Design On High Resolution Sigma-Delta Modulator

Combining oversampling, feedback and digital filter technique together effectively attenuates the noise that is in the signal bandwidth and makes Sigma-Delta converter achieve at least 14-bit resolution. Oversampling Sigma-Delta data converters is becoming increasingly popular for digital audio circuit and ADSL system.This thesis studies the basics of Sigma-Delta modulator and designs analog circuits with EDA tools. The thesis mainly includes the content as follows.1. The non-linear characteristics of Sigma-Delta modulator are studied, and the study mainly focuses on the stability of high-order Sigma-Delta modulator and the mismatch problem of MASH modulator. When analyzing the stability of high-order Sigma-Delta modulator, the non-linear models of the quantizer is improved, the effect of the highly nonlinearity of the quantizer on the system is discussed, and the NPG criterion for stability is given at last. When analyzing the mismatch of MASH modulator, the thesis focuses on the effect of the noise leakage from the first stage that degrades the performance of the modulator.2. Detailed analytical models of the basic building blocks (integrators and operational-amplifiers) and the sampling switches of 2-1 MASH Sigma-Delta modulator are presented by Simulink. The non-ideal factors are taken into account, such as thermal noise of switched capacitor (SC) circuits and operational amplifier parameters (noise, finite gain, finite bandwidth, slew rate and output swing). Circuit parameters and specs are gained by behavioral simulation of the nonidealities.3. The Sigma-Delta modulator is implemented by analog circuits and verified in CMOS 0.35um. In the analog part, the performance of the operational amplifier, the first integrator and the switches is optimized and improved so that the modulator can achieve 16-bit resolution with 20kHz signal bandwidth and 256 oversampling ratio.The research contributions of this work include: (1) Analyze the stability of high order Sigma-Delta modulator with non-linear models; (2) Determine the sampling capacitors of every stage to optimize the power of the modulator through the behavioral simulation; (3) Improve the architecture of the operational-amplifiers to increase the speed and minimize the power; (4) Use bootstrapped switch to eliminate the effect of sampling process on the modulator; (5) Apply correlated-double-sampled (CDS) technique in the first integrator to attenuate the effect of offset and flicker noise of the operational amplifier.