An apparatus includes a current source, an electronic circuit and a circuit board. The current source is configured to flow an electrical current having a selected frequency between a pair of electrodes coupled to a medical probe. The electronic circuit is configured to measure a single-ended voltage relative to ground that is formed on at least one of the electrodes in the pair in response to the electrical current, and, based on the measured voltage, to assess physical contact between the at least one of the electrodes and tissue. The circuit board includes the current source and the electronic circuit, and includes a layout that produces, at the selected frequency, a predefined capacitance between the current source and ground, thus forming a reference for measurement of the single-ended voltage.
