Embodiments of a wearable cardioverter defibrillator (WCD) system are configured to monitor a patient's ECG for shockable arrhythmias and deliver a shock to the patient in response to such a detection. To monitor the patient's ECG with reduced signal noise to improve the system's performance, the system includes a cable assembly having: a signal line; an inner shield and an outer shield; an ECG electrode electrically connected to the signal line of the cable assembly; and an amplifier having first and second input nodes respectively connected to the signal line and the outer shield of the cable assembly. The amplifier's output node is electrically connected to the inner shield of the cable assembly to reduce the reactive load seen by the patient's heart in driving the ECG sensing circuitry, which reduces the noise on the ECG signal outputted by the amplifier.