Bioimpedance Circuit Design Challenges for Body-Worn Systems

Introduction

Wearable devices for vital-sign monitoring (VSM) are transforming the healthcare industry, allowing us to monitor our vital signs and activity anytime, anywhere. The most relevant information about some of these key parameters can be obtained by measuring body impedance.

To be effective, wearable devices must be small, low cost, and low power. In addition, measuring bioimpedance entails challenges related to the use of dry electrodes and safety requirements. This article provides some solutions to these issues.

Electrode Half-Cell Potential 

The electrode is an electrical transducer that makes contact between an electronic circuit and a nonmetallic object such as human skin. This interaction produces a voltage, known as the half-cell potential, which reduces the dynamic range of the ADC. The half-cell potential varies with the electrode material, as shown in Table 1.

Table 1. Half-Cell Potentials for Common Materials

Metal and Reaction
Half-Cell Potential (V)
Al→Al3++3e
–1.706
Ni→Ni2++2e
–0.230
Ag+Cl+→AgCl+e
+0.223
Ag→Ag++e
+0.799
Au→Au++e
+1.680

Electrode Polarization

When no current flows through the electrode, the half- cell potential is observed. The measured voltage increases when dc current flows. This overvoltage impedes current flow, polarizing the electrode, and diminishing its performance, especially under conditions of motion. For most biomedical measurements, nonpolarizable (wet) electrodes are preferred over polarizable (dry) electrodes,  but portable and consumer devices typically use dry  electrodes due to their low cost and reusability.

Electrode-Skin Impedance

Figure 1 shows an equivalent circuit of the electrode. Rd and Cd represent the impedance associated with the electrode-skin interface and polarization at this interface, Rs is the series resistance associated with the type of electrode materials, and Ehc is the half-cell potential.

Figure 1a
Figure 1. Equivalent circuit model for biopotential electrode.

The electrode-skin impedance is important when designing the analog front end due to the high impedances involved. Dominated by the series combination of Rs and Rd at low frequencies, the impedance decreases to Rd at high frequencies due to the capacitor’s effect. Table 2 shows typical values for Rd, Cd, and the impedance at 1 kHz.

Table 2. Typical Electrode-Skin Impedance

Material
Rd
Cd
|Rd//Cd| @ 1 kHz
Wet Ag/AgCl
350 kΩ
25 nF
6 kΩ
Metal Plate
1.3 MΩ
12 nF
13 kΩ
Thin Film
550 MΩ
220 pF
724 kΩ
MEMS
650 kΩ
Negligible
650 kΩ

IEC 60601

IEC 60601 is a series of technical standards for the safety and effectiveness of medical electrical equipment published by the International Electrotechnical Commission. It specifies 10 µA maximum dc-leakage current through the body under normal conditions and 50 µA maximum under worst-case, single-fault conditions. The maximum ac-leakage current depends on the excitation frequency. If the frequency (ƒE) is less than or equal to 1 kHz, the maximum allowed current is 10 µA rms. If the frequency is greater than 1 kHz, the maximum allowed current is

Equation 1

These patient current limits are important circuit design parameters.

Circuit Design Solution

The impedance measurement requires a voltage/current source and a current/voltage meter, so DACs and ADCs are commonly used. A precision voltage reference and voltage/current control loops are essential, and a microcontroller is typically required to process data and obtain the real and imaginary parts of the impedance. Additionally, wearable devices are typically powered by a unipolar battery. Finally, integration of as many components as possible in a single package is very beneficial. The ADuCM350 ultralow-power, integrated, mixed-signal meter-on-a-chip includes a Cortex-M3 processor and a hardware accelerator that can perform a single-frequency discrete Fourier transform (DFT), making it a powerful solution for wearable devices.

To meet IEC 60601 standards, the ADuCM350 is used with the AD8226 instrumentation amplifier to make high-precision measurements using a 4-wire technique, as shown in Figure 2. Capacitors CSIO1 and CISO2 block dc current flow between the electrode and the user, eliminating the polarization effect. An ac signal generated by the ADuCM350 is propagated into the body.

Capacitors CSIO3 and CSIO3 block the dc level from the ADC, solving the half-cell potential problem and maintaining maximum dynamic range at all times. CSIO1, CSIO2, CSIO3, and CSIO4 isolate the user, ensuring zero dc current in normal mode and in the first case of failure, and zero ac current in the first case of failure. Finally, resistor RLIMIT is designed to guarantee that the ac current in normal operation is below the limit. RACCESS symbolizes the skin-electrode contact.

The ADuCM350 measures the current from the transimpedance amplifier (TIA) and the output voltage of the AD8226 to calculate the unknown bodyimpedance. RCM1 and RCM2must be as high as possible to ensure that most  of the current flows through the unknown impedance  and the TIA. The recommended value is 10 MΩ.

Figure 2a
Figure 2. Four-wire isolated measurement circuit using ADuCM350 and AD8226.

Design Limitations

This design presents some limitations when the eletrode-skin impedance is close to 10 MΩ at the excitation frequency. The electrode-skin impedance must be significantly smaller than RCM1 and RCM2 (10 MΩ), or VINAMP+ will not be equal to A and VINAMP– will not be equal to B, and the measurement accuracy will be degraded. The electrode-skin impedance is typically much smaller than 1 MΩ when  the excitation frequency is greater than 1 kHz, as shown  in Table 2.

Validation

To prove the accuracy of this design, the system was tested with different unknown impedances, with the results compared to those measured by an Agilent 4294A impedance analyzer. The magnitude error was less than ±1% in all the tests. The absolute phase error was less  than 1° at 500 Hz and 5 kHz. The 9° phase offset error at 50 kHz could be corrected in software.

Conclusion

Designs for battery-powered, body-worn devices that measure bioimpedance must consider low power, high SNR, electrode polarization, and IEC 60601 safety requirements. A solution using the ADuCM350 and AD8226 was described here. Additional details, including complete design equations, can be found at www.analog.com/media/en/analog-dialogue/volume-48/number-4/articles/bio_imp.pdf.

References

Neuman, Michael R. “Biopotential Electrodes.” The Biomedical Engineering Handbook, Fourth Edition. CRC Press, 2015

Chi, Mike Yu, Tzyy-Ping Jung, and Gert Cauwenberghs.  “Dry-Contact and Noncontact Biopotential Electrodes: Methodological Review.” IEEE Reviews in Biomedical Engineering, Volume 3, 2010.

http://en.wikipedia.org/wiki/IEC_60601

Author

jose-conchell

Jose Carlos Conchell

José Carlos Conchell received his degree in electronic engineering from Universidad de Valencia, Spain, in 2010, then joined ADI as a system applications engineer in the Healthcare Group.