High Input Impedance Amplifier Research Articles

Design and implementation of low-power CMOS biosignal amplifier for active electrode in biomedical application using subthreshold biasing strategy

Active Electrodes (AEs) are electrodes which have integrated bio-amplifier circuitry and are known to be less susceptible to motion artifacts and environmental interference. In this work, a low-power and high-input impedance amplifier for active electrode application is implemented based on subthreshold biasing strategies. In this proposed Application Specific Integrated Circuit (ASIC) device was versatile and numerical to achieve a high degree of programmability. It could be adapted to any other external part of one cochlear prosthesis, the sound analyzer that could be driven by a Digital Signal Processor (DSP). This research work also discusses the measurement of the electrode-skin impedance mismatch between two electrodes while concurrently measuring a bioelectrical signal without degradation of the performance of the amplifier, the efficient, noise-optimized analysis of bioelectrical signals utilizing two-wired active buffer electrodes. The reduction of power-line interference when using amplifying electrodes employing autonomous adaption of the gain of the subsequent differential amplification. The amplifier’s features include offset compensation, Common Mode Rejection Ratio (CMRR) improvement in software and a bandwidth extending down to DC. The proposed active electrode amplifier is designed using 90 nm CMOS technology. Simulation results exhibit up to the change in noise immunity and lessening in power utilization contrasted with the traditional bio-amplifier design at a similar delay.

Measurement of capacitive coupled ECG from the car seat

Abstract The topic of the article is the design and construction of the device for measurement of ECG in the car. This device is divided into two parts: sensing part and processing part. These parts are described in detail in separated sections. The sensing part of the device was incorporated into the driver’s seat. The ECG signal is measured through capacitive coupled electrodes installed in the seat that includes high-input impedance amplifier and sensing electrode surface. This sensing part of the device is capable of recording electrocardiogram data through clothing. Part of the hardware responsible for processing of the recorded signal was placed out of the seat and it is connected through shielded cable with sensing electrodes in order to reduce noise. The main part of the hardware dealing with signal digitalization and filtration utilizes analog front-end ADS1191 by Texas Instrument. It is possible to set up sampling frequency, gain of channel and configuration of RLD circuit during data acquisition. The experimental measurement and ability of the device to measure capacitive coupled ECG while driving are presented at the end of the article.

A wearable H-shirt for exercise ECG monitoring and individual lactate threshold computing

Millions of people worldwide are involved in daily physical exercise and fitness, which implies the growing demand for wearable devices and gadgets monitoring these activities. To ensure the comfort of exercise, wearable health monitoring devices are the most lucrative for real-time and continuous electrocardiogram (ECG) monitoring, which is essential for making individualized fitness plans and avoiding the occurrence of cardiac arrest. Moreover, the ECG monitoring data obtained are instrumental in the computation of the individual lactate threshold heart rate, which is the inflection point of the heart rate-time curve and indicates the exercise intensity level, at which the blood concentration of lactate and lactic acid begins to exponentially increase and will result in overstraining, unless the exercise efforts (running speed or muscle intensity) are temporarily reduced. The respective monitoring and overstrain-warning devices have a large potential for the individual assessment of the endurance performance, as well as for prescribing intensities in the endurance training. However, the common drawbacks of commercial wearable devices used in healthcare and medical fields for continuous monitoring of ECG, electroencephalogram, respiration, temperature, SpO2, and human activity, such as smart helmets, wearable patches, and activity-tracking bands, are motion artifacts and unstable skin-electrode contact, which jeopardize the accurate measurements during exercise. Given this, the Health shirt (H-shirt) integrated with conductive fabric ECG electrodes enabling the accurate ECG monitoring during physical exercise has been introduced and validated by the authors. In this paper, the modified design of the sports H-shirt with the lactate threshold heart rate computing is proposed and implemented. High elasticity of the H-shirt not only ensures a stable skin-electrode contact but also avoids the discomfort caused by the measurement. A highly integrated printed circuit board is used to filter, amplify, digitalize, and transmit the ECG signal sampled from the electrodes, while a high input impedance amplifier circuit is designed to improve its signal-to-noise ratio. The Bluetooth low energy (BLE) is used as the data transmission protocol between the H-shirt and the mobile phone. A mobile phone-based ECG analysis and exercise evaluation platform have been developed, which allow one to detect six types of abnormal ECG (tachycardia, bradycardia, a dropped beat, premature beat, bigeminy, and trigeminy) and individual lactate threshold, as well as to provide real-time health warning and exercise optimization hints through mobile phone voice messages. Being powered by a 150mAh 3.7V lithium battery, the H-shirt operated for at least 24h and was tested on four volunteers of both genders and age from 14 to 32 years, whose ECG and the lactate threshold heart rate values were accurately measured during their test exercise on a treadmill with running speed up to 16km/h and used as a reference for the users to adjust their exercise intensity. The available options of adding sensors for measurement of more physiological parameters by the proposed sports H-shirt make it quite lucrative for the rehabilitation exercise monitoring.

