AD6676 FAQ: Why are the performance specifications of the AD6676 different than the ones used to specify a typical ADC?

Document created by sko on Nov 13, 2014
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I am familiar with typical ADC ac performance specifications such as SNR, SINAD, THD, SFDR, and ENOB when comparing ADC’s with each other. The AD6676 uses different specifications to quantify its ac performance characteristics. Why?


Since the AD6676 is a wideband IF receiver subsystem and utilizes a band-pass Σ-Δ ADC, other ac performance metrics that have more significance in the design of IF receiver are used.


For example, the “shaped” noise floor as described in AD6676 FAQ: Why isn't the noise floor of the AD6676 flat? occurs over a user-specified IF pass band that can be much less than the output bandwidth supported by the AD6676’s complex output data rate (FDATA_IQ). This makes SNR impractical since SNR for a typical ADC is defined over its entire Nyquist bandwidth (FADC/2).   The Noise Spectral Density (NSD) characteristics as well as its integrated in-band noise (IBN) performance (measured over a specified bandwidth and IF pass band region) provide more insight to a user on how the AD6676’s noise performance will affect their system performance in the demodulation of narrow band or wide band signal situated across the IF pass band. Since the AD6676 defines its full-scale level in dBm (PIN_0dBFS), an equivalent NF can be easily determined by the “spot noise” or NSD measured at a particular point in the pass band region.