A two-path bandpass sigma-delta modulator with extended noise shaping

A three-stage bandpass sigma-delta (Sigma Delta) analog-to-digital converte r has been designed specifically for operation at low oversampling ratios. In the proposed architecture, the center frequency of the third stage is sh ifted slightly from that of the first two stages to achieve more efficient noise shaping across the signal band. An experimental modulator based on th e proposed topology has been integrated in a 0.25-mum CMOS technology and a chieves a dynamic range of 75 dB with a maximum signal-to-noise-plus-distor tion ratio (SNDR) of 70 dB when digitizing a 2-MHz signal band centered at 16 MHz. This circuit implements an f(s)/4 bandpass architecture and thus op erates at 64-MHz clock rate. It dissipates 110 mW from a 2.5-V supply, and its active area is 4 mm(2).