The Research and Design of An Interpolation Filter Used in an Audio DAC

Interpolation Filter plays an important part in the Sigma-Delta DAC. This paper describes a low-pass interpolation filter with adjustable oversampling ratio applied in a 24-bit audio DAC. The interpolation filter we design is a 3-stage structure with two stages half-band pass filter and a novel high ratio interpolation filter. We apply a novel structure of hardware to implement the half-band pass filter. Compare with traditional method, the new method only costs four group adders and the same size ROM and RAM to complete the work. The third stage of the interpolation filter we design to implement high ratio interpolation. We just use normal logic and time units to replace the CIC (Cascaded Integrator-Comb) filer without any adders applied. The in-band SNR and THD can achieve about 131.9 dB and - 90.5 dB in the actual work situation. We design it aided by the Matlab&Simulink. The filter has been verified by FPGA and synthesized by DC tools. The performance of the filter can meet the audio specifications.