Bioimpedance measurement systems often use the Howland current sources to excite the biological material under study. Usually, difference or instrumentation amplifiers are used to measure the resulting voltage drop on this material. In these circuits, common mode voltage appears as artifacts in the measurement. Most researches on current sources are focused on improving the output impedance, letting other characteristics aside. In this paper, it is made a brief review on the load common mode voltage and output swing of various topologies of Howland current sources. Three circuits are proposed to reduce load common mode voltage and enhance load capability by using a fully differential amplifier as active component. These circuits are equated, simulated and implemented. The three proposed circuits were able to deliver an output current with cut-off frequency (-3dB) higher than 1 MHz for loads as big as 4.7 kΩ. The worst measured load common mode voltage was smaller than 24 mV for one of the circuits and smaller than 8 mV for the other two. Consequently, it could be obtained increases in the Common Mode Rejection Ratio (CMRR) up to 60 dB when compared to the Enhanced Howland Current Source (EHCS).
For Electrical Impedance Spectroscopy (EIS) applications, the Enhanced Howland Current Source (EHCS) is a popular choice as an excitation circuit due to its simplicity, reliability, and safety. However, its output impedance degradation at high frequency leads to errors that are unacceptable for high load impedance applications, such as the ones which use dry or microelectrodes. Based on a proposed mathematical model, this work designed an EHCS circuit which includes an output current buffer and frequency compensation. PSpice simulations were performed as proof of concept, and then the measured data were collected for comparison. For the proposed circuit, called here Load-in-the-Loop Compensated Enhanced Howland Source (LLC-EHCS), the results showed that the output current errors are lower than 1% up to 3.7 MHz over the load range of 560-2200 Ω and 1.2 MHz with 5.6 kΩ. On the other hand, for the case of the standard EHCS circuit, these frequencies are 170 and 80 kHz, respectively. Also, the output linear swing was found to be 3 times higher than the EHCS. It can be concluded that the proposed LLC-EHCS may be widely used as an excitation circuit for high load and wide bandwidth EIS applications.
In order to test and calibrate an EIT (Electrical Impedance Tomography) system, many researchers rely on phantoms mimicking breast tissues. These phantoms are usually made of saline solutions, agar and/or vegetables, allowing the user to set the conductivity of the material by changing the salt concentration. Due to that fact that the dispersion behavior in the vicinity of megahertz is fundamental to detect carcinoma, this work aims to propose a phantom composed by a mixture of agar and gelatin (emulating the normal tissue) and a piece of carrot (emulating the carcinoma). It also investigates the frequency dependence from 1 to 10,000 kHz. The proposed phantom showed capability to mimic some absolute and relative electrical parameters used to detect cancer according to the literature. The differences in the impedance modulus were found to be more dispersive in the mimic carcinoma tissue type than the normal mimic one. It is simple to prepare, low cost, has similar electrical properties to the ones that have been used in the literature, better mechanical properties and longer life time. It can be concluded that gelatin-agar gel may have a high potential to be used as a breast tissue phantom.
Electrical Bioimpedance Spectroscopy (EIS) is a technique used to assess passive electrical properties of biological materials. EIS detects physiological and pathological conditions in animal tissues. Recently, the introduction of broadband excitation signals has reduced the measuring time for application techniques such as Electrical Bioimpedance Myography. Therefore, this work is aimed at proposing a prototype by using discrete interval binary sequences (DIBS), which is based on a system that holds a current source, impedance acquisition system, microcontroller and graphical user interface. Measurements between 5 Ω to 5 kΩ had impedance acquisition and phase angle errors of aproximately 2% and were lower than 3 degrees, respectively. Based on a proposed circuit, bioimpedance of the chest muscle (Pectoralis Major) was measured during isotonic exercise (push-up). As a result, our analyses have detected tiredness and fatigue. We have explored and proposed new parameters which assess such conditions, as both the maximum magnitude and tiredness coefficient. These parameters decrease exponentially with consecutive push-ups and were convergent in the majority of the sixteen days of measurement.
An impedance spectrum is calculated by the ratio between an injecting current and a resulting measured voltage, which allows the extraction of electrical properties from the material under study. The current source is considered an essential block to deliver a controlled current to a wide range of working loads and large bandwidth. To comply with such requirements, the current source output impedance must be much higher than the load impedance at each discrete frequency within the range. However, stray capacitance from cables and circuitry reduce the output impedance, especially at higher frequencies. We proposed a modified mirrored enhanced Howland current source (MEHCS) by using the feedback technique for a wide frequency range applications on electrical bioimpedance. We implemented four MEHCS circuits [with/without multiplexer (MUX) and with/without feedback], and then the output current and impedance were measured up to 20 MHz. The proposed current source showed an improvement in the frequency response at lower and higher frequencies when compared to the standard circuit. The measured output impedance was 10 times higher in the proposed circuit than in the standard MEHCS. The use of a feedback also increased the bandwidth in almost one decade in low and high frequencies when loaded with a resistor of about 1 kΩ.
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