a method for position sensing includes inserting a probe having a first probe - electrode and a second probe - electrode into a body of a subject , and coupling body - surface electrodes to a surface of the body . currents passing between the first probe - electrode and the body - surface electrodes are measured , using first circuitry coupled to at least the first probe - electrode and having a first electrical ground , and position coordinates of the probe are determined responsively to the measured currents . second circuitry , having a second electrical ground , is coupled to at least the second probe - electrode , and the first electrical ground is isolated from the second electrical ground .