Physiological monitoring is provided through a lightweight wearable monitor (12) that includes a flexible extended wear electrode patch (15) and a reusable monitor recorder (14) that removably snaps into a receptacle (25) on the electrode patch (15). The wearable monitor (12) sits centrally along the sternum (13) oriented top-to-bottom. Placement of the wearable monitor (12) in a location at the sternal midline (16), with its unique narrow "hourglass"-like shape, significantly improves the ability of the wearable monitor (12) to cutaneously sense cardiac electrical potential signals, particularly P-wave and QRS interval signals indicating ventricular activity in ECG waveforms. The monitor recorder (12) includes an ECG sensing circuit that measures raw cutaneous electrical signals using a driven reference containing power supply noise and system noise to the reference lead, which is critical to preserving characteristics of low amplitude cardiac action potentials, particularly P-waves (121).
