The Research And Implement Of High-performance Sigma-delta Modulator

Sigma Delta modulation is a robust means of implanting the high-resolution Analog-to-Digital converters in a VLSI technology. By employing over-sampling and quantization noise shaping, Sigma Delta modulators can achieve a good signal-to-noise ratio (SNR) because the noise in-band is depressed. Moreover, it is very suitable for the application of mixed signal interfaces due to low sensitivity to analog component imperfections and easy implementation in standard CMOS technologies.In this thesis, the methodology of Top-Down design is adopted. Basic knowledge of Sigma Delta modulators and the design flow of high-performance modulators are firstly provided, and then noise transfer function (NTF) of modulator is optimally designed, involving the optimization of zeros and poles and trade-offs among out-of-band, input signal range and SNR. After that, combining macro-models with Verilog-A language, behavioral modelings of non-ideal Switched-Capacitor modulator are expounded. Among those, the critical modules are modeled in macro-models, and the functional modules are modeled in Verilog-A. Based on the SPEC given by behavioral modeling, several circuits are detailedly designed. Two-stage fully-differential amplifier with Miller compensation capacitor and zero-adjusting resistance is proposed, which is followed by a common-mode feedback circuit (CMFB) to ensure common-mode stability at the output ports. Finally, layout of modulator is designed in Virtuoso environment, and it is verified by Calibre.When adding a sine wave with frequency of 20KHz and amplitude of 0.75V, the overall simulation results show that the designed Sigma Delta modulator has achieved high performance with 78.2dB of SNR, 12.7bits of ENOB, 0.88mW of power dissipation, and 3.31 of FOM. Implemented with Huahong NEC 0.25um CMOS process and taped out in forms of MPW, the modulator is under testing now.