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We analyze the in-band thermal noise of continuous-time delta sigma modulators. The manifestation of noise in converters with time-invariant loop filters is straight forward and well known. However, several practically useful modulators, like those with switched-capacitor-resistor (SCR) and return-to-open (RTO) feedback DACs, have a time-varying loop filter. We give a framework for analyzing the noise performance of such designs, and show that it is important to consider the aliasing of noise from around multiples of the sampling frequency. Our results compare well with those obtained from a circuit simulator.