Movement artifacts can be diminished by shielding the electrode cables. Shielding means that the electrode cable consists of a small coax cable, where the outer layer is “shielding” the inner signal wire. Two types of shielding are available:
- Patient ground shield
- True Active Signal Shield (TAS2)
When the patient ground shield is used, the shield of the coax electrode cable is connected to the isolated zero of the power supply, the patient ground. The signal wire is protected against the influence of the mains power, but there is a considerable capacitance between the inner signal wire and the outer shield. So when performing an ambulatory measurement, or any other measurement with moving cables, a large amount of movement artifacts will be seen, caused by variation in the capacitance of the cable.
In the case of true active signal shield, the signal of the inner wire is put on the shield using the low impedance output of an amplifier. The shield has exactly the same potential as the signal wire in that case. There is no influence from the outside and there is no capacitance of the cable. Actually, it appears as if the cable is not there. The active shield technology can be used for bipolar, common reference and average reference measurements.