Stability Analysis for Frequency Tunable Bandpass Delta-Sigma ADC Architectures

Abstract

This paper presents bandpass delta-sigma modulator (BP-Δ∑M) structures, which are specifically suited for frequency tuning. The structures are identified through stability analyses and comparisons between chain of resonators feedforward (CRFF) and chain of resonators feedback (CRFB) delta-sigma modulator (Δ∑M) configurations. Frequency tuning is predominantly achieved through altering the resonator coefficients. Therefore, the paper guides designers though the analysis process for identifying suitable tunable BP-Δ∑M structures. When moderately changing the resonators the structures perform almost identical to a non-tunable modulator optimized for the same frequency. For large frequency shifts, the out-of-band gain (OOBG) of the signal transfer function (STF) and noise transfer function (NTF) in the CRFF structure, drastically increases and the control loop becomes unstable. It is shown how utilizing CRFB structures is advantageous for frequency tuning and that the OOBG in CRFF structures can be compensated through a unity STF (USTF) Δ∑M structure.