A medical device includes multiple electrodes and a processor. The multiple electrodes are coupled to a substrate attached externally to a patient and are configured to sense multiple respective electrical signals from an organ of the patient. The processor is configured to estimate two or more noise levels associated with the electrical signals sensed by two or more respective pairs of the electrodes, to select the electrical signals having a smallest noise level from among the two or more noise levels, and to provide only the selected electrical signals for analysis.