Electrical apparatus includes a probe, having a proximal end and a distal end. The probe includes a sensor, which outputs a sensor signal, and a first connector at the proximal end of the probe, electrically coupled at least to the sensor. A probe adapter includes a second connector, which is arranged to mate with the first connector, and a third connector, for coupling to a console. A shield includes a material of high magnetic permeability, which is configured to enclose an internal volume containing the first and second connectors when the probe is connected to the probe adapter.