Howland circuits have been widely used as powerful source for exciting tissue over a wide frequency range. When a Howland source is designed, the components are chosen so that the designed source has the desired characteristics. However, the operational amplifier limitations and resistor tolerances cause undesired behaviors. This work proposes to take into account the influence of the random distribution of the resistors in the modified Howland circuit over the frequency range of 10 Hz to 10 MHz. Both output current and impedance of the circuit are deduced either considering or the operational amplifiers parameters. The probability density function due to small changes in the resistors of the circuit was calculated by using the analytical modeling. Results showed that both output current and impedance are very sensitive to the resistors variations. In order to get higher output impedances, high operational amplifier gains are required. The operational amplifier open-loop gain increases as increasing the sensitivity of the output impedance. The analysis done in this work can be used as a powerful co-adjuvant tool when projecting this type of circuit in Spice simulators. This might improve the implementations of practical current sources used in electrical bioimpedance.

https://file.scirp.org/pdf/CS_2013110810122254.pdf

High Accurate Howland Current Source: Output Constraints Analysis

Current sources are widely used in bio-impedance spectroscopy (BIS) measurement systems to maximize current injection for increased signal to noise while keeping within medical safety specifications. High-performance current sources based on the Howland current pump with optimized impedance converters are able to minimize stray capacitance of the cables and setup. This approach is limited at high frequencies primarily due to the deteriorated output impedance of the constant current source when situated in a real measurement system. For this reason, voltage sources have been suggested, but they require a current sensing resistor, and the SNR reduces at low impedance loads due to the lower current required to maintain constant voltage. In this paper, we compare the performance of a current source-based BIS and a voltage source-based BIS, which use common components. The current source BIS is based on a Howland current pump and generalized impedance converters to maintain a high output

Performance evaluation of wideband bio-impedance spectroscopy using constant voltage source and constant current source

High accurate and wideband current excitation for bioimpedance health monitoring systems

Smart cities and communities (SCC) constitute a new paradigm in urban development. SCC ideate a data-centered society aimed at improving efficiency by automating and optimizing activities and utilities. Information and communication technology along with Internet of Things enables data collection and with the help of artificial intelligence (AI) situation awareness can be obtained to feed the SCC actors with enriched knowledge. This paper describes AI perspectives in SCC and gives an overview of AI-based technologies used in traffic to enable road vehicle automation and smart traffic control. Perception, smart traffic control and driver modeling are described along with open research challenges and standardization to help introduce advanced driver assistance systems and automated vehicle functionality in traffic. To fully realize the potential of SCC, to create a holistic view on a city level, availability of data from different stakeholders is necessary. Further, though AI technologies provide accurate predictions and classifications, there is an ambiguity regarding the correctness of their outputs. This can make it difficult for the human operator to trust the system. Today there are no methods that can be used to match function requirements with the level of detail in data annotation in order to train an accurate model. Another challenge related to trust is explainability: models can have difficulty explaining how they came to certain conclusions, so it is difficult for humans to trust them.

https://www.mdpi.com/2624-6511/4/2/40/pdf

AI Perspectives in Smart Cities and Communities to Enable Road Vehicle Automation and Smart Traffic Control

Comprehensive study of Howland circuit with non-ideal components to design high performance current pumps

In this work, the single Op-Amp with load-in-the-loop topology as a current source is revisited. This circuit topology was already used as a voltage-controlled current source (VCCS) in the 1960s but was left unused when the requirements for higher frequency arose among the applications of electrical bioimpedance (EBI). The aim of the authors is not only limited to show that with the currently available electronic devices it is perfectly viable to use this simple VCCS topology as a working current source for wideband spectroscopy applications of EBI, but also to identify the limitations and the role of each of the circuit components in the most important parameter of a current for wideband applications: the output impedance. The study includes the eventual presence of a stray capacitance and also an original enhancement, driving with current the VCCS. Based on the theoretical analysis and experimental measurements, an accurate model of the output impedance is provided, explaining the role of the main constitutive elements of the circuit in the source's output impedance. Using the topologies presented in this work and the proposed model, any electronic designer can easily implement a simple and efficient current source for wideband EBI spectroscopy applications, e.g. in this study, values above 150 kΩ at 1 MHz have been obtained, which to the knowledge of the authors are the largest values experimentally measured and reported for a current source in EBI at this frequency.

