On continuous-time ΔΣ modulators with return-to-open DACs

We examine the alias rejection of continuous-time ΔΣ modulators (CTDSMs) with return-to-open (RTO) feedback digital-to-analog converters (DACs). We show that using an RTO DAC can compromise the alias rejection of the CTDSM around odd multiples of the sampling rate. The intuition and theory behind this curious phenomenon are given. A circuit technique that mitigates this problem is proposed, and its efficacy is verified with simulations. © 2011 IEEE.