AD6676 FAQ: Why isn't the noise floor of the AD6676 flat?

Document created by sko on Nov 13, 2014
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I am looking at the noise spectral density (NSD) plots shown in NOMINAL PERFORMANCE characteristics of the datasheet and noticed that they are not flat and can vary depending on the AD6676 configuration.  Why?

The AD6676 includes a tunable continuous-time band-pass Σ-Δ ADC.  The very nature of this ADC architecture results in an NSD that is “shaped” rather than flat.  The degree of which the NSD is shaped is primarily dependent on the oversampling ratio (OSR = FADC/(2*BW) with a higher OSR resulting a flatter noise floor.  Other parameters that can influence the NSD (as well as signal frequency response) are listed in Table 7 of the datasheet and are referred to as User Specified Application Parameters.   Refer to the datasheet to learn how all these parameters impact the response of the Σ-Δ ADC.