Firstly,two structures of multi-stage capacitor array were proposed,based on the voltage calculation at the output node of the capacitor array and the equivalent calculation of the capacitors' series and parallel.Secondly,applicable occasions for both multi-stage capacitor arrays structures were determined by the effect of parasitic capacitance and mismatch on the linearity of the multi-stage capacitor arrays.Finally,a capacitor array for 16 bit SAR ADCs was implemented with the proposed design method.The simulation result of the designed capacitor array showed that its nonlinear error was less than 0.3LSB with no gain error,and the sampling capacitance accounted for 92.42% of the total capacitance.Compared with traditional design method of capacitor arrays and other design methods in related researches,the proposed method effectively reduced nonlinear errors,improved SNR and optimized the area of SAR ADCs.To achieve low power,a calibration circuit was designed with the near-threshold technology while the calibration circuit was a unique sub-module of high precision SAR ADCs.Calibration circuit improved the precision of SAR ADCs,but it brought an additional power overhead.A low power calibration circuit could reduce the overall power of SAR ADCs.So,a calibration circuit based on near-threshold technology was implemented.The design flow of the near-threshold digital circuit was explained at first.Besides,circuit and layout of near-threshold cells,which can form the calibration circuit,were designed based on TSMC40 process.After that,a multiplier with near-threshold technology was implemented to verify the correctness of the design flow and test the performance of the designed near-threshold cells.Finally,a digital calibration circuit with near-threshold technology was implemented,based on the design flow of the near-threshold digital circuit and the designed near-threshold cells.The designed near-threshold calibration circuit for SAR ADCs can achieve the expected function at 0.5V supply voltage.Compared with the calibration circuit composed of standard cells at standard voltage,the designed circuit reduced the typical power by 75.44%.To implement a complete SAR ADC,circuit and layout of the main modules that make up the SAR ADC were designed and simulated.Bootstrap technology in sampling switch reduces the effect of the input signal on the resistance of the sampling switch.A comparator with the dynamic latch structure was designed.And a delay between two clock signals was added to improve the precision of the comparator.The simulation results showed that effective number of bits of the designed SAR ADC without calibration circuit was 13.8bit,Fo M was 115.2f J/c·s,and layout area was 323.92?m*291.39?m.For the complete ADC that includes calibration circuit,if the calibration circuit adopts near-threshold technology,the overall power of ADC can be reduced to 83.87% of the power of ADC without near-threshold technology.Therefore,the SAR ADC designed in this paper met the requirements of the audio system for the precision and power.