SLAS510G March 2007 – February 2021 TLV320AIC3104
The digital interpolation filter upsamples the output of the digital audio processing block by the required oversampling ratio before data is provided to the digital delta-sigma modulator and analog reconstruction filter stages. The filter provides a linear phase output with a group delay of 21 / fS. In addition, programmable digital interpolation filtering is included to provide enhanced image filtering and reduce signal images caused by the upsampling process that are below 20 kHz. For example, upsampling an 8-kHz signal produces signal images at multiples of 8-kHz (i.e., 8 kHz, 16 kHz, 24 kHz, etc.). The images at 8 kHz and 16 kHz are below 20 kHz and still audible to the listener; therefore, they must be filtered heavily to maintain a good quality output. The interpolation filter is designed to maintain at least 65-dB rejection of images which are below 7.455 fS. In order to use the programmable interpolation capability, fS(ref) should be programmed to a higher rate (restricted to be in the range of 39 kHz to 53 kHz when the PLL is in use), and the actual fS is set using the NCODEC divider, where NCODEC = NDAC = NADC. For example, if fS = 8 kHz is required, then fS(ref) can be set to 48 kHz, and the DAC fS set to fS(ref) / 6. This ensures that all images of the 8-kHz data are sufficiently attenuated well beyond a 20-kHz audible frequency range.