Research And Design Of Sigma-Delta DAC Digital Circuits

With the development of multimedia and various electronic products,there is an increasing demand for high-resolution,high-reliability digital-to-analog converters(DAC).Nyquist DAC and oversampling DAC are two common types of DAC,which are widely used in many occasions.The working principle of the Nyquist DAC is simple and the data conversion speed is fast.However,the internal components of the Nyquist DAC have matching accuracy problems,which makes it difficult to realize a high-resolution DAC.The oversampling DAC,on the other hand,is able to realize high-resolution DAC.Nevertheless,the current interpolation filters of oversampling DAC mostly use simple zero-interpolating method,which has the following shortcomings:(1)The zero-interpolating method will produce image frequencies.In order to suppress the image frequencies,it must add in multi-level high-order low-pass filter to filter out the image frequency;this leads to an increase of chip area and power consumption.Nonetheless,there exist a problem with this multilevel filter that when the requirement of the input signal bandwidth is to be increased,the multi-level filter has to be redesigned,which is not feability in finished chip.(2)The oversampling signal reproduced by the zero-interpolating method often does not resemble the original signal causing the Sigma-Delta DAC to output misinterpreted signal.To solve the above shortcomings,this thesis proposes a cubic-spline-interpolating Sigma-Delta DAC.This zero-interpolating method Sigma-Delta DAC uses oversampling and noise-shaping method to realize a high-resolution DAC,which avoides the component matching problem.To solve the zero-interpolating method problems,this DAC,incorporating cubic-spline-interpolating method,not only does not produce any image frequencies,it is also able to reproduce and recover the original signal fully without the need of multi-level filter.Hence,there is no need for redesign even if the input signal bandwidth increases.The design of the zero-interpolating and cubic-spline-interpolating Sigma-Delta DAC has been verified using FPGA and Matlab.With a sinusoidal input signal of amplitude 0.5 and frequency of 31.25 k Hz,the results show a signal-to-noise distortion ratio(SNDR)of 145.2 d B and an effective number of bits(ENOB)of 23.83 bits are achieved.