On the parasitic-sensitivity of switched-capacitor summing-integrator structures for ΣΔ modulators

This brief focuses on the comparative analysis of the parasitic sensitivity of different switched-capacitor summing-integrator structures employed in ΣΔ modulators. One summing-integrator structure with cross-coupled switches is found to be parasitic sensitive, whereas that without cross-coupled switches is found to be parasitic insensitive. The effect of parasitic-sensitive and parasitic-insensitive summing integrators on the performance of a second-order ΣΔ modulator is reported. Circuit simulations indicate a degradation of the signal-to-noise-plus-distortion ratio (SNDR) of the modulator from 89 dB to as low as 51 dB in the presence of certain parasitic capacitances when the parasitic sensitive summing integrator is used. This is primarily due to a prominent second-harmonic component in the quantizer output. Simulations using the parasitic-insensitive summing-integrator structure are found to cause no prominent harmonic distortion and, hence, no degradation of the SNDR in the presence of any parasitic capacitances.