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Title: 15-nW Biopotential LPFs in 0.35- μm CMOS Using Subthreshold-Source-Follower Biquads With and Without Gain Compensation
Authors: Zhang, Tantan
Mak, Pui In(麥沛然)
Vai, Mang I(韋孟宇)
Mak, Peng Un(麥炳源)
Law, Man Kay(羅文基)
Pun, Sio Hang(潘少恒)
Wan, Feng(萬峰)
Martins, Rui P.
Issue Date: 12-Mar-2013
Publisher: IEEE Xplore
Citation: IEEE Transactions on Biomedical Circuits and Systems, Oct 2013, Vol 7, Issue 5, p. 690-702
Abstract: Most biopotential readout front-ends rely on the gm-C lowpass filter (LPF) for forefront signal conditioning. A small gm realizes a large time constant (τ=C/gm) suitable for ultra-low-cutoff filtering, saving both power and area. Yet, the noise and linearity can be compromised, given that each gm cell can involve one or several noisy and nonlinear V-I conversions originated from the active devices. This paper proposes the subthreshold-source-follower (SSF) Biquad as a prospective alternative. It features: 1) a very small number of active devices reducing the noise and nonlinearity footsteps; 2) No explicit feedback in differential implementation, and 3) extension of filter order by cascading. This paper presents an in-depth treatment of SSF Biquad in the nW-power regime, analyzing its power and area tradeoffs with gain, linearity and noise. A gain-compensation (GC) scheme addressing the gain-loss problem of NMOS-based SSF Biquad due to the body effect is also proposed. Two 100-Hz 4th-order Butterworth LPFs using the SSF Biquads with and without GC were fabricated in 0.35-µm CMOS. Measurement results show that the non-GC (GC) LPF can achieve a DC gain of -3.7 dB (0 dB), an input-referred noise of 36 µVrms (29 µVrms), a HD3@60 Hz of -55.2 dB (-60.7 dB ) and a die size of 0.11 mm2 (0.08 mm2). Both LPFs draw 15 nW at 3 V. The achieved figure-of-merits (FoMs) are favorably comparable with the state-of-the-art.
ISSN: 1932-4545
Keywords: Biomedical
Body effect
Gain compensation
Harmonic distortion
Lowpass filter
Source follower
Time constant

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