## 4 Discussion
### Reduction of Power Line Interference  
With respect to the attenuation of power line interference, three key biopotential signals (ECG, EOG, and EMG) were considered and recorded under the same laboratory settings which have been designated as Experiment 1, Experiment 2 and Experiment 3 in the current study. The analyses of the biopotential signals obtained have been shown in Figures 6-8. The waveforms as well as spectrum characteristics shown in the Figures indicate that the proposed shielded drive methods could effectively suppress power line interference. Also, the effect of Method 4 (Active-Shield) is considered to be the best since it suppressed almost all of the power line interference of 50 Hz as well as its harmonics in the ECG, EOG, and EMG to 32.0 dB, 45.8 dB, and 36.9 dB respectively, as shown in Table 1. Table 1 shows the average dB values across subjects for the three different shielded methods (GND-Shield, Bias-Shield, and Active-Shield) against No-Shield method in suppressing the 50 Hz power line frequency in Experiment 1, Experiment 2, and Experiment 3. It is noteworthy that the greater the number, the better the performance of the methods in suppressing the 50 Hz power line interference. Although the designed notch filter was able to eliminate the 50 Hz frequency inherent in the raw signal's (i.e., No-Shield signal) spectrum, useful components of the target signal near 50 Hz were destroyed, and also the associated harmonic components were still present in the target signal. [PERSON] et al. designed an automatic interference control device (AICD) for the elimination of power line interference, and the AICD was able to suppress the 60 Hz frequency in biopotential signal recordings to 30 dB, yet their method could not eliminate the associated harmonic components [4]. However, by utilizing the proposed shielded drive method, the 50 Hz frequency and its harmonic components were significantly reduced, thus leading to improved quality of the target signals. Figure 11A showed an intensive model of the No-Shield method for measuring ECG signals, similar to the interference model in [PERSON]'s study [19]. Because of the capacitive coupling to the main power (C\({}_{\text{pow}}\)) and the capacitive coupling to the safety earth ground (C\({}_{\text{body}}\)), the power line interference is present at the patient's body as a common-mode signal and it might not be the same as the isolated ground of the power supply, causing a leakage current (I\({}_{\text{diff}}\)) between the patient and the power-supply ground. When the leakage current flowed through the contact impedance of \(Z_{\text{pgn}}\) \begin{table}
\begin{tabular}{l c c c} \hline \hline  & **M1–M2 (dB)** & **M1–M3 (dB)** & **M1–M4 (dB)** \\ \hline ECG Experiment & 19.3 & 23.5 & 32.0 \\ EOG Experiment & 12.6 & 17.3 & 45.8 \\ EMG Experiment & 15.5 & 18.0 & 36.9 \\ \hline \hline \end{tabular} Note: M1, M2, M3 and M4 represent the spectrum values at the 50 Hz power line interference for the signal recordings of Method 1, Method 2, Method 3 and Method 4, respectively.  
\end{table} 
Table 1: Performance of three shielded methods (Method 2, Method 3, and Method 4) relative to the unshielded method (Method 1) in suppressing the 50 Hz power line interference for different experiments, the ECG, EOG and EMG Experiments represent recording sessions of electrocardiogram, electro-oculogram, and electromyography signals, respectively. it would introduce a 50 Hz signal visible on both inputs of the amplifier. However, this is not the only source of the 50 Hz disturbance. In case the cables all have a different capacitive coupling to the mains and to ground, the voltage difference between the two cables of the positive and negative inputs will not be zero, causing a current (I\({}_{\text{dcable}}\)) from one cable to the other through the electrodes.