A medical apparatus includes a shaft, an expandable frame,a membrane, a diagnostic electrode, a reference electrode, and aprocessor. The shaft is configured for insertion into an organ ofa patient. The expandable frame is coupled to a distal end of theshaft. The diagnostic electrode, which is disposed on an externalsurface of the expandable frame, is configured to sense diagnosticsignals when in contact with tissue. The reference electrode isdisposed on a surface of the expandable frame directly oppositethe diagnostic electrode, wherein the reference electrode iselectrically insulated from the tissue and is configured to senseinterfering signals.
