A 1-V, 6.93-nW subthreshold source-follower biopotential LPF with combined gain compensation and current cancellation in 65-nm CMOS

This paper presents a 4th-order 100-Hz low-pass filter (LPF) for biopotential signal acquisition in 65-nm CMOS. A second-order subthreshold source-follower (SSF) biquad is proposed in which gain compensation (GC) and current cancellation (CC) act simultaneously on the same NMOS pair, addressing two fundamental limitations of the NMOS SSF biquad: the body-effect-induced DC gain loss and the large floating capacitance required at low cutoff frequencies. A small-signal analysis of the combined topology yields closed-form expressions for DC gain, cutoff frequency, quality factor, input-referred noise, and harmonic distortion. The 4th-order filter is realized by cascading a conventional PMOS-SSF biquad with the proposed GC+CC NMOS biquad, operating from a 1.0 V supply in TSMC 65-nm CMOS. Post-layout simulation reports a − 3 dB cutoff of 97.43 Hz, a DC gain of + 0 . 20 dB, total power of 6.93 nW, integrated input-referred noise of 32.46 μ V rms over 0.1–100 Hz, and worst-case HD3 of − 55 . 8 dB at 10 mV pp differential input.