A Subthreshold Folded-Cascode Lowpass Biquad for Biopotential Acquisition

This paper presents a subthreshold CMOS 2nd-order lowpass filter for low-frequency biomedical signals discrimination. We modify the folded-cascode (FC) OTA to combine with two floating capacitors in a unity-gain negative feedback configuration. Circuit simulation using 0.35 μm CMOS confirms that the proposed biquad, operated from a 1 V dc supply, consumes 15.15 nW to obtain a cutoff frequency of 152 Hz. With 52.5 pF total capacitance, the filter contributes an input-referred noise of 31.6 μVrms. At cutoff frequency, when a 100 mVppsinusoidal input signal is applied, the filter produces 1.7% total harmonic distortion. This results in a filter's dynamic range of 61 dB.