Design Of18-bits Sigma-delta DAC

Data converters has become the tache between the real world and the virtualworld. The most important factor to to evaluate the performance of the dataconverters is there resolution. With the popularity of electronic products, desire ofresolution of DAC becomes higher and higher. Compared with the traditionalNyquist sampling DAC, sigma-delta DAC can obtain higher precision in cost oflower hardware consumption. Therefore, researches on sigma-delta DAC havebecome much more popular.Firstly, the background, the development and research status at home andaboard are introducded in this paper. Then, the working principle of DAC isexplained and behavioral simulation of each block has been done. Finally, thearchitecture of each block is chosen. A fifth-order full feed-forward modulatorwhich is single loop and one bit quantized is adopted after the comparison andanalyze of all kinds of modulator. By analyzing the signal to noise ratio of thesystem,128is determined as the over sampling ratio and three kinds of FIR filter arecascaded to achieve high times interpolation. As to analog low-pass filter,sixth-order Bessel low-pass filter is adopted by comparison the simulation result ofdifferent types of analog low-pass filters. The modulator is realized in Verilog whilethe analog low-pass filter is achieved by switch-capacitor circuits. Systemspecifications are as follows: input signal bandwidth--100kHz; over sampling ratio–128; clock frequency--25.6MHz.GF0.35μm CMOS technology is employed by the sigma-delta DAC.Specifically, digital circuits are synthesized and automatically placed and routedunder this process; the circuit design and manually layout of the analog circuits arealso under the process condition. The digital circuits consist of digital interpolationfilter and modulator. The analog circuits include the single-to-double transitionblock,1bit D/A, two phase non overlapping clock, reference circuit and operationamplifier.Mix signal simulation has been done after the circuit design. The sigma-deltaDAC achieves98.26dB-SNR when the magnitude of input sin wave is-6dB.