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Why am I getting high noise out of my DAC when operating at low sample rates?
Regards,Collin WellsPrecision ADC Applications
When operating a sigma-delta DAC, care must be taken to ensure that the out-of-band noise falls as far as possible from the audible frequency range. Noise might be audible if the oversampling rate of the converter is too low, which might be the case for 8kHz sampling. The optimal DAC oversampling frequency for, say, the AIC3204 is around 6MHz (or 48kHz*128). In order to achieve such oversampling rate for 8kHz sampling, the DOSR must be set to 768.
The images below show an example taken from TLV320AIC3204 CS, where MCLK is 4.096 MHz. The values for MDAC and MADC below are chosen to be compatible with all processing blocks as well as miniDSP programs for TLV320AIC3254. This may not be the optimal values from a power consumption standpoint -- once a processing block is seleted for you system (say, only 3 biquads are needed per channel, as for PRB_P1 which has a resource class (RC) of 8) the MDAC and NDAC values can be tuned for that particular resource class. Please refer to http://www.ti.com/litv/pdf/slaa404c for further details. Page 20 has a sample script related to 8kHz sampling.
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