A 30MHz 28dBm-IIP3 3.2mW fully-differential Sallen-Key 4th-order filter with out-of-band zeros cancellation

Abstract

In this paper a 4th-order 30MHz Butterworth low-pass analog filter is presented, exploiting the Sallen-Key (SK) biquadratic cell circuit. The out-of-band zeros typically present in SK cells, are cancelled by using a low-power auxiliary path, resulting in a significant improvement of the stopband rejection, at the cost of a small power budget for the same auxiliary path biasing. An efficient unity gain buffer has been used, based on super-source-follower stage, providing very large in-band IIP3 over the entire filter bandwidth (21.5dBm for 25MHz&26MHz input tones), at 3.2mW power consumption from a single 1.8V supply voltage. The filter prototype has been designed in CMOS 0.18μm tech. The total area occupancy is 0.12mm2, the in-band integrated noise is 197μVRMS.