Feature: Medical


A flexible and high-precision AFE Analog Devices has a broad portfolio of products for bioimpedance analysis, including devices such as the ADuCM35x, a highly-integrated system-on-chip (SoC) designed specifically for impedance spectroscopy. Recently announced is the AD5940, a high-precision, low-


Figure 3: Bioelectrical impedance vector analysis


capacity to evaluate the variations in the ICW. Te MF-BIA technique overcomes the limitations of SF-BIA by


performing the measurement at both low and high frequencies. Te low-frequency measurement allows for a more accurate estimate of ECW, whereas at high frequency an estimate of the TBW is obtained, with ICW being the difference between the two estimates. However, this technique is not perfect and shows its limitations in the estimation of body fluids in the elderly affected by disease. Finally, the BIS is based on the measurement of impedance


at zero frequency, which, according to the model of Figure 1, is the resistance RE


frequency, which is the parallel of RE


due to the extracellular fluids, and, at infinite with RI


. At these frequency


extremes, the capacitance due to the cell membrane behaves like either an open or short circuit. Intermediate frequency (IF) measurements provide information related to the capacitance value. BIS provides more detailed information than other techniques do, but, in this case the measurement takes longer. Bioimpedance vector analysis (bioelectrical impedance vector


analysis, or BIVA) is a human-health assessment technique based on the absolute measurement of bioimpedance. It uses a graph that shows a vector representation of the impedance with the value of the resistance on the abscissa and the capacitive reactance on the ordinate, both normalised with respect to the patient’s height. Te method is based on the formulation of three tolerance


ellipses: 50%, 75% and 95%. Te tolerance ellipse of 50% defines the population with average body composition. Moving along the horizontal axis of the ellipse, individuals with a low percentage of lean mass are identified on the right, and vice versa. Moving along the vertical axis identifies the level of hydration, with levels below the norm toward the top of the ellipses. Te observation of fluctuations in the components of the


human body for example, the deviation from normal values of lean body mass, fat mass and total body water, are key factors in establishing the patient’s state of health. A significant loss of lean mass and an imbalance of body fluids are the main parameters used to diagnose diseases.


24 March 2021 www.electronicsworld.co.uk


power consumption analogue front end (AFE), ideal for portable applications. Designed for measuring bioimpedance and skin conductivity, the AD5940 comprises two excitation loops and a common measurement channel. Te first excitation loop generates signals with a maximum frequency of 200Hz, and can be configured as a potentiostat for the measurement of electrochemical cells of different types. Its basic components are a dual-output DAC, a precision amplifier that provides the excitation signal, and a transimpedance amplifier for current measurements. Working at a low frequency, this loop consumes little power and is therefore also referred to as the low-power loop. Te second loop has a similar configuration but works with


signals to 200kHz, hence its name “high-speed loop”. Te device is equipped with an acquisition channel with a 16-


bit, 800kSPS SAR-type ADC and an analogue signal processing chain upstream of the converter, which includes a buffer, a programmable gain amplifier (PGA) and a programmable antialiasing filter. Te architecture also includes a switching matrix mux which connects the ADC with the signals coming from multiple internal or external sources to the device. In this way, in addition to its primary impedance measurement function, accurate system diagnostics can be performed to verify the instrument’s full functionality. Figure 4 shows the connection of the AD5940 for the absolute


impedance measurement of the human body using a four-wire configuration. For this type measurement, the high frequency loop is used, with the excitation signal supplied by a programmable AC voltage generator. A second generator supplies the common-mode voltage – useful for a correct measurement. Te current resulting from the impedance of the human body is measured by the transimpedance amplifier and converted with the 16-bit ADC. Te system measures to a frequency of 200kHz and provides a signal- to-noise ratio (SNR) of 100dB at 50kHz. Te digital data is sent to a hardware accelerator for extraction of the quantities of interest; that is, the real part and the imaginary part of the impedance. As a medical device, the bioimpedance analyser must comply


with the IEC 60601 standard, which sets limits for the voltages and currents that can be applied to the human body. For this reason, a resistance, Rlimit


maximum current, as have four coupling capacitors, Ciso


, has been provided to limit the X, to


prevent a DC component from being applied to the human body. Bioimpedance measurement is a versatile, fast, non-invasive


and low-cost tool for assessing the composition of the human body and diagnosing diseases. Current technology enables the development of compact, high-performance, low-power bioimpedance analysers that can be battery-powered. Te AD5940’s features make it particularly suitable for wearable applications.


Page 1  |  Page 2  |  Page 3  |  Page 4  |  Page 5  |  Page 6  |  Page 7  |  Page 8  |  Page 9  |  Page 10  |  Page 11  |  Page 12  |  Page 13  |  Page 14  |  Page 15  |  Page 16  |  Page 17  |  Page 18  |  Page 19  |  Page 20  |  Page 21  |  Page 22  |  Page 23  |  Page 24  |  Page 25  |  Page 26  |  Page 27  |  Page 28  |  Page 29  |  Page 30  |  Page 31  |  Page 32  |  Page 33  |  Page 34  |  Page 35  |  Page 36  |  Page 37  |  Page 38  |  Page 39  |  Page 40  |  Page 41  |  Page 42  |  Page 43  |  Page 44