/pdf/simple-voltage-controlled-current-source-for-wideband-50ljqxp8m5.pdf

Simple voltage-controlled current source for wideband electrical bioimpedance spectroscopy: circuit dependences and limitations

Nowadays, the road safety is one of the most important priorities in the automotive industry. Many times, this safety is jeopardized because of driving under inappropriate states, e.g. drowsiness, drugs and/or alcohol. Therefore several systems for monitoring the behavior of subjects during driving are researched. In this paper, a device based on a contactless electrical bioimpedance system is shown. Using the four-wire technique, this system is capable of obtaining the heart rate and the ventilation of the driver through multiple textile electrodes. These textile electrodes are placed on the car seat and the steering wheel. Moreover, it is also reported several measurements done in a controlled environment, i.e. a test room where there are no artifacts due to the car vibrations or the road state. In the mentioned measurements, the system response can be observed depending on several parameters such as the placement of the electrodes or the number of clothing layers worn by the driver.

/pdf/ventilation-and-heart-rate-monitoring-in-drivers-using-a-40l61griy5.pdf

Ventilation and Heart Rate Monitoring in Drivers using a Contactless Electrical Bioimpedance System

In recent years, Electrical Bioimpedance (EBI) methods have gained importance. These methods are often based on obtaining impedance spectrum in the range of β-dispersion, i.e. from a few kHz up to some MHz. To measure EBI a constant current is often injected and the voltage across the tissue under study is recorded. Due to the performance of the current source influences the performance of the entire system, in terms of frequency range, several designs have been implemented and studied. In this paper the basic structure of a Voltage-Controlled Current Source based on a single Op-Amp in inverter configuration with a floating load, known as load-in-the-loop current source, is revisited and studied deeply. We focus on the dependence of the output impedance with the circuit parameters, i.e. the feedback resistor and the inverter-input resistor, and the Op-Amp main parameters, i.e. open loop gain, CMRR and input impedance. After obtaining the experimental results, using modern Op-Amps, and comparing to the theoretical and simulated ones, they confirm the design under study can be a good solution for multi-frequency wideband EBI applications because of higher values of the output impedance than 100kΩ at 1MHz are obtained. Furthermore, an enhancement of the basic design, using a current conveyor as a first stage, is proposed, studied and implemented.

/pdf/performance-of-the-load-in-the-loop-single-op-amp-voltage-473tetohkt.pdf

Performance of the load-in-the-loop single Op-Amp voltage Controlled current source from the Op-Amp Parameters

Given the advances in the technology known as smart textiles, the use of textile electrodes is more and more common. However this kind of electrodes presents some differences regarding the standard ones as the Ag-AgCl electrodes. Therefore to characterize them as best as possible is required. In order to make the characterization reproducible and repetitive, a skin dummy made of agar-agar and a standardized measurement set-up is used in this article. Thus, some dependencies in the skin-electrode interface are described. These dependencies are related to the surface of the textile electrode, the conductive material and the applied pressure. Furthermore, the dependencies on clothing in the skin-textile electrode interface are also analyzed. Thus, based on some parameters such as textile material, width and number of layers, the behavior of the interface made up by the skin, the textile electrode and clothing is depicted.

https://iopscience.iop.org/article/10.1088/1742-6596/434/1/012024/pdf

Textile electrode characterization: dependencies in the skin-clothing-electrode interface

As improving road safety is one of the first aims in the automotive world, several new techniques and methods are being researched in recent years. Some of them consist of monitoring the driver behavior to detect non-appropriate states for driving, e.g. drowsy driving or drunk driving. Usually, the appearance of these non-appropriate states is related to changes in several physiological parameters. This work is divided into two main parts. The first one presents an electrical bioimpedance system capable of monitoring the ventilation using textile electrodes. Apart from describing the system, in this part some tests done in a controlled environment are also shown. In the second part of this paper, an enhancement of the system is described and checked using a patient simulator.

https://www.springer.com/cda/content/document/cda_downloaddocument/9783662444849-c2.pdf?SGWID=0-0-45-1486652-p176885589

Raul Macias

Papers

Simple voltage-controlled current source for wideband electrical bioimpedance spectroscopy: circuit dependences and limitations

Ventilation and Heart Rate Monitoring in Drivers using a Contactless Electrical Bioimpedance System

Performance of the load-in-the-loop single Op-Amp voltage Controlled current source from the Op-Amp Parameters

Textile electrode characterization: dependencies in the skin-clothing-electrode interface

Monitoring Drivers’ Ventilation Using an Electrical Bioimpedance System: Tests in a Controlled Environment