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Nahmil Koo

Bio: Nahmil Koo is an academic researcher from KAIST. The author has contributed to research in topics: Electronic oscillator & Amplifier. The author has an hindex of 2, co-authored 5 publications receiving 8 citations.

Papers
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Journal ArticleDOI
Nahmil Koo1, SeongHwan Cho1
TL;DR: A biopotential amplifier that has a large tolerance to common-mode interference (CMI) for a reliable two-electrode electrocardiogram (ECG) recording and the effects of CMCP under non-ideal conditions are analyzed.
Abstract: This article presents a biopotential amplifier that has a large tolerance to common-mode interference (CMI) for a reliable two-electrode electrocardiogram (ECG) recording. To withstand large CMI without affecting the ECG signal, a common-mode charge pump (CMCP) that absorbs displacement current from the CMI and keeps the common-mode input within the desired range is proposed. This article also analyzes the effects of CMCP under non-ideal conditions and provides design guidelines for improved signal quality. A prototype chip fabricated in 180-nm CMOS achieves CMI tolerance of 15 $\text{V}_{\text {PP}}$ at 60 Hz while consuming 24.8 $\mu \text{W}$ from a 1.2-V supply.

18 citations

Proceedings ArticleDOI
Nahmil Koo1, SeongHwan Cho1
20 Feb 2019
TL;DR: The analog front-end of the ECG device must be immune to large CMI, because common-mode interference caused by powerline coupling to the human body can be as large as a few tens of volts.
Abstract: Two-electrode ECG devices have gained popularity in the recent past to enable comfortable and long-term monitoring of cardiovascular health. As a ground or bias electrode is not used in a two-electrode ECG device, common-mode interference (CMI) caused by powerline coupling to the human body can be as large as a few tens of volts. Such a large CMI ruins the ECG recording, and thus the analog front-end of the ECG device must be immune to large CMI.

5 citations

Journal ArticleDOI
TL;DR: In this article , an adaptive TCMRR enhancing loop is implemented in parallel with a charge-pump-based common-mode suppressing loop (CMSL), which adjusts the CMI current through each contact impedance so that commonmode (CM) to differential-mode (DM) conversion is minimized.
Abstract: In this article, we present an electrocardiogram (ECG) amplifier that has a large total CMRR (TCMRR) regardless of contact impedance mismatch and a large tolerance to common-mode interference (CMI). To achieve these features, an adaptive TCMRR enhancing loop is implemented in parallel with a charge-pump-based common-mode suppressing loop (CMSL). It adjusts the CMI current through each contact impedance so that common-mode (CM) to differential-mode (DM) conversion is minimized. We also propose a fast settling technique for the adaptive loop so that contact impedance variation can be tracked fast enough to allow robust ECG acquisition. A prototype chip fabricated in 180-nm CMOS achieves TCMRR larger than 105 dB even when there is contact impedance mismatch of up to 30%. It also achieves tolerance to CMI of 18 VPP at 60 Hz and input-referred noise of 1.90 ${\mu }\rm V_{rms}$ while consuming 43.3 $\mu \text{W}$ .

2 citations

Proceedings ArticleDOI
06 Nov 2022
TL;DR: In this paper , it was shown that neural activity monitoring and functional localization of the active peripheral nerve at the same time can be achieved using fast neural impedance tomography (EIT) with far enhanced temporal resolution.
Abstract: Electrical impedance tomography (EIT) is widely used for functional imaging of the bio-impedance of body parts for various applications, such as lung ventilation monitoring [1]. It was recently shown that ‘fast neural EIT’ with far enhanced temporal resolution (frame rate) can provide the neural activity monitoring and functional localization of the active peripheral nerve at the same time [2]. In an EIT system, to reconstruct an impedance tomography image (Fig. 1(a)), an AC current is injected from a current generator $(\text{I}_{\text{CG}})$ into the target bio-impedance network $\text{Z}_{\text{BIO}}$ through an electrode pair (channel) in a rotational manner, while demodulating the voltages appeared at all the other channels. The I/Q demodulation is the most popular way to extract the resistance and reactance information of $\text{Z}_{\text{BIO}}$ [1]. For the neural EIT, however, this method cannot support a high enough frame rate, failing to acquire neural activities, mainly due to the down-conversion to DC and low-pass filtering. As shown in Fig. 1(a), many cycles of the AC input signal are needed for the I and Q outputs to be well settled to their final values. A higher excitation frequency $(\text{f}_{\text{CG}})$ can be used for faster settling in conventional applications, but in the neural EIT, $\text{f}_{\text{CG}}$ should be $\lt 20$ kHz for high SNR image acquisition [2]. Alternatively, peak detection can be used [3], but it needs a much faster sampling clock than $\text{f}_{\text{CG}}$, consuming a large dynamic power in all the demodulation channels.

1 citations

Proceedings ArticleDOI
22 May 2021
TL;DR: Under frequent shorting, the analysis shows that mismatch- induced DC current is small compared to IOC-induced DC current, and a biphasic pulse scheme which compensates the IOC is proposed which reduces the FC.
Abstract: In this paper, we analyze faradaic DC current (FC) generated by inherent offset charge (IOC) and charge mismatch. Under frequent shorting, the analysis shows that mismatch- induced DC current is small compared to IOC-induced DC current. To reduce the FC, we propose a biphasic pulse scheme which compensates the IOC. Simulation results show the proposed pulse scheme reduces the FC by about 58 times compared to a conventional biphasic pulse.

Cited by
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Proceedings ArticleDOI
01 Feb 2020
TL;DR: This paper presents a multimodal biosensing SoC that addresses issues to enable reliable Photoplethysmography (PPG), Electrocardiography (ECG), and Bio-impedance (BIOZ) acquisition under highly variable conditions in wearable devices.
Abstract: Recently, biosensors have become widely deployed in consumer wearable devices, e.g., smart watches and wrist bands, to track user health conditions during sport, sleep, and daily activity. However, unlike biosignal acquisition in well-controlled medical settings, signal acquisition in consumer devices inevitably suffers from degraded signal quality due to poor sensor interfaces, motion artifacts, and environmental interferences, such as ambient light and power-line coupling, which can be >60dB stronger than the wanted signals. These challenges result in demanding AFE specifications, even under tight constraints on sensor area, chip size, and battery capacity, which are all necessary for wearable devices. This paper presents a multimodal biosensing SoC that addresses these issues to enable reliable Photoplethysmography (PPG), Electrocardiography (ECG), and Bio-impedance (BIOZ) acquisition under highly variable conditions in wearable devices. It contains a 130dB-DR PPG readout, which is 11dB higher than the state of the art [1], to tolerate strong ambient light; an ECG AFE with a pseudo Right-Leg-Drive (RLD) tolerant of >130V PP common-mode interference (CMI) from power-line coupling, which is 3× higher than the technique presented in [2]; and a BIOZ AFE for body-fat measurement using 1cm2 electrodes rather than the typically >10cm2 electrodes in conventional body-fat meters. The area for the entire PPG/ECG/BIOZ AFE is ~3mm2, which is around 50% smaller than prior AFEs with similar features [3], [4].

27 citations

Journal ArticleDOI
TL;DR: In this paper, a compact DDA-based fully-differential CMOS instrumentation amplifier dedicated for micro-power ECG monitoring is presented, where only eight transistors are employed to realize a power-efficient current-sharing DDA A RC network (using MOS pseudo resistors and poly capacitors) forms feedback loops around the DDA creating an ac-only amplification.
Abstract: This paper presents a compact DDA-based fully-differential CMOS instrumentation amplifier dedicated for micro-power ECG monitoring Only eight transistors are employed to realize a power-efficient current-sharing DDA A RC network (using MOS pseudo resistors and poly capacitors) forms feedback loops around the DDA creating an ac-only amplification The proposed amplifier is dc-coupled via gate terminals of the p-channel input transistors It thus achieves sufficiently high input impedance over the entire ECG frequency range Fabricated in a 035-μm CMOS process, the proposed amplifier occupies 00712 mm2 It operates from a 2 V dc supply with 336 nA current consumption Measurements show that the amplifier attains its input impedance of 575 MΩ at 150 Hz and achieves 154 μVrms input-referred noise over 01-300 Hz Noise and power efficiency factors are 202 and 816, respectively At 50 Hz, the mean CMRR of 8324 dB is obtained from 11-chip measurement Experiments performed on a human subject confirm the functionality of the proposed amplifier in a real measurement scenario

20 citations

Journal ArticleDOI
TL;DR: In this article, a review of the physics of body-electrode interfaces is presented in the context of biopotential sensing and human body communication, and the effects of such interfaces on the application's performance are discussed.
Abstract: Several on-body sensing and communication applications use electrodes in contact with the human body. Body-electrode interfaces in these cases act as a transducer, converting ionic current in the body to electronic current in the sensing and communication circuits and vice versa. An ideal body-electrode interface should have the characteristics of an electrical short, i.e., the transfer of ionic currents and electronic currents across the interface should happen without any hindrance. However, practical body-electrode interfaces often have definite impedances and potentials that hinder the free flow of currents, affecting the application's performance. Minimizing the impact of body-electrode interfaces on the application's performance requires one to understand the physics of such interfaces, how it distorts the signals passing through it, and how the interface-induced signal degradations affect the applications. Our work deals with reviewing these elements in the context of biopotential sensing and human body communication.

6 citations

Journal ArticleDOI
TL;DR: In this paper , a low-noise resistive bridge sensor analog front-end (AFE) using a chopper-stabilized multipath current feedback instrumentation amplifier (CFIA) and an automatic offset cancellation loop was proposed.
Abstract: Resistive bridge sensors are used in many application areas to measure changes in physical parameters. To amplify the resistive changes from sensing elements with high precision, various offset contributors in the resistive bridge and amplifiers should be minimized. This study proposes a low-noise resistive bridge sensor analog front-end (AFE) using a chopper-stabilized multipath current feedback instrumentation amplifier (CFIA) and an automatic offset cancellation loop. The proposed circuit exploits a multipath chopper-stabilized architecture for obtaining low noise performance and wide bandwidth characteristics. This circuit can minimize the offsets in the bridge and the high frequency and low frequency amplifiers, while achieving high precision resistive signal acquisition. The high frequency path of the multipath amplifier uses the CFIA topology with class-AB output stage. The offset in the high frequency path is stabilized by the low frequency path amplifier with a high gain and low noise chopper amplifier. The up-modulated offset in the low frequency chopper amplifier path is reduced by the AC-coupled ripple reduction loop (RRL). An automatic offset calibration loop (AOCL) circuit was designed to calibrate the offset due to the bridge mismatch. The AOCL reduces the bridge offset using a successive approximation register (SAR)-based binary-search algorithm. The gain of the proposed circuit is adjustable from 15.56 dB to 44.14 dB. The AFE is implemented in a $0.18~ \boldsymbol {\mu } \text{m}$ CMOS process and draws $123~ \boldsymbol {\mu } \text{A}$ current from a 3.3 V supply. The input referred noise and noise efficiency factor (NEF) are 14.6 nV/ $\boldsymbol {\surd } $ Hz and 6.1, respectively.

5 citations