A medical apparatus includes a shaft, an expandable frame, a membrane, a diagnostic electrode, a reference electrode, and a processor. The shaft is configured for insertion into an organ of a patient. The expandable frame is coupled to a distal end of the shaft. The diagnostic electrode, which is disposed on an external surface of the expandable frame, is configured to sense diagnostic signals when in contact with tissue. The reference electrode is disposed on a surface of the expandable frame directly opposite the diagnostic electrode, wherein the reference electrode is electrically insulated from the tissue and is configured to sense interfering signals.