## 4 Discussion
### Elimination of Electrode Lead Jitter Interference  
By conducting an additional experiment that involved the recording of EMG signals in an electromagnetic shielded room designated as Experiment 4, the effect of electrode lead jitter was studied as shown in Figure 10. By examining the results in Figure 10, one can see that all the shielded drive methods could significantly eliminate the interference in the EMG recordings caused by electrode lead jitter. Of the three shielded drive methods, the Active-Shield (Method 4) had the best performance, which almost completely eliminated the electrode lead jitter interference. It is conceivable that the Active-Shield method could be applied not only to EMG signals but also to the other biopotential signals.  
For shielded technologies, the electrode cable consists of a small coax cable, where the outer layer is shielding the inner signal wire. Figure 13 shows the transmission line models of the electrode cable of the two different shielded methods, in which the outer shield is connected to different shielding signals (only one of the two input electrodes of each channel is plotted for simplicity).  
As seen from Figure 13, the outer shield is connected to the isolated ground of the power supply for the GND-Shield method. In this way, the inner signal wire is protected against the capacitive coupling from the mains power, since the potential between the cables becomes zero and there is no current from one cable to the other through the electrodes, leading to no differential interference input. However, there is a considerable capacitance (\(\mathrm{C_{L}}\)) and resistance (\(\mathrm{R_{L}}\)) between the inner signal wire and the outer shield. Consequently, when there were some movements of the electrode cables, a large amount of movement artifacts will be seen in the output, caused by variation in the impedance (\(\mathrm{C_{L}}\) and \(\mathrm{R_{L}}\)) of the cable. The situation was similar for the case of Bias-Shield method. In contrast, for the Active-Shield method, the signal of the inner wire is put on the outer shield using a low impedance output of an amplifier, so the shield has almost the same potential as the inner signal wire in this case. Therefore, the change in \(\mathrm{C_{L}}\) and \(\mathrm{R_{L}}\) caused by cable movements will no longer have any influence on the potential of the inner wire and the system output. The model in Figure 13 provides the explanation for why the proposed Active-Shield circuit is effective in eliminating the electrode lead jitter interference (Figure 10).  
The developed shielded drive circuit in the current study exhibits a simple structure and low cost, and is effective in suppressing power line interference as well as eliminating electrode lead jitter interference which simplifies the subsequent signal processing and analysis tasks. It could prevent the signal from extraneous noises without any requirement of hardware or software filtering. These  
Figure 13: Transmission line model of the electrode cable for GND-Shield and Active-Shield methods.  
characteristics of the proposed method are significantly important in developing biopotential signal acquisition and processing methods needed in wearable devices and other types of portable devices. It is noteworthy that the proposed shielded drive circuit built in the current study is independent of the acquisition system and thus we could integrate the shielded drive circuit into different acquisition devices to improve the signal quality in our future work.