Sigma-Delta DAC Digital Modulator Research, Design And FPGA Realization

High-precision DAC is one of the hot research today, and Sigma-Delta modulation technique is a predominant implementation of data converters which about high-resolution of the low-speed, it stands out in the various structures of the converter, and becomes the most popular structure of high-precision data converters.Sigma-Delta modulation is a way which transforms speed into accuracy, it is shaping the quantization noise to the high frequency part out of the pass band by the technology of oversampling and noise shaping, which greatly reduces the noise within the signal bandwidth and improved Signal to Noise Ratio(SNR). Sigma-Delta modulator is less dependent on the design of analog circuit requirements and very low noise mismatch. It suppress noise mainly through digital processing technology, which is easy to achieve by CMOS technology, and can easily be embedded integration.The structure of Sigma-Delta data converter can reach high accuracy which other structures cannot be, the highest precision ADC and DAC are almost Sigma-Delta type, so it has been very widely used among high-precision nuclear detector, digital audio and video, automotive electronics, radar applications, wireless communications, oil and gas exploration, and other fields.Sigma-Delta DAC’s digital circuit includes a digital interpolation filter and modulator, and the analog circuit section includes a low-bit DAC and reconstruction filters. It will increase the complexity、the chip area and pin count、the costs and the application difficult of the DAC by integrating interpolator and digital modulator in the Sigma-Delta DAC. FPGA can easily implement and integrate a variety of digital logic circuits by logic programming as the mainstream of today’s digital signal processing technology. Currently, a large number of systems use the FPGA, transfer those digital circuits such as interpolator and digital modulator in Sigma-Delta DAC that from the DAC to the FPGA can simplify the design of DAC without increasing the complexity of the design of the system, it can also improve integration of system and reduce costs. Some manufacturers launched Sigma-Delta DAC that only contains low-bit DAC and reconstruction filter to adapt to such demands, such as TI’s DAC1280, these chips have features such as a small footprint, low cost, simple to use. The structure model and implementation technology of interposer are relatively mature, so the digital modulator plays a decisive role for DAC accuracy, which is main research direction and difficulties of design about high-precision Sigma-Delta DAC.This paper mainly researches the Sigma-Delta digital modulator of Sigma-Delta DAC. The first chapter is the background. The second chapter is the basic principles and structure of Sigma-Delta modulator. The third chapter is the design of Sigma-Delta Modulator. First, I described a complete design Process of higher-order high-precision stable Sigma-Delta modulator, and according to the stability requirements proposed to optimize the design method of modulator, the method according to the simulation results to determine the stability of the modulator. Then details frequency selection and simulation length these two factors of the simulation to be considered when determining the stability, then elaborated the specific design of a Sigma-Delta modulator under the condition of fifth order, OSR value of64, and1-bit according to the paper proposed method and design process. Finally, gives the method to implement digital Sigma-Delta modulator in FPGA, and made the modulator designed in this paper implemented in the FPGA. The fourth chapter design the circuit for testing the performance of Sigma-Delta modulator. The fifth chapter design software for the circuit to control the circuit to complete the test. The fifth chapter first compares advantages and disadvantages of different calculation methods to obtain zero-pole by test the modulator performance, the results show that the table look-up method is better than Chebyshev type II filter method in the zero method, then compares pros and cons between the method of this paper and the traditional Lee Criterion method, the test results show that the stability required by simulation-based method is superior to the traditional Lee Criterion method, the final test of the design of the1-bit fifth-order modulator shows that the SNR is more than110dB,the THD is less than-122dB,which meets high-precision Sigma-Delta DAC’s performance requirements.