What is the STF response of a Σ-Δ ADC and why does it matter?

Document created by Anthony.DeSimone Employee on Dec 4, 2014
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The STF refers to the signal transfer function of the modulator or in practical terms, its input frequency response.  Traditional high-speed ADC’s based on a pipeline architecture typically have a low pass frequency response with the roll-off in response being attributed to its internal sample-and-hold (SHA) circuit or a buffer that may precede it. The AD6676 is based on a 6th order continuous-time band pass architecture displaying a wideband bandpass frequency response relative to its IF bandwidth setting (region of low noise spectral density). Large signals falling outside the IF passband region may not experience significant attenuation (before digital filtering).  As a result, one also should consider the impact of any large out-of-band signal on the AD6676’s performance when optimizing IF filtering and AGC operation.