Noise in mixed continuous-time switched-capacitor sigma-delta modulators

It is commonly accepted that oversampling/noise-shaping is the most suitable technique for implementing high-resolution data converters in MOS technologies. Most existing modulators use switched-capacitor (SC) techniques because they provide a highly controllable design. When compared to continuous-time (CT) techniques, SC techniques have two main drawbacks: settling time limitation and thermal noise folding. In this paper the performance of mixed continuous-time/switched-capacitor (CT-SC) and SC modulators is compared by analytically evaluating the thermal and jitter noise contributions. Relationships are derived which define the conditions for designing mixed modulators with lower levels of noise power than their SC counterparts. The main conclusion is that in mixed modulators the thermal noise can be significantly reduced, while the jitter noise could be a severe limit on the realisation of high resolution converters