A 3.47 NEF 175.2dB FOMs Direct Digitization Front-End Featuring Delta Amplification for Enhanced Dynamic Range and Energy Efficiency in Bio-Signal Acquisition
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2025
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Bio-signal monitoring systems have recently been widely applied to implantable and wearable devices for healthcare applications. However, they require extremely low-noise front-end circuits to ensure reliable recording while receiving a very small signal (e.g., the amplitude of the EEG ranging from 1 to 100µV [1]). A conventional analog front-end typically achieves a low input-referred noise (IRN) of $50-60\text{nV}/\sqrt{\text{Hz}}$ and a low noise efficiency factor (NEF) around 3 to 5 by adopting a low-noise amplifier (LNA) with a low NEF and large gain, followed by the rest of amplifier chain and an ADC [2]–[5]. Nevertheless, a high LNA gain significantly reduces input range, making these circuits vulnerable to large interference caused by motion and stimulation artifacts. For instance, transcranial direct-current stimulation applies a 1-to-2mA current to the scalp, resulting in an artifact of about $100\text{mV}_{\text{PP}}$ [6], [7]. Conventional structures are easily saturated by such artifacts and require a few seconds of recovery time [8]. Recently, an ADC-first front-end utilizing delta-sigma modulator (DSM) has been proposed to increase dynamic range (DR) for handling substantial artifacts by directly digitizing the input without a gain [9]–[12]. VCO-based DSMs further enhanced both DR and power efficiency [13]–[18]. However, these ADC-first front-ends commonly suffer from higher input-referred noise levels of 80 to 120 $\text{nV}/\sqrt{\mathrm{H}}_{\overline{l}}$ compared to the ExG recording requirement of 50 to $60\text{nV}/\sqrt{\text{Hz}}$ The main reason for this problem is the trade-off between the linear range and the IRN of the first integrator. $\vert \mathrm{n}$ [13], [16], [18], a source-degenerated amplifier helps extend the linear input range while their NEF is higher than a common-source amplifier due to the reduced $\mathrm{g}_{\mathrm{m}}. \vert \mathrm{n}$ [14], [15], a common-source amplifier is used to drive the VCO (Fig. 15.6.1). However, they can use only a single differential pair that has a higher NEF than the inverter-based amplifier.
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2025 IEEE International Solid-State Circuits Conference (ISSCC)
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276 - 278
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IEEE
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IEEE International Solid-State Circuits Conference (ISSCC 2025)
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09647 - Jang, Taekwang / Jang, Taekwang