A 0.9-nW, 101-Hz, and 46.3- $\mu$ Vrms IRN Low-Pass Filter for ECG Acquisition Using FVF Biquads

This paper presents a differential fourth-order low-pass filter suitable for electrocardiography (ECG) acquisition. It is formed by cascading two compact and power-efficient biquads operating in the subthreshold region. Each biquad combines two capacitors and a flipped voltage follower circuit. The filter attains a cutoff frequency adjustable to cover the entire range of ECG (150–250 Hz). The filter prototype has been fabricated in a 0.35-<inline-formula> <tex-math notation="LaTeX">$\mu \text{m}$ </tex-math></inline-formula> CMOS technology. It occupies an area of <inline-formula> <tex-math notation="LaTeX">$362\,\,\mu \text {m} \times 466 \,\,\mu \text{m}$ </tex-math></inline-formula> and operates from a 0.6-V supply. Measurements confirm that the filter consumes 0.9-nW static power for a 101-Hz cutoff frequency and contributes the input-referred noise of 46.27 <inline-formula> <tex-math notation="LaTeX">$\mu \text{V}_{\mathbf {rms}}$ </tex-math></inline-formula>. For a 60-Hz input frequency, the filter achieves a dynamic range of 47 dB where the third-harmonic distortion of −60 dB is produced. This leads to the figure of merit of <inline-formula> <tex-math notation="LaTeX">$46.5 \times 10^{-18}$ </tex-math></inline-formula> J. When the chip area is also concerned, the proposed filter performs comparably to the recent state-of-the-art nanowatt-class low-pass filter.