| In order to process the analog signals in nature with digital circuits and digital signal processing technology,it is necessary to use an ADC(analog-to-digital converter)to convert the analog signals into digital signals.Compared with the traditional Nyquist ADC,the Σ-ΔADC adopts oversampling and noise shaping technology,which has a good suppression effect on in-band noise.On the one hand,high conversion accuracy can be achieved with less hardware overhead and low device matching requirements;on the other hand,a lot of design work is shifted from complex analog circuit design to digital circuit design,which reduces the difficulty of design.Therefore,in recent years,Σ-Δ ADC have become the mainstream choice for ADC in high-precision applications such as audio and biomedicine.The Σ-Δ ADC is mainly composed of two core parts,a Σ-Δ modulator and a digital decimation filter.The sigma-delta modulator was mainly studied,and a feedback type 2-order2-bit quantization Σ-Δ modulator circuit with input feedforward for audio codec was designed.The research status and development trend of Σ-Δ ADC,were firstly introduced.And then the performance index,working principle and basic structure of Σ-Δ ADC were analyzed in detail.On this basis,the system model of the Σ-Δ modulator was established,and then the nonideal factors that affect the performance of the modulator were modeled and analyzed,and the design index was determined for the actual circuit module design.Next,according to the proposed modulator model and sub-module indicators,based on the 0.35μm CMOS process,the circuit design and pre-simulation of each modulator sub-module,including integrator,2 bit quantizer,two phase non-overlapping clock and feedback digital to analog converter(DAC)were completed.Finally,the layout design and post-simulation of the modulator were completed.The post-simulation results show that under the condition of 24 k Hz signal bandwidth and256 times oversampling rate,the Signal to Noise and Distortion Ratio(SNDR)of the designed sigma-delta modulator circuit reaches 91.2d B,and the Effective Number of Bits(ENOB)reaches 14.86 bit.Compared with the common second-order structure sigma-delta modulator,this work achieves a higher SNDR through a higher 256 times oversampling rate and 2 bit quantization technology.Compared with the common sigma-delta modulators that use highorder structures to achieve high SNDR of higher than 90 d B,the circuit structure of this work is simpler and the hardware overhead is smaller.The performance of the modulator designed in this thesis meets the requirements of highfidelity audio signal processing,can be used in high-fidelity audio codecs,and has practical engineering application value. |
