A 0.9V 75MHz 2.8mW 4th-order analog filter in CMOS-bulk 28nm technology

Abstract

In this paper a 75MHz 4th order low-pass analog filter in CMOS-28nm is presented. The filter is composed by the cascade of two Active-RC Rauch biquadratic cells. The hereby proposed design manages a very low Vdd/Vth ratio (<2) for Standard-Process (SP) MOS transistors by means of a dedicated input Common-Mode (CM) feedback circuit (I-CMFB), that operates the OPAMP input differential pair in weak inversion region independently on common mode variations. The single Rauch cell exploits a feed-forward compensated OPAMP, where an improved output CM feedback (O-CMFB) circuit has been adopted reducing this way the parasitic poles of the CM loop, and improving O-CMFB phase margin. The OPAMP has been designed using a folded topology for stable operating point. The design prototype achieves 75MHz–3dB frequency at 2.8mW power consumption from a single 0.9V supply voltage, including also I-CMFB and O-CMFB power. In-band integrated noise is 320uVrms. The filter performs 20dBm and 13dBm IIP3 at 6&11MHz and 50&55&MHz, respectively. Thanks to the very large linearity achieved at edge of the signal pass-band this design has one of the highest Figure-of-Merit for Active-RC filters.