A Nanopower Folded-Cascode EEG Lowpass Filter

Abstract

This brief presents a continuous-time lowpass biquadratic cell developed from a folded-cascode OTA operating in the subthreshold region. The proposed cell has been implemented by configuring the OTA in a unity-gain feedback manner and inserting two individual capacitors into the output and internal folding nodes. This allows the proposed biquadratic cell to benefit from the high loop gain that maintains good linearity at passband frequencies well below its cutoff. As an application of this biquadratic cell, a pseudo-differential lowpass filter for electroencephalogram detection is designed and implemented in a 0.18-µm standard CMOS technology. The custom filter occupies a silicon area of 230 µm × 300 µm and operates from a 0.8-V dc supply with 7 nA total current consumption. Multi-chip measurements exhibit the average cut-off frequency at 151.2 Hz, input-referred noise of 35 µVrms, and linear range of 110 mVP. This brief achieves the best FoM to date among recent relevant nano-power filters.