Test & measurement


significance of bioimpedance parameters is summarised in Table 1.


CURRENT BIOIMPEDANCE MEASUREMENT SOLUTION AND ITS PAIN POINTS


In light of the importance of bioimpedance, it is necessary to measure all bioimpedance parameters accurately. According to Ohm’s law, the measurement of an impedance usually uses voltammetry. Therefore, we can use a bioimpedance measurement integrated circuit (IC) to send a current signal to the human body as the stimulus and measure the response voltage at the same time, as shown in Figure 3.


TABLE 1. PHYSIOLOGICAL SIGNIFICANCE OF BIOIMPEDANCE PARAMETERS BIOIMPEDANCE PARAMETER ECF equivalent resistance RE


Microcosmic Parameters Z = (RI + CM)||RE


ICF equivalent resistance RI


Cell membrane equivalent capacitance CM


Resistor R


Macrocosmic Parameters Z = R + jX = |Z|∠φ


Absolute value of reactance X


Modulus |Z|


Absolute value of phase angle φ


Figure 3. Voltammetry bioimpedance measurement.


Traditional bioimpedance measurement often uses the single-frequency measurement method. It only uses a sinusoidal signal of one fixed frequency as the stimulus. This method is simple to operate, but cannot obtain the details of bioimpedance that changes with frequency. As previously shown, bioimpedance involves a complex number that changes with frequency. Therefore, in order to accurately measure bioimpedance in the whole frequency domain, the frequency of the stimulus signal must cover the range from direct current (DC) to a relatively high frequency, rather than a fixed one. In the face of this pain point, most current measurement solutions use a periodic square wave pulse with one fixed frequency as the stimulus. A typical solution is shown in Figure 4. The system power supply comes from the universal serial bus (USB) interface or battery,


Figure 4. Current bioimpedance measurement system. 30


and outputs stable power by a low dropout (LDO) regulator. The microcontroller unit (MCU) sends square wave pulses to the body through skin electrodes and measures the response with a built-in analogue-to-digital converter (ADC). The measurement results can be transmitted and displayed on mobile phones, computers, and other terminals by the Bluetooth module. The first advantage of this solution is that the stimulus signal is easy to generate in a simple system configuration. It can be easily done based on the MCU. The second advantage is that, in the frequency domain, the square wave signal is actually a superposition of sinusoidal signals of many frequencies. Therefore, only using a square wave stimulus of one frequency can achieve the effect of multifrequency sinusoidal wave measurement, as shown in Figure 5. Despite many advantages, this solution still has many disadvantages. First, according to Figure 5, compared with the amplitude of the fundamental wave and the second harmonic wave, the higher harmonics of the square wave decreases very quickly. This means that high frequency signals will be interfered with by more noise, making it difficult for the ADC to extract effective response signals. Second, the actual stimulus frequencies of the square wave measurement are only the integer


PHYSIOLOGICAL SIGNIFICANCE


High: dehydration


Low: edema/ascites/ organ failure


High: high body fat Low: low body fat


High: good cell function Low: poor cell function


High: low overall water content


Low: high overall water content


High: high overall tissue density


Low: low overall tissue density


High: dehydration/tissue necro- sis/tissue damage


Low: high water content/ tumor/inflammation


High: good nutritional status Low: malnutrition


Figure 5. The time/frequency-domain waveform of a periodic square wave.


multiples of its fundamental frequency. If it is required to study a more universal case where the stimulus frequency is not the integer multiple, it may be necessary to adjust the stimulus frequency, which may result in re- development of the firmware. Third, limited to


May 2024 Instrumentation Monthly


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