An IF input continuous-time sigma-delta analog-digital converter with high image rejection

A novel resistor time-sharing technique is proposed to achieve higher image rejection (IR) in the design of an intermediate frequency (IF) continuous time (CT) sigma delta (/spl Sigma//spl Delta/) modulator with integrated IF mixers. A third order CT /spl Sigma//spl Delta/ modulator with current feedforward compensation is designed, as well as current input comparator, digital tunable clock tree, etc. It is also found that, for the first stage of a modulator, the gain of input signal and feedback signal can be scaled down to relieve the harsh requirement of active components' input/output swing. This design is implemented in a 0.35 /spl mu/m double-poly four metal layer CMOS technology. Active area is 0.4 mm/sup 2/ and consumes 14.8 mW from a 3.3 V power supply. Postlayout simulation with 25 MHz IF shows no image signal is present for 4096 output data.