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In this paper, passive continuous-time (CT) Sigma Delta modulators are briefly reviewed and compared with conventional active CT Sigma Delta modulators. A fifth-order CT Sigma Delta modulator with a hybrid active-passive loop filter is realized with only three active integrators. The hybrid CT Sigma Delta modulator is robust to the excess loop delay, clock jitter, and RC product variations. The prototype chip is designed in a 0.25- mum CMOS technology targeting for GPS or WCDMA applications. The experimental results show that the prototype Sigma Delta modulator achieves a 68-dB dynamic range and a - 75 dB IM3 over a 2-MHz bandwidth with a 150-MHz clock, consuming 1.8 mA from a 1.5-V supply.