Study On The Decimation Filter In Sigma-Delta ADC

This thesis focuses on the study and design a digital decimation filter in the Sigma-Delta ADC which used in the high-end audio device. Because of the merits, such as high-linearity, high-resolution and easy integratoin with digital circuit, it is widely used in the area of audio process, wireless communication and precision measurement. As the advance of the technology, Sigma-Delta ADC will be used in the wideband field, such as the digital video process. The Sigma-Delta ADC has two main parts, the frontend modulator and backend digital decimation filter. The modulator has two functions, the first is oversampling the input, the second is moving the qualitazation noise to higher frequency which called noiseshaping. The backend decimation filter downsamples the signal to the Nyquist Rate,at the same time,filters out the out-of-band quantization noise which be shaped by the modulator. So,the SNR in the baseband rises.The followings are the main content done in this thesis.Firstly, the whole design adopt a Top-down approach. Base on the specification that system must meet, the stucture and type of the filter need to be choosen in the beginning. The filter is implement with multistage multirate stucture. The CIC filter is choosen to be the first stage, followed by two stage of halfband filter and one CIC compensation filter. After comparing and analysis, the CIC compensation filter locates between the two halfband filters is the best choice for calculation efficient. At the same time, for further increase the calculation efficient, the last three stage use a two-phase structure which let the operation of the filter at the downsampled rate.Secondly, the filter is designed in the Matlab with FDAtool toolbox and Fdesign toolbox. The stopband attenuation of the filter is 120dB, passband ripple less than 0.01dB. Also the filter supports 24/20/16 bits output wordwidth, 96/48 kHz output frequency. After the coefficients of the flilter is calculated, they need to be coded into CSD. Due to the wordlength of the coefficient and the output have the effect on the resolution of the filter, after analysis, this design adopt 24 bit coefficient quantization and the most 24 bit output wordlength for meeting the design specifications.Thirdly, the design and testbench are written by Verilog HDL. Using Simulink which embeded in the Matlab and Sdtoolbox to build the model of the Sigma-Delta modulator. This model is used to generate the dataflow of output of the modulator which is used to simulate and validate the function of the filter in the Modelsim.Finally, after validation the code, the next step of the design is synthesis the Verilog HDL by Design Compiler to get the netlist. Then the layout of the design can be achieved by the Auto-Place-and-Route tool, Astro. The technology library in my design is 0.18 um standard cell library. The area of the chip is 1.7mm*1.7mm. As such design adopts the top-down design method, it has good capability of duplication and transplantation. The operation of digital filter is a pure DSP process, so it is suitable for the use of FPGA to implement the filter. At last, Quartus, a FPGA software, is used to simulate the implement of the filter in the FPGA.