A 0.5-V 3.69-nW Complementary Source-Follower-C Based Low-Pass Filter for Wearable Biomedical Applications

Ultra-low-power circuits that can work under a low-voltage supply are in great demand in future wearable biomedical applications, which tend to be integrated with low-output-voltage energy harvesting devices. In this paper, we present a low-voltage low-power continuous-time low-pass filter (CT-LPF), which is indispensable in biomedical systems. When a low-voltage supply is used, it is necessary to make the output quiescent voltage (VQ) stable in the LPF, otherwise the dynamic range will be reduced. Conventional Source-follower (SF) based topologies can achieve ultra-low-power consumption. However, the difference of the input and output VQis sensitive to process and temperature variations. In this work, a complementary SF based topology with a bulk-common-mode-feedback (B-CMFB) circuit is proposed to keep the output VQtracking the input VQ and immune to the process and temperature variations. A 4th-order LPF using the proposed topology has been implemented in a standard 0.18 μm CMOS process, which achieves a power consumption of only 3.69-nW under a 0.5-V voltage supply with a bandwidth of 200 Hz. Measurement results show that the input-referred noise is 91.9 μVrms. The IIP3 is 5.0 dBm and the dynamic range (DR) is 48.5 dB. The active chip area is only 0.074 mm2. The proposed LPF achieves both ultra-low power consumption with a 0.5-V supply and a stable output VQimmune to process and temperature variations, which is suitable for low-supply-voltage biomedical systems.