In my application I have an issue with ground loops and to solve this I require to have devices isolated from their drivers.

The system operates with a 13.56MHz signal and a wide bandwidth is not of concern.

Capacitive coupling between transformer windings is the greatest concern.

How should I select a transformer 1:1 ratio that will provide the best isolation and allow operation at 13.56MHz?

  • \$\begingroup\$ 13.56 MHz, are you working at High Frequency RFID? \$\endgroup\$
    – Kortuk
    Commented Jun 14, 2010 at 14:03

1 Answer 1


The gap they put between the two coils is the most important part.

The larger the gap, the less efficient your transformer will be due to magnetic fields that do not share between the coils. The larger the gap, the larger the isolation. This also stop arcing from high high voltage differences, it both puts a larger dielectric and increases the distance in the epsilon * A / d equation for capacitance. Make sure your coils have some sort of magnetic core to increase efficiency.

If you are really trying to keep capacitive coupling to a minimum by making sure the two devices have a shield between the coils. A shield grounded will stop them from capacitivly coupling to each other.

Secondly, some differential noise can couple through if you are working about differential noise, then you actually need to put a filter on the feed side, this will be a decent bit of work and probably not something you need to deal with.

Hope I helped, let me know if there is anything else I can help with.

  • \$\begingroup\$ The capacitive coupling is likely to be a killer. We must minimize the ac current that can flow across the transformer so there is no ground loop. We have prototyped a solution with antennas and a 1cm air gap. This works well. Just need to miniaturize this into a PCM component. Selecting the actual component and identifying manufacturers is the problem. We are using unbalanced differential signals so noise isn't a real concern. \$\endgroup\$
    – Callum
    Commented Jun 14, 2010 at 16:33
  • \$\begingroup\$ At 1cm can you really call it an antenna? It is intentionally in the near field. The major thing it seems you need to me is to place a piece of metal between the two devices and ground it well. This will block capacitive coupling, but your inductive coupling of the transformer should work well. \$\endgroup\$
    – Kortuk
    Commented Jun 14, 2010 at 16:40
  • \$\begingroup\$ My lunch break starts now, but I will be back a bit later and will add more and look up a few components if that will help. \$\endgroup\$
    – Kortuk
    