Examples presented herein generally include systems and methods which can contact electrodes to cardiac tissue while sensing electrical potential from fluids near the tissue. The electrodes in contact with the tissue can be carried by an end effector and can be used to sense and/or ablate the tissue they contact. A reference electrode can be insulated by a shaft connected to the end effector so that the reference electrode can sense the electrical potentials from the fluids near the tissue while being prevented, by the shaft geometry, from contacting the cardiac tissue. In some examples, the reference electrode can be tubular, having an inner surface that is configured to contact and sense electrical potentials from the fluids and an electrically insulated outer surface. Configured as such, the reference electrode can serve as an extension of an irrigation tube extending through the shaft.