Ramon Pallas-Areny

Skin impedance from 1 Hz to 1 MHz

The impedance of skin coated with gel but otherwise unprepared was measured from 1 Hz to 1 MHz at ten sites on the thorax, leg, and forehead of ten subjects. For a 1-cm/sup 2/ area, the 1 Hz impedance varied from 10 k Omega to 1 M Omega , which suggests that the bipotential amplifier input impedance should be very high to avoid common-mode-to-differential-mode voltage conversion. The 1-MHz impedance was tightly clustered about 120 Omega . The 100-kHz impedance was about 220 Omega , which suggests that the variation in skin impedance can cause errors in two-electrode electrical impedance tomographs.<<ETX>>

AC-coupled front-end for biopotential measurements

AC coupling is essential in biopotential measurements. Electrode offset potentials can be several orders of magnitude larger than the amplitudes of the biological signals of interest, thus limiting the admissible gain of a dc-coupled front end to prevent amplifier saturation. A high-gain input stage needs ac input coupling. This can be achieved by series capacitors, but in order to provide a bias path, grounded resistors are usually included, which degrade the common mode rejection ratio (CMRR). This paper proposes a novel balanced input ac-coupling network that provides a bias path without any connection to ground, thus resulting in a high CMRR. The circuit being passive, it does not limit the differential dc input voltage. Furthermore, differential signals are ac coupled, whereas common-mode voltages are dc coupled, thus allowing the closed-loop control of the dc common mode voltage by means of a driven-right-leg circuit. This makes the circuit compatible with common-mode dc shifting strategies intended for single-supply biopotential amplifiers. The proposed circuit allows the implementation of high-gain biopotential amplifiers with a reduced number of parts, thus resulting in low power consumption. An electrocardiogram amplifier built according to the proposed design achieves a CMRR of 123 dB at 50 Hz.

Wireless Magnetic Sensor Node for Vehicle Detection With Optical Wake-Up

Vehicle detectors provide essential information about parking occupancy and traffic flow. To cover large areas that lack a suitable electrical infrastructure, wired sensors networks are impractical because of their high deployment and maintenance costs. Wireless sensor networks (WSNs) with autonomous sensor nodes can be more economical. Vehicle detectors intended for a WSN should be small, sturdy, low power, cost-effective, and easy to install and maintain. Currently available vehicle detectors based on inductive loops, ultrasound, infrared, or magnetic sensors do not fulfill the requirements above, which has led to the search for alternative solutions. This paper presents a vehicle detector which includes a magnetic and an optical sensor and is intended as sensor node for use with a WSN. Magnetic sensors based on magnetoresistors are very sensitive and can detect the magnetic anomaly in the Earths magnetic field that results from the presence of a car, but their continuous operation would drain more than 1.5 mA at 3 V, hence limiting the autonomy of a battery-supplied sensor node. Passive, low-power optical sensors can detect the shadow cast by car that covers them, but are prone to false detections. The use of optical triggering to wake-up a magnetic sensor, combined with power-efficient event-based software, yields a simple, compact, reliable, low-power sensor node for vehicle detection whose quiescent current drain is 5.5 μA. This approach of using a low-power sensor to trigger a second more specific sensor can be applied to other autonomous sensor nodes.

Bioelectric impedance measurements using synchronous sampling

Synchronous sampling has been applied to the demodulation of bioelectric impedence signals. This overcomes the need for analog demodulators in bioimpedance measurements. The sampling rate is determined by signal bandwidth, rather than by the highest frequency component before demodulation.<<ETX>>

Common mode rejection ratio in differential amplifiers

The common mode rejection ratio (CMRR) of a differential amplifier (DA) using a single operational amplifier and an instrumentation amplifier (IA) using three operational amplifiers is analyzed, and the complete equations are derived for the case when op amps have finite differential and common mode gains. Amplitude and phase measurements support the theoretical predictions. It is concluded that, at low frequencies, for the single-op-amp DA the use of a trimming potentiometer is better than relying on low-tolerance resistors, because of the higher CMRR achieved. The DA yields a fixed 90 degrees phase shift for the CMRR at frequencies above 1 kHz. For the three-op-amp 1A, it is important that the input buffers are coupled and that they are built from a matched op amp pair. The best CMRR is obtained when the differential gain is concentrated in the input stage, but in any case it decreases at frequencies above 1 kHz because of the reduced CMRR for the differential stage at these frequencies. >