Space Electromagnetic and Plasma Sensor (SEAPS): A Laboratory Prototype for a Space Payload

Space electromagnetic and plasma sensor is a proposed space payload consisting of an electric field vector sensor—a tri-axial arrangement of monopoles in a mutually orthogonal configuration—and a magnetic field vector sensor, which is a tri-axial arrangement of mutually perpendicular loop antennas. Both operate between 0.5 and 20 MHz in order to acquire the complete 3D polarization matrix. The antennas, which are meant to be electrically short, are matched with a high input impedance amplifier, followed by gain blocks, filters and a six-channel data acquisition system. Some of the proposed goals of the project include study of the low frequency Sun, plasma interactions between the solar wind and Earth’s magnetosphere, lunar atmosphere, and radio emission from other planets within the solar system. This article describes the construction of a laboratory prototype and preliminary calibration results.

Electrophysiological Effects of Dexmedetomidine on Sinoatrial Nodes of Rabbits.

The purpose of this study was to investigate the electrophysiological effects of dexmedetomidine on pacemaker cells in sinoatrial nodes of rabbits. Healthy rabbits were anesthetized intravenously with sodium pentobarbital, and the hearts were quickly dissected and mounted in a tissue bath. Machine-pulled glass capillary microelectrodes which were connected to a high input impedance amplifier and impaled in dominant pacemaker cells. Thereafter, an intracellular microelectrode technique was used to record action potential. The amplitude of action potential, velocity of diastolic (phase 4) depolarization, and rate of pacemaker firing in normal pacemaker cells in sinoatrial node were decreased by use of dexmedetomidine (0.5 ng/ml, 5 ng/ml, 50 ng/ml) in a concentration-dependent manner. Pretreatment with yohimbine (1 μM), did not alter the effects of dexmedetomidine (5 ng/ml) on sinoatrial node pacemaker cells. Pretreatment with CsCl (2 mmol/L), dexmedetomidine (5 ng/ml) decreased the amplitude of action potential, but had no significant effect on other parameters of action potential. Dexmedetomidine exerts inhibitory electrophysiological effects on pacemaker cells in sinoatrial nodes of rabbits in a concentration-dependent manner, which may not be mediated by alpha 2-adrenoreceptor. Action potential; Cardiology; Dexmedetomidine; Pacemaker activity; Sinoatrial node.

Analysis of pacemaker activity in the human stomach

Extracellular electrical recording and studies using animal models have helped establish important concepts of human gastric physiology. Accepted standards include electrical quiescence in the fundus, 3 cycles per minute (cpm) pacemaker activity in corpus and antrum, and a proximal-to-distal slow wave frequency gradient. We investigated slow wave pacemaker activity, contractions and distribution of interstitial cells of Cajal (ICC) in human gastric muscles. Muscles were obtained from patients undergoing gastric resection for cancer, and the anatomical locations of each specimen were mapped by the operating surgeon to 16 standardized regions of the stomach. Electrical slow waves were recorded with intracellular microelectrodes and contractions were recorded by isometric force techniques. Slow waves were routinely recorded from gastric fundus muscles. These events had similar waveforms as slow waves in more distal regions and were coupled to phasic contractions. Gastric slow wave frequency was significantly greater than 3 cpm in all regions of the stomach. Antral slow wave frequency often exceeded the highest frequency of pacemaker activity in the corpus. Chronotropic mechanisms such as muscarinic and prostaglandin receptor binding, stretch, extracelluar Ca(2+) and temperature were unable to explain the observed slow wave frequency that exceeded accepted normal levels. Muscles from all regions through the thickness of the muscularis demonstrated intrinsic pacemaker activity, and this corresponded with the widespread distribution in ICC we mapped throughout the tunica muscularis. Our findings suggest that extracellular electrical recording has underestimated human slow wave frequency and mechanisms of human gastric function may differ from standard laboratory animal models.

