Test & measurement
ADOPTING A BETTER BIOIMPEDANCE ANALOGUE FRONT END FOR NEXT- GENERATION VITAL SIGNS MONITORING DEVICES
Vital signs monitoring (VSM) devices perform real-time measurement of the user’s electrocardiogram (ECG), photoplethysmogram (PPG), bioimpedance, and other parameters. Among them, bioimpedance is an important electrical parameter that characterises the user’s physiological characteristics and vital signs, and is widely used in many detection fields. This article from Yi Xin, senior central applications engineer at Analog Devices, introduces the basic concepts, application scenarios, and current pain points of bioimpedance measurement. A new measurement solution is also introduced in this article, and the analogue front end (AFE) used in the solution supports outputting sinusoidal sweep stimulus and quadrature demodulation, thereby improving the accuracy and convenience of bioimpedance
measurement. At the same time, the AFE can also perform synchronous measurements with other biosensors, simplifying the design of next-generation VSM systems.
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ioimpedance refers to the impedance change of organisms or their tissues, organs, and cells, when a stimulus below their safe threshold is applied. It is usually a complex impedance that changes
with frequency. Bioimpedance measurement is noninvasive and low cost, simple to operate, and supports a wide range of functions. It is easy for doctors and patients to accept and has a high level of clinical applications.
At present, VSM has gradually extended from medical healthcare equipment to portable consumer products. People can now understand their own physiological conditions anytime and anywhere with VSM devices. VSM has been widely used in remote medical care, disease prevention, auxiliary diagnosis, and health fitness. It can measure various physiological signals such as ECG, PPG, bioimpedance, and skin temperature. These signals can be used to calculate and analyse heart rate, peripheral oxygen saturation, blood pressure trending, respiration, mental stress, and so on. Bioimpedance is usually combined with other physiological signals to evaluate the user’s health status completely and comprehensively.
HUMAN BODY BIOIMPEDANCE AND ITS PHYSIOLOGICAL SIGNIFICANCE All substances have a certain opposition effect to the current passing through them. This effect is called the impedance of the substance. To achieve accurate calculation and measurement of human body bioimpedance, it is necessary to model the human body as a circuit composed of passive devices such as resistors and capacitors. That is the equivalent model of human body bioimpedance. So how can human body bioimpedance be translated into an equivalent circuit model? First, we study the components of the equivalent model. Human tissue is composed of cells. Since the intracellular fluid (ICF) and extracellular fluid (ECF) have good electrical conductivity, they can be equivalent to two
resistors as RI and RE. The cell membrane is mainly composed of lipids, and there is a transmembrane voltage difference between its inside and outside, so it can be equivalent to a
capacitor as CM. Therefore, the human body bioimpedance has both resistance and capacitance components, which give it a complex impedance. Then, we study the specific structure of the equivalent model.
Figure 1. A human body bioimpedance equivalent circuit model.
As shown in Figure 1, the cell membrane can be regarded as a capacitor. Low frequency current (purple dotted line) has difficulty passing through the capacitor, so it cannot pass through cells and can only flow in the ECF. At the same time, also due to the capacitor, high frequency current (green solid line) can flow in the ICF through cells directly. Therefore, we can express the bioimpedance equivalent model of the human body as the circuit shown in Figure 1. High frequency
May 2024 Instrumentation Monthly
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