Researches On Design And Implement Of 16 Bit Audio ADC Based On Sigma-delta Modulation Technology

In this paper, the sigma-delta modulation technology based on oversampling and noise shaping is studied thoroughly, after theoretical analysis and computer simulation, some significant conclusions are obtained, the whole scheme for designing this type of ADCs is also presented.The high order modulator is the main difficult of the design. First, the linear model of the modulator is discussed, the relationship between the coefficients of the modulator and the noise transfer function is also studied; afterward, we use butterworth highpass filter to approach the modulator coefficients, including: choose the poles and the out of band gain to satisfy the requirements of stability and dynamic range,optimize the zeros of the transfer function to increase the signal-to-noise ratio in the band of interest,change the gain of the signal transfer function to extend the allowed range of the input signal,add dither circuit to eliminate the impact of idle-tone and use root locus method to validate the stabilization of the modulator. After accomplishing the design of the modulator, the scheme of using three cascaded filters of comb filter and halfband filter as decimation filter is presented, the work principle,character and design procedure of these two types of filter are introduced particularly, besides that, the method of using canonical signed digits(CSD) to represent the filter coefficients for the purpose of reducing the complexity of the hardware and the consume is discussed.Simulating the scheme using MATLAB tool, the results indicate that the signal-to-noise ratio of the output signal is above 110 dB, this can absolutely satisfy the precision requirement of the audio ADC with the analog circuit being quite insensitive to mismatch and other circuit imperfections.