Bioimpedance circuit design challenges in wearable systems

Wearable vital signs monitor (VSM) devices is changing the health care industry, so that we can at any time vital signs and monitor their activities. Information that is most relevant to some of these important parameters can be obtained by measuring body impedance.

In order to operate effectively, wearable devices must be small in size, low in cost, and low in power consumption. In addition, measuring bioimpedance faces challenges associated with the use of dry electrodes and safety requirements. This article proposes some solutions to these problems.

Electrode half cell potential

An electrode is an electrical sensor that establishes contact between an electronic circuit and a non-metallic object such as human skin. This interaction produces a voltage called the half-cell potential that reduces the dynamic range of the ADC. The half cell potential varies depending on the electrode material, as shown in Table 1.

Electrode polarization

When no current is passed through the electrode, a half cell potential can be observed. When there is a direct current, the measured voltage will rise. This overvoltage condition prevents current flow, polarizes the electrode, and reduces its performance, especially during motion. For most biomedical measurements, non-polar (wet) electrodes are better than polarized (dry) electrodes, but both dry and consumer devices typically use dry electrodes because dry electrodes are low cost and reusable.

Electrode skin impedance

Figure 1 shows the equivalent circuit of the electrode. Rd and Cd represent the impedance associated with electrode-to-skin contact and polarization at the contact, Rs is the series impedance associated with the electrode material type, and Ehc is the half-cell potential.

Figure 1. Equivalent circuit model of a biopotential electrode

When designing the analog front end, electrode-to-skin impedance is very important due to the high impedance involved. Under low frequency conditions, the impedance is mainly determined by the series combination of Rs and Rd, and at high frequencies, the impedance drops to Rd due to the influence of capacitance. Table 2 shows the typical values ​​of Rd and Cd and the impedance at 1 kHz.

IEC 60601

IEC 60601 is a series of technical standards issued by the International Electrotechnical Commission for the safety and effectiveness of medical electrical equipment. The standard states that the maximum DC leakage current through the human body is 10 μA under normal conditions and 50 μA in the worst single fault condition. The maximum AC leakage current depends on the excitation frequency. If the frequency (fE) is less than or equal to 1 kHz, the maximum allowable current is 10 μA rms. If the frequency is greater than 1 kHz, the maximum allowable current is

These are critical circuit design parameters for patient current limits.

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