A 0.5-V Ultra-Low-Power Low-Pass Filter with a Bulk-Feedback Technique

In wearable biomedical applications, a constant dc common-mode (CM) voltage of low-pass filters (LPFs) is needed to boost the dynamic range when low supply voltage is used for high power efficiency. Traditional source-follower based LPFs consume very low power but introduce a CM voltage difference between input and output. This paper presents an ultra-low-power LPF, which features a source-follower based topology with a bulk-feedback technique. The bulk-feedback technique is proposed to keep a constant dc CM voltage, which is robust to process and temperature variations. In order to validate the proposed concept, a fourth-order LPF has been designed as a prototype. Implemented in a TSMC 180 nm CMOS process, the filter occupies an active area of only 0.048 mm2. Simulation results show that the filter consumes 9.0-nW static power from a 0.5-V voltage supply when the cutoff frequency is set to about 200 Hz. The filter achieves an input-referred noise in −3 dB bandwidth of 41.21 µVrms and an in-band IIP3 of 4.0 dBV, which corresponds to a spurious free dynamic range (SFDR) of 61.1 dB. The proposed LPF achieves a competitive FOM among the reported works.