Test & measurement


the clock frequency, the output frequency and sampling frequency of the MCU are not high enough, which may not be suitable for some application scenarios that require high stimulus frequencies. Fourth, the main function of the MCU is control or calculation. Compared with application-specific ICs, the accuracy of its internal ADC is very limited. These shortcomings make the current measurement solution impossible to accurately, conveniently, and


completely measure R, X, |Z|, φ, and their relations to the stimulus frequency. It brings inconvenience to in-depth assessment of human health status. Last but not least, VSM devices are becoming more and more integrated, and the current solution can only measure bioimpedance but cannot measure other physiological signals simultaneously.


HIGHLIGHTED IC AND ITS BENEFITS However, the solution using a sinusoidal sweep stimulus can avoid the previous four inferiorities well, such as poor high frequency characteristics, inability to coordinate measurements with other sensors, and so on. This solution uses a specific purpose AFE to output sinusoidal stimulus currents whose frequencies cover the range from DC to a high frequency. It has many benefits. First, the stimulus frequency can be flexibly configured and is no longer limited to a specific fundamental frequency or its multiples. Second, the solution using square wave has the shortcoming of reduced stimulus amplitude in the high frequency band. In view of this, the stimulus amplitude in this solution can be flexibly adjusted to improve the measurement accuracy in the high frequency band. In addition, considering that the human body can be seen as a complex impedance, the resistance and reactance are orthogonal on the complex plane. This solution can also conveniently demodulate the response signal into two channels with a phase difference of 90° by quadrature demodulation, to calculate bioimpedance parameters conveniently.


A typical improved solution is shown in Figure 6. The system uses a power management integrated circuit (PMIC) to manage the power rails and MCU with Bluetooth and data security to control the bioimpedance AFE MAX30009 and other biosensors. In addition to the advantages mentioned before, this system has many other benefits. First, the AFE can work with ECG or PPG biosensors to achieve synchronous measurements, thereby realising the function of measuring multiple vital signs in one system. Second, the MCU integrates Bluetooth and security functions, so there is no need to use additional Bluetooth or secure authentication modules. It can ensure the secure transmission of private health information. Third, the PMIC integrates a charger, a fuel gauge, LDO regulators, and DC-to-DC converters, which can integrate the functions of many different kinds of power ICs into one single IC to save the system size.


Here, we use the internal block diagram of the bioimpedance AFE to illustrate the basic steps and


Instrumentation Monthly May 2024 Figure 6. Sinusoidal sweep bioimpedance measurement system.


Figure 7. Bioimpedance transmit channel.


principles of sinusoidal sweep bioimpedance measurement. Figure 7 shows the AFE transmit channel that outputs the stimulus current. The AFE uses an internal direct digital synthesizer (DDS) and digital-to-analogue converter (DAC) to generate a sinusoidal sweep voltage with an adjustable frequency, which is converted into a current stimulus by a bias resistor and then applied to the human body. The response signal is measured by the AFE receiving channel through the receiving pins. The amplitude of the stimulus current can be controlled by four internal bias resistors or one external bias resistor. Internal bias resistors are 552.5 kΩ, 110.5 kΩ, 5.525 kΩ, and 276.25 kΩ, respectively. These four internal bias resistors correspond to four stimulus current amplitudes. The smaller the resistance, the larger the amplitude. In addition to the internal bias resistor, the user can also adopt an external bias resistor to determine the stimulus current amplitude freely. The AFE also supports quadrature


demodulation of the response signal. Quadrature demodulation divides the response voltage v(t) into two channels with a phase difference of 90°,


thereby obtaining |Z| and φ. Figure 8 shows the demodulation process. The receiving channel mainly consists of a bypassable and programmable analogue high-pass filter (HPF), an instrumentation amplifier (INA) with programmable gain, two quadrature demodulators, two antialiasing filters (AAF), two programmable gain amplifiers (PGA) and two ADCs. HPF and INA are used to reduce the noise and improve common-mode rejection ratio


(CMRR). Two quadrature demodulators respectively multiply the received response voltage v(t) with two square waves, which have the same frequency as v(t) but a phase difference of 90°, to generate voltages of two channels. They are vI (t) of in-phase channel (I-channel) and vQ (t) of quadrature channel (Q-channel), respectively. Since the AAF is a two-pole, low-pass filter with a corner frequency that is much smaller than the signal frequency, it can extract the average value of vI (t) or vQ (t) to the next-stage ADC for sampling, as shown in Figure 9. For convenience, we can ignore the gain of filters and amplifiers in the signal chain here, so the output signal of the INA is still v(t). We use VI and VQ to represent the output voltages of the AAF, respectively, and can derive that |Z| and φ are


Therefore, we can calculate |Z| and φ at different frequencies according to equations 8 and


9 by sending sinusoidal sweep stimulus currents to the bioimpedance and then quadrature demodulation. Then we can calculate other


parameters of bioimpedance, namely RE, RI, CM, R, and X, as well as their relations to the angular frequency like the one shown in Figure 2, according to equations 1 to 7. The most significant advantage of this solution is that all parameters can be measured or calculated accurately and completely, and the stimulus frequency and


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