Test & measurement TABLE 2. BIOIMPEDANCE MEASUREMENT APPLICATIONS PARAMETER Bioimpedance Range Stimulus Frequency Range Stimulus Type Bioimpedance Signal Band Electrode Placement BIS/BIA 10 Ω to 5 kΩ 1 kHz to 1 MHz (5 to 15 frequencies, typ) Sinusoidal wave DC


4 dry 4 wet


Arm to arm Leg to leg Arm to leg


Wrist to wrist


amplitude can be freely determined, compared with traditional solutions having low signal-to-noise ratio (SNR) in the high frequency band. In addition, the AFE has many other advantages. First, the AFE still retains the function of outputting a square wave stimulus, which helps with easy upgrading and replacement. Second, as shown in Figure 6, compared with traditional solutions, the AFE’s dedicated synchronisation pins can share clock signals with other ECG or PPG biosensors to achieve synchronous measurement of multiple vital signs. This advantage can improve the integration and scalability of VSM devices. Third, the AFE can reuse the electrodes of the ECG biosensor to reduce the size of the system and improves portability.


BIOIMPEDANCE MEASUREMENT APPLICATIONS


The bioimpedance measurement solution built by the MAX30009 has a wide range of application scenarios. They can be mainly divided into four categories: bioimpedance spectroscopy (BIS) or bioimpedance analysis (BIA), respiration, galvanic skin response (GSR) or electrodermal analysis (EDA), and impedance cardiogram (ICG). Each of these categories require a different impedance range, stimulus frequency range, stimulus type, impedance signal band, electrode type, and placement, as shown in Table 2.


Among them, BIS or BIA should reflect the details of all bioimpedance parameters in a wide frequency range accurately, and generate their plot looks like the one in Figure 2. Therefore, we usually select a dozen frequencies to perform sinusoidal sweep bioimpedance measurement in the frequency range from 1 kHz to 1 MHz. BIS or BIA is commonly used in body fat analysis, body moisture analysis, and so on.


The principle of respiration measurement is that the chest impedance changes periodically during breath. Since people usually breathe between 10 to 60 times per minute, the respiration bioimpedance is a slow changing


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signal in a low frequency band between 0.05 Hz and 4 Hz. Either a square or sinusoidal signal can be selected as the stimulus, at a typical frequency of 75 kHz. GSR or EDA are commonly used in mental stress monitoring devices like lie detectors that measure changing impedance caused by hand sweat when a person is tense. The devices usually requires dry electrodes, and a single frequency stimulus between DC and 400 Hz. ICG is a noninvasive method that can indirectly calculate stroke volume and cardiac output based on the thoracic cavity impedance change during a cardiac cycle. It can be used together with ECG to evaluate cardiac function. Usually, four wet electrodes are placed on the neck and abdomen of the patient. A sinusoidal signal between 20 kHz and 200 kHz (typically 75 kHz) is used as the stimulus. With synchronisation pins, the MAX30009 can work with other ECG biosensors to achieve a comprehensive assessment of heart health. It can also reuse the skin electrodes of ECG biosensors.


Figure 9. Quadrature demodulation. CONCLUSION


Bioimpedance is one of the most important parameters for VSM. It is closely related to people’s health status. Therefore, accurate bioimpedance measurement is of great benefit to maintaining health. The MAX30009 is a specific purpose device that can output many stimulus signals such as sinusoidal sweep. It supports the detailed measurement of bioimpedance parameters by quadrature demodulation, so has wide application scenarios. It can also achieve synchronous measurement with other biosensors and therefore has many advantages that traditional solutions do not have. It is an ideal product for constructing VSM applications. For further bioimpedance measurement products and information, please refer to the Bioimpedance Sensors Parametric Search and Body Composition, Hydration, Bioimpedance Analysis.


Analog Devices www.analog.com May 2024 Instrumentation Monthly RESPIRATION 10 Ω to 5 kΩ


50 kHz to 500 kHz (75 kHz, typ)


Sinusoidal wave Square wave


0.05 Hz to 4 Hz


2 wet 4 dry 4 wet


Either side of chest GSR/EDA 50 kΩ to 10 MΩ


DC to 400 Hz (Single frequency)


Sinusoidal wave DC to 1 Hz 2 dry


Across palm Across finger Across wrist


ICG 5 Ω to 50 Ω


20 kHz to 200 kHz (75 kHz, typ)


Sinusoidal wave DC to 20 Hz 4 wet Neck to abdomen


Figure 8. Bioimpedance receiving channel.


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