A Smart Health Monitoring Chair for Nonintrusive Measurement of Biological Signals

We developed nonintrusive methods for simultaneous electrocardiogram, photoplethysmogram, and ballistocardiogram measurements that do not require direct contact between instruments and bare skin. These methods were applied to the design of a diagnostic chair for unconstrained heart rate and blood pressure monitoring purposes. Our methods were operationalized through capacitively coupled electrodes installed in the chair back that include high-input impedance amplifiers, and conductive textiles installed in the seat for capacitive driven-right-leg circuit configuration that is capable of recording electrocardiogram information through clothing. Photoplethysmograms were measured through clothing using seat mounted sensors with specially designed amplifier circuits that vary in light intensity according to clothing type. Ballistocardiograms were recorded using a film type transducer material, polyvinylidenefluoride (PVDF), which was installed beneath the seat cover. By simultaneously measuring signals, beat-to-beat heart rates could be monitored even when electrocardiograms were not recorded due to movement artifacts. Beat-to-beat blood pressure was also monitored using unconstrained measurements of pulse arrival time and other physiological parameters, and our experimental results indicated that the estimated blood pressure tended to coincide with actual blood pressure measurements. This study demonstrates the feasibility of our method and device for biological signal monitoring through clothing for unconstrained long-term daily health monitoring that does not require user awareness and is not limited by physical activity.

MEMSエレクトレット・コンデンサ・マイクロホンによる脈波測定

A micro Electret Condenser Microphone (ECM) fabricated by Micro Electro Mechanical System (MEMS) technology was employed as a novel apparatus for human pulse wave measurement. Since ECM frequency response characteristic, i.e. sensitivity, logically maintains a constant level at lower than the resonance frequency (stiffness control), the slightest pressure difference at around 1.0Hz generated by human pulse wave is expected to detect by MEMS-ECM. As a result of the verification of frequency response of MEMS-ECM, it was found that -20dB/dec of reduction in the sensitivity around 1.0Hz was engendered by a high input-impedance amplifier, i.e. the field effect transistor (FET), mounted near MEMS chip for amplifying tiny ECM signal. Therefore, MEMS-ECM is assumed to be equivalent with a differentiation circuit at around human pulse frequency. Introducing compensation circuit, human pulse wave was successfully obtained. In addition, the radial and ulnar artery tracing, and pulse wave velocity measurement at forearm were demonstrated; as illustrating a possible application of this micro device.

Application of a taste evaluation system to the monitoring of Kimchi fermentation☆

As an objective method, taste evaluation with an instrument is able to supplement the subjective sensory evaluation and to be applied to the optimization of food processing. Kimchi, a Korean traditional pickle fermented with lactic acid bacteria, is expanding its consumption worldwide. The fermentation control of it has been routinely done by measuring titratable acidity and pH. In this study, an eight-channel taste evaluation system was prepared, followed by an application to the monitoring of Kimchi fermentation. Eight polymer membranes which individually responded to cationic or anionic substances were prepared by mixing electroactive materials such as tri-n-octylmethylammonium chloride, bis(2-ethylhexyl)sebacate as the plasticizer and polyvinyl chloride in the ratio of 1:66:33. Each membrane prepared was separately installed onto the sensitive area of an ion-selective electrode to produce the respective taste sensor. The eight-channel sensor array and a double junction reference electrode were connected to a 16-channel high input impedance amplifier. The amplified sensor signals were stored to a personal computer via a multi-channel A/D converter. Two sensor groups composed of the cation-selective and anion-selective polymer membrane electrodes showed characteristic concentration-dependency to various artificial taste substances. As a whole, the response potentials of the sensor array increased during the fermentation period at 4, 10 and 25 degrees C. Even the response potentials of the anion-selective taste sensors slightly increased possibly due to the protonation of anions by liberated H+ ions, thereby leading to a decrease in the anion concentration. When the signal data were interpreted by principal component analysis (PCA), the first PC at 4 degrees C explained most of the total data variance. A close correlation was found between the values of titratable acidity and the first PC, which indicated a possible applicability of the multi-channel taste sensor of this study to the process monitoring of various pickle.

Design of an electronic interface for capacitively coupled four-electrode conductivity detection in capillary electrophoresis microchip

In this paper, we present an electronic interface dedicated to capacitively coupled four-electrode liquid conductivity detection in micro capillary electrophoresis devices. Due to the small dimensions of the detector (106μm×170μm), it is necessary to adapt the read-out interface. Our study focuses on two major aspects of the four-electrode setup: First, the current which flows between the two outer electrodes must be known. Second, the two inner electrodes must be connected to a high input impedance and high common mode rejection ratio (CMRR) differential amplifier. The results obtained in a miniaturized CE channel show a linear response with liquid impedances within a range of 250kΩ to 3.45MΩ (for a MES/His buffer, this compares to a concentration range of 0.2–20mM).

Scalp electrode impedance, infection risk, and EEG data quality

Objectives: Breaking the skin when applying scalp electroencephalographic (EEG) electrodes creates the risk of infection from blood-born pathogens such as HIV, Hepatitis-C, and Creutzfeldt–Jacob Disease. Modern engineering principles suggest that excellent EEG signals can be collected with high scalp impedance (≈40 kΩ) without scalp abrasion. The present study was designed to evaluate the effect of electrode-scalp impedance on EEG data quality.Methods: The first section of the paper reviews electrophysiological recording with modern high input-impedance differential amplifiers and subject isolation, and explains how scalp-electrode impedance influences EEG signal amplitude and power line noise. The second section of the paper presents an experimental study of EEG data quality as a function of scalp-electrode impedance for the standard frequency bands in EEG and event-related potential (ERP) recordings and for 60 Hz noise.Results: There was no significant amplitude change in any EEG frequency bands as scalp-electrode impedance increased from less than 10 kΩ (abraded skin) to 40 kΩ (intact skin). 60 Hz was nearly independent of impedance mismatch, suggesting that capacitively coupled noise appearing differentially across mismatched electrode impedances did not contribute substantially to the observed 60 Hz noise levels.Conclusions: With modern high input-impedance amplifiers and accurate digital filters for power line noise, high-quality EEG can be recorded without skin abrasion.

Impedance Spectroscopy Of Biological Cells

ABSTRACTA six-electrode impedance chamber is used to perform wide-bandwidth (10 Hz − 3 MHz), sensitive impedance measurements to evaluate the electrical properties of living cells under normal physiologic conditions. The high sensitivity in these measurements, compared to cell suspension techniques, is accomplished by embedding the cells into the pores of a filter, creating a “pseudoepithelium” and markedly reducing the current shunt pathways around the cells. The shunt resistance for each cell is between 6 − 7 × 107 Ω. The cellular geometry and the portion of cell membrane under measurement are precisely controlled in this method. This technique produces a multi-parallel, whole-cell, patch-clamp like structure for 3.3 × 105 cells. The advantages of this technique, general insights, and improvements in impedance measurements will be discussed. Effects of bandwidth mismatch between high-input impedance amplifiers, reference point drift due to solution resistivity changes, and the use of porous electrodes will be covered. Lastly, the ability to electroporate the cell membrane contributes another unique way to study the impedance of living cells.

Rotating condenser method to measure two-dimensional biophysical potentials

A new device has been constructed to measure bioelectric potentials in plants. It measures, simultaneously, the electric length potentials along the shoot and the potentials across the shoot. Such potentials arise in plants in response to external physical stimuli. The instrument measures the potentials without touching the plants. The electric fields are detected capacitively by rotating two discs on two sides of the shoots. Each disc has four symmetrical 'vanes'; the discs are fixed and rotated 45 degrees with respect to each other. All vanes are electrically connected. The sensor arrangement will separate the length and the transverse potentials by generating two different frequencies (8 and 4 times the disc rotating frequency). The signal so generated is capacitively coupled to a high input impedance (1013 Omega ) amplifier. Two selective amplifying systems are used to generate feedback voltages and the overall equipment will work as a zero instrument. The equipment has been tested and used to record potentials in plants stimulated for 30 min in horizontal position. Transverse potentials recorded under these circumstances were of the order of 50 mV.

Estimation of Ventricular Repolarization in Man by Monophasic Action Potential Recording Technique

Myocardial monophasic action potentials (MAP) can be recorded with the aid of suction or contact electrodes applied endocardially via a catheter. The technique necessitates high input impedance amplifiers with infinite time constant. A bipolar technique improves signal quality with regard to electrical contamination around the rapid upstroke of the MAP. Mechanical artefacts in the recordings are common and may be explained by catheter movement induced by atrial or ventricular contractions. The MAP signal can be used for a precise measurement of time of local excitation and for the study of atrial as well as ventricular repolarization. The technique has mostly been applied in the exploration of atrial and ventricular repolarization in healthy hearts and during different cardiac arrhythmias. Furthermore, several studies have documented the electrophysiological action of antiarrhythmic drugs upon the human heart. Concluding from 576 different investigations we consider the technique to have no serious side-effects.

A low noise instrumentation amplifier

Difficulty in obtaining very low noise difference amplification with gain stability using conventional operational amplifier circuitry is considered. A design procedure eliminating the incompatibility of gain stability and low noise is discussed. The design, circuit details and performance of low noise, high input impedance amplifier with good gain stability and wide offset range is described.

Absolute calibration of antennas at extremely low frequencies

To calibrate electric-field sensors at extremely low frequency (ELF) the measured terminal voltage <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">V</tex> at a receiving antenna is compared with the Analytically predicted <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">V</tex> for a precisely known incident field. That incident field is generated by an electrically very short thin linear radiator. Formulas are given for the field in the immediate vicinity of such a radiator above a perfectly conducting ground. Also given is an expression for the integral of this field along a vertical path starting at ground level. The expressions involve the magnitude of the current at the base of the radiator and methods are described for establishing that quantity precisely by measuring the base-to-ground voltage and the very small capacitance (about 20 pF) of the radiator. The methods for small capacitance measurement are also applied to linear and spherical receiving antennas, and experiments are described in which it was possible to predict the terminal voltage at a receiving antenna to within <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">\pm3</tex> percent when the voltage applied to the transmitting radiator, located at a relatively short distance ( <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">\approx 16</tex> to 21 m) was known. Details of measurement circuitry are given including descriptions of an extremely high input impedance ( <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">\approx 5000 M\Omega</tex> ) amplifier and a circuit for the compensation of cable capacitance. For a uniform incident field the effective height of both, a vertical linear receiving antenna and of a spherical antenna, is equal to the distance between ground level and the center of the conducting part of the antenna. Typical values for the natural, ambient ELF field in the frequency range 3 to 30 Hz are shown to be in the vicinity of 0.1 mV/m/Hz <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1/2</sup> .

Electrocardiogram Recording with Pasteless Electrodes

Experimentation regarding various aspects of a technique for recording electrocardiographic potentials from unprepared skin, without the use of conventional paste, is described. Because of the relatively high skin-to-electrode impedances encountered without electrolytic paste, high input impedance amplifiers must be utilized for acquisition of the signal. In order to minimize susceptibility to external electrostatic and electromagnetic interference, an inherent problem with high input impedance amplifiers, buffer amplifiers were incorporated directly within the electrode housing. Of the different metals tested, stainless steel proved to be the most stable skin contact material for pasteless operation. The integrated electrode-buffer amplifiers described comply with specifications of the American Heart Association and should prove useful as a direct replacement for conventional paste-type electrodes in existing clinical EKG equipment as well as for long-term applications such as space missions and intensive-care-unit patient monitoring, where frequent attention to the electrodes is inconvenient.

A high input impedance, low level d.c. amplifier utilizing a solid-state operational amplifier

A high input impedance amplifier suitable for different kinds of conventional electrophysiological measurements is described.

Investigation of a resonant sample and hold circuit

A resonant sample and hold circuit is analyzed which consists of a periodically operated switch, an inductor L and a capacitor C in series. Dissipative elements associated with these are represented by Rs, which also includes any source impedance, RL and Rc, respectively, and are taken into account. The switch is closed for an interval equal to one half the natural period of oscillation T2 of the circuit. This is assumed to be much smaller than the sampling interval TS. The impedance of the open switch is taken as infinite. The circuit has the following advantages. It produces a peak near one half the sampling frequency which can be controlled by changing the resistances Rs, RL, Rc singly or in any combination. This can be used in pulse amplitude or pulse code modulation systems to equalize the cutoff of the reconstruction low-pass filters. The circuit also exhibits maximum attenuation at the carrier frequency ws. The current through the switch is sinusoidal with a peak amplitude several times smaller than what it would be if no resonant inductor were used. The voltage across the capacitor during charging is sinusoidal of frequency ω22. This makes possible the reduction of the bandwidth requirements of the high input impedance amplifier through which, for example, the capacitor voltage is coupled to an analog-to-digital (A/D) converter.

The field-effect transistor-its characteristics and applications

The characteristics of the junction gate field-effect transistor and the basic relations among its parameters are described. Some figures of merit are derived in terms of its physical parameters and the limitations considered. Applications in nuclear instrumentation are discussed. Some advantages of field-effect transistors are in high input impedance, low noise at very low and medium frequencies, and a wide operating temperature range (-200°C to +100°C). In addition, the field-effect transistor represents a new circuit element - voltage (charge) controlled conductance with fast response. Field-effect transistors are particularly suitable for low noise amplifiers at low and medium frequencies, for high input impedance amplifiers (as for integrators), for charge-sensitive amplifiers, and for analogue multipliers. They can also be applied as controllable feedback elements.