A transformer (10) includes a substrate (12) having an input and an output, the input being connectable to one section of a medical electrical device, the output being connectable to another section of the medical electrical device. An input ground system and an output ground system, which is electrically isolated from the input ground system, provide continuous return paths for an input signal and an output signal, respectively. The transformer (10) includes a primary winding (60) electrically connected to the input, a secondary winding (70) electrically connected to the output, and a core (30) transferring data from the primary winding (60) to the secondary winding (70) using magnetic field coupling. A middle portion of the primary winding (60) and a middle portion of the secondary winding (70) wrap around the core (30) and are twisted together. Other portions of the primary winding (60) are twisted with one another, while other portions of the secondary winding (70) are twisted with one another.