Key result
A combined voltage and charge switchable amplifier was superior to individual amplifiers by adapting to signal quality and artifacts, and estimating body-to-electrode capacitance without extra hardware.
Why the study?
Due to small body-to-electrode capacitance, front-end amplifier design for capacitive ECG remains challenging, and comparing voltage and charge amplifier responses to artifacts and integrating them was needed.
A novel VA-CA switchable amplifier improves capacitive ECG measurements by adapting to signal quality and artifacts while estimating body-to-electrode capacitance.
May improve capacitive ECG signal quality without extra hardware; leaves open clinical validation before practice adoption.
Objective:Sensing with capacitive electrodes is of interest for long-term, comfortable bio-potential measurements (e.g., ECG). However, due to the small body-to-electrode capacitance (Ce), the design of the associated front-end amplifier remains a challenge. Both voltage amplifiers (VA) and charge amplifiers (CA) can be employed. While basic comparisons of both typologies were done before, this paper extends the comparison to their responses to artifacts (caused by motion or interference). Further, a VA-CA-switchable amplifier is proposed, allowing to adapt the amplifier type to different situations, and enabling to estimate the body-to-electrode capacitance Cein a passive way.Methods:A VA-CA switchable amplifier was implemented in a 180 nm CMOS process. The responses to artifacts for VA and CA were studied by modelling, simulations and experiments using the custom IC. The proposed Ceestimation method was validated by electrical tests and in-vivo tests.Results:VAs are less affected by Cevariation artifacts, while CAs recover faster from triboelectricity artifacts. In a VA, these two artifacts are multiplicative and get modulated if they occur simultaneously, but in a CA they remain independent.Conclusion:The combined VA-CA amplifier has the potential for optimal amplifier selection according to the properties of the recorded signal, the value of Ceand the actual presence of artifacts. Moreover, it can estimate Cewithout extra hardware.Significance:The proposed VA-CA switchable structure is superior to an individual VA or CA, thanks its adaptability to signal quality and artifacts, and it provides extra information on the body-to-electrode interface quality (Ce).
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Zhang et al. (2022) studied Capacitive ECG measurements. VA-CA switchable amplifier vs. Individual voltage amplifier (VA) or charge amplifier (CA) was evaluated on Responses to artifacts and estimation of body-to-electrode capacitance (Ce). A combined voltage and charge switchable amplifier was superior to individual amplifiers by adapting to signal quality and artifacts, and estimating body-to-electrode capacitance without extra hardware.
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