Charge pump optimization and output spur reduction in VCO-based OTAs for active-RC analog filters

This paper presents performance improvement techniques for VCO-based OTAs (VCO-OTAs). An on-off charge pump(CP) topology is used as an alternative to power-hungry current-steering charge pump used in earlier VCO-OTAs. This brings down the power consumption of the CP and also the phase frequency detector (PFD) used in VCO-OTAs. The paper also presents the use of inherent low pass filtering in VCO-OTAs for out-of-band spur reduction. The techniques have been demonstrated by prototyping a 4-MHz 4th order active-RC filter using VCO-OTAs. The filter prototype, fabricated in 180nm CMOS technology, shows a power reduction of 41% in the CP and 25% in the PFD, achieved by trading off in-band IIP3 from 21dBm to 19dBm and noise from 0.71mVrms to 0.75mVrms.