The bioimpedance system uses ADI’s highly integrated analog front-end (AFE) chip to measure the signal as it offers several advantages.
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Wide Configurable Current Source
The transmit channel integrates a sinusoidal current source, capable of delivering an AC excitation current across a wide frequency range from 16 Hz to 500 kHz, with a current amplitude range from 16 nA rms to 1.28 mA rms. Both frequency and current are fully configurable. -
In addition, its flexible input/output multiplexer (mux) supports either bipolar or tetrapolar electrode measurement structures. For the receive channel, it features
- High input impedance
- Low noise
- High CMRR
- Programmable gain
- Configurable low-pass/high-pass filters
- Dual 20-bit ADCs
- Synchronous IQ signal acquisition
- Built-in Calibration Options
In applications requiring high absolute impedance accuracy (such as bioimpedance analysis/spectroscopy (BIA/BIS) or body impedance measurement in automated external defibrillators (AEDs)), the MAX30009 offers multiple calibration options. By connecting an external precision resistor to the dedicated four-wire calibration port, the highest accuracy can be achieved.
Alternatively, its internally trimmed precision resistors also provide excellent calibration performance
- As shown in Figure 5, the system is a complete bioimpedance measurement solution.
The system employs the highly integrated and programmable power management IC, the MAX20356, to manage the power rails and communicate with the bio-impedance AFE MAX30009 via a low power Bluetooth®-enabled microcontroller unit (MCU).
The transmit channel of the AFE uses an internal direct digital synthesizer (DDS) and digital-to-analog converter (DAC) to generate a sine wave sweeping voltage with adjustable frequency.
This voltage is converted into a current stimulus through a bias resistor and applied to the human body. The response signal is obtained by the AFE receive channel through measurement at the input pins.
In ICG measurements, the commonly used excitation signal frequency range is 20 kHz to 200 kHz, with an excitation current rms value of about 0.12 mA, corresponding to a human body AC impedance range of 0.1 Ω to 0.4 Ω.
In this work, an excitation signal of 65.536 kHz and 256 μA is applied to the human body for impedance measurement.
This reduces system complexity, PCB size, and power consumption — making it ideal for wearable cardiovascular monitoring devices.
Figure 5. Block diagram of a bioimpedance measurement system with the MAX32666, MAX20356, and MAX30009.
Applicable Part Numbers
| DigiKey Part Number | Manufacturer Part Number |
|---|---|
| 175-MAX30009ENA+TTR-ND,175-MAX30009ENA+TCT-ND,175-MAX30009ENA+TDKR-ND | MAX30009ENA+T |
| 505-MAX30009ENA±ND | MAX30009ENA+ |
| 175-MAX20356AEWY+TR-ND | MAX20356AEWY+ |
| 505-MAX20356AEWY+TTR-ND | MAX20356AEWY+T |