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 ( $\text{V}_{\mathrm {Q}}$ ) 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 $\text{V}_{\mathrm {Q}}$ is 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 $\text{V}_{\mathrm {Q}}$ tracking the input $\text{V}_{\mathrm {Q}}$ and immune to the process and temperature variations. A 4th-order LPF using the proposed topology has been implemented in a standard $0.18~\mu \text{m}$ 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 $91.9~\mu \text{V}_{\mathrm {rms}}$ . 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 $\text{V}_{\mathrm {Q}}$ immune to process and temperature variations, which is suitable for low-supply-voltage biomedical systems.