A novel fully differential biopotential amplifier with DC suppression

Fully differential amplifiers yield large differential gains and also high common mode rejection ratio (CMRR), provided they do not include any unmatched grounded component. In biopotential measurements, however, the admissible gain of amplification stages located before dc suppression is usually limited by electrode offset voltage, which can saturate amplifier outputs. The standard solution is to first convert the differential input voltage to a single-ended voltage and then implement any other required functions, such as dc suppression and dc level restoring. This approach, however, yields a limited CMRR and may result in a relatively large equivalent input noise. This paper describes a novel fully differential biopotential amplifier based on a fully differential dc-suppression circuit that does not rely on any matched passive components, yet provides large CMRR and fast recovery from dc level transients. The proposed solution is particularly convenient for low supply voltage systems. An example implementation, based on standard low-power op amps and a single 5-V power supply, accepts input offset voltages up to /spl plusmn/500 mV, yields a CMRR of 102dB at 50 Hz, and provides, in accordance with the AAMI EC38 standard, a reset behavior for recovering from overloads or artifacts.

A comprehensive model for power-line interference in biopotential measurements

Power line interference is a major problem in high-resolution biopotential measurements. Because interference coupling is mostly capacitive, shielding electrode leads and a high common-mode rejection ratio (CMRR) are quite effective in reducing power-line interference but do not completely eliminate it. We propose a model that includes both interference external to the measuring system and interference coming from its internal power supply. Moreover, the model considers interference directly coupled to the measuring electrodes, because, as opposed to connecting leads, electrodes are not usually shielded. Experimental results confirm that reducing interference coupled through electrodes yields a negligible interference. The proposed model can be applied to other differential measurement systems, particularly those involving electrodes or sensors placed far apart.

AC instrumentation amplifier for bioimpedance measurements

The input impedance and common-mode rejection ratio requirements for an amplifier for bioimpedance measurements are analyzed, considering the capacitive components of the electrode-skin contact impedance. An AC-coupled instrumentation amplifier that fulfills those requirements, and provides both interference and noise reduction and a zero phase shift over a wide frequency band without using broadband operational amplifiers, is described.<<ETX>>

Heart Rate Detection From Plantar Bioimpedance Measurements

The heart rate is a basic health indicator, useful in both clinical measurements and home health care. Current home care systems often require the attachment of electrodes or other sensors to the body, which can be cumbersome to the patient. Moreover, some measurements are sensitive to movement artifacts, are not user-friendly and require a specialized supervision. In this paper, a novel technique for heart rate measurement for a standing subject is proposed, which is based on plantar bioimpedance measurements, such as those performed by some bathroom weighting scales for body composition analysis. Because of the low level of heart-related impedance variations, the measurement system has a gain of 1400. We have implemented a fully differential AC amplifier with a common-mode rejection ratio (CMRR) of 105 dB at 10 kHz. Coherent demodulation based on synchronous sampling yields a signal-to-noise ratio (SNR) of 55 dB. The system has a sensitivity of 1.9 V/Omega. The technique has been demonstrated on 18 volunteers, whose bioimpedance signal and ECG were simultaneously measured to validate the results. The average cross-correlation coefficient between the heart rates determined from these two signals was 0.998 (std. dev. 0.001)

The effect of respiration-induced heart movements on the ECG

A model describing the rotation of the cardiac vector as a possible mechanism for the presence of respiratory information in the ECG is discussed. The way in which this information is revealed is analyzed, and the predictions subjected to qualitative experimental assessment via spectral analysis. The results show that respiratory frequencies occur in the ECG spectrum as a result of heart movement. Measurements on a patient wearing a pacemaker and ventilated to control respiratory rate show that even in the absence of respiratory sinus arrhythmia there is baseband information in the ECG spectrum, attributable neither to electrode artifacts nor to EMG, and sidebands from the respiratory cycle.<<ETX>>