Biomedical subjects

Khalil Najafi

Publications and source records attributed to Khalil Najafi.

5 recordsLinked to original sources

PubMed2005

A compact large voltage-compliance high output-impedance programmable current source for implantable microstimulators.

A new CMOS current source is described for biomedical implantable microstimulator applications, which utilizes MOS transistors in deep triode region as linearized voltage controlled resistors (VCR). The VCR current source achieves large voltage compliance, up to 97% of the supply voltage, while maintaining high output impedance in the 100 MOmega range to keep the stimulus current constant within 1% of the desired value irrespective of the site and tissue impedances. This approach improves stimulation efficiency, extends power supply lifetime, and saves chip area especially when the stimulation current level is high in the milliampere range. A prototype 4-channel microstimulator chip is fabricated in the AMI 1.5-microm, 2-metal, 2-poly, n-well standard CMOS process. With a 5-V supply, each stimulating site driver provides at least 425-V compliance and > 10 MOmega output impedance, while sinking up to 210 microA, and occupies 0.05 mm2 in chip area. A modular 32-site wireless neural stimulation microsystem, utilizing the VCR current source, is under development.

Computer-Aided Design↗

PubMed2005

A 1.48-mW low-phase-noise analog frequency modulator for wireless biotelemetry.

This paper presents a low-phase-noise, hybrid LC-tank, analog frequency modulator for wireless biotelemetry employing on-chip NMOS varactors in the inversion region as the frequency tuning element. We demonstrate that a correct estimate for the destination signal-to-noise ratio, which quantifies the quality of the wirelessly received signal in a frequency-modulated biotelemetry system, is only achieved after taking into account the large-signal oscillation effect on the tank varactor. A prototype chip is fabricated using AMI 1.5-microm double-poly double-metal n-well CMOS process, and exhibits a measured gain factor of 1.21 MHz/V in the mid-range of the tuning voltage and a phase noise of -88.6 dBc/Hz at 10-kHz offset from the 95.1-MHz carrier while dissipating 1.48 mW from a 3 V power supply leading to a figure of merit (FOM) of -166.5 dBc/Hz. The VCO is successfully interfaced with a penetrating silicon microelectrode with 700 microm2 iridium recording sites for wireless in vitro recording of a 50 Hz simulated normal sinus rhythm signal from saline over a distance of approximately 0.25 m. Given a typical gain of approximately 40 dB for fully integrated front-end bioamplifiers, a wireless recording microsystem employing this VCO would be capable of detecting input biopotentials down to the submilivolt range.

Electronics, Medical↗

PubMed2005

Wireless multichannel biopotential recording using an integrated FM telemetry circuit.

This paper presents a four-channel telemetric microsystem featuring on-chip alternating current amplification, direct current baseline stabilization, clock generation, time-division multiplexing, and wireless frequency-modulation transmission of microvolt- and millivolt-range input biopotentials in the very high frequency band of 94-98 MHz over a distance of approximately 0.5 m. It consists of a 4.84-mm2 integrated circuit, fabricated using a 1.5-microm double-poly double-metal n-well standard complementary metal-oxide semiconductor process, interfaced with only three off-chip components on a custom-designed printed-circuit board that measures 1.7 x 1.2 x 0.16 cm3, and weighs 1.1 g including two miniature 1.5-V batteries. We characterize the microsystem performance, operating in a truly wireless fashion in single-channel and multichannel operation modes, via extensive benchtop and in vitro tests in saline utilizing two different micromachined neural recording microelectrodes, while dissipating approximately 2.2 mW from a 3-V power supply. Moreover, we demonstrate successful wireless in vivo recording of spontaneous neural activity at 96.2 MHz from the auditory cortex of an awake marmoset monkey at several transmission distances ranging from 10 to 50 cm with signal-to-noise ratios in the range of 8.4-9.5 dB.

Action Potentials↗

PubMed2004

A fully integrated neural recording amplifier with DC input stabilization.

This paper presents a low-power low-noise fully integrated bandpass operational amplifier for a variety of biomedical neural recording applications. A standard two-stage CMOS amplifier in a closed-loop resistive feedback configuration provides a stable ac gain of 39.3 dB at 1 kHz. A subthreshold PMOS input transistor is utilized to clamp the large and random dc open circuit potentials that normally exist at the electrode-electrolyte interface. The low cutoff frequency of the amplifier is programmable up to 50 Hz, while its high cutoff frequency is measured to be 9.1 kHz. The tolerable dc input range is measured to be at least +/- 0.25 V with a dc rejection factor of at least 29 dB. The amplifier occupies 0.107 mm2 in die area, and dissipates 115 microW from a 3 V power supply. The total measured input-referred noise voltage in the frequency range of 0.1-10 kHz is 7.8 microVrms. It is fabricated using AMI 1.5 microm double-poly double-metal n-well CMOS process. This paper presents full characterization of the dc, ac, and noise performance of this amplifier through in vitro measurements in saline using two different neural recording electrodes.

Action Potentials↗

PubMed2003

Long-term recordings from afferent taste fibers.

The receptor cells of taste buds have a life span of about 10 days but it is not known if response characteristics of these receptors alter during the turnover cycle. To examine taste cell responses over time, a micromachined polyimide sieve electrode array was implanted between the cut ends of the rat chorda tympani nerve, which then regenerated through the electrode array. Long-term stable recordings from regenerated single afferent fibers innervating taste buds were possible using this technique for up to 21 days. Responses to taste stimuli recorded from the same fiber changed with time. The changes occurred in both the magnitude of response and the relative response profiles to four chemical stimuli, NaCl, sucrose, HCl, and quinine HCl. These changes in response characteristics were hypothesized to result from changes in the taste receptor cells as the receptor cells turnover in the taste buds.

Action Potentials↗