A compact Gm-C filter architecture with an ultra-low corner frequency and high ground-noise rejection

Active filters with a very low corner frequency (of only a few hertz or below) are usually demanded at the frontend circuitry of biomedical instruments. This paper presents a novel circuit architecture for implementing the Bessel low-pass filter with an ultra-low corner frequency and negligible interferences from the ground. Basing on the transconductance-capacitor (Gm-C) architecture, the proposed filter incorporates a differential amplifier into the negative feedback loop to scale down the corner frequency, as well as to eliminate noise coupling from the ground. To demonstrate the design concept, a second-order Bessel filter is fabricated with the 0.35μm CMOS technology. With a corner frequency of around 1Hz, the filter consumes only 1.2μW and a chip area of 0.089mm2. Moreover, the 60-Hz interference from the ground is proved to be attenuated by more than 36dB.