Parasitic Effects on Electrical Bioimpedance Systems: Critical Review

Abstract

Parasitic capacitance represents the main error source in measurement systems based on electrical impedance spectroscopy. The capacitive nature of electrodes' impedance in tetrapolar configuration can give origin to phase errors when electrodes are coupled to parasitic capacitances. Nevertheless, reactive charges in tissue excitation systems are susceptible to instability. Based on such a scenario, mitigating capacitive effects associated with the electrode is a requirement in order to reduce errors in the measurement system. A literature review about the main compensation techniques for parasitic capacitance was carried out. The selected studies were categorized into three groups: (i) compensation in electronic instrumentation; (ii) compensation in measurement processing, and (iii) compensation by negative impedance converters. The three analyzed methods emerged as effective against fixed capacitance. No method seemed capable of mitigating the effects of electrodes' capacitance, that changes in the frequency spectrum. The analysis has revealed the need for a method to compensate varying capacitances, since electrodes' impedance is unknown.

Keywords: critical review; electrical bioimpedance; error compensations; parasitic capacitances.

Figure 1

Simplified impedance measurement system of a homogeneous electrolyte.

Figure 2

Electric equivalent of electrode impedance based on the element of the constant phase $Z_{CPE}$.

Figure 3

Parasitic impedance of the excitation and measurement system, where $V_{out}=G$. $(V_A-V_B)$ and G is the voltage gain of the instrumentation amplifier.

Figure 4

Internal compensation to the instrumentation amplifier. $C_{sp+},C_{sp-}$ are the parasitic capacitances observed in the instrumentation amplifier’s input nodes. $I_{test}$ is the current source used to calibrate the compensation system, $C_{comp}$.

Figure 5

Compensation for NIC use in a tetrapolar EIT. The box highlighted in blue is the negative impedance converter (capacitance). $Z_B$, $Z_o$, $C_{s1}$ and $C_{s2}$, are the body impedance, output impedance (total) of the current sources $I_{+}$ and $I_{-}$, and the parasitic capacitances of the excitation source, respectively. Source: Adapted from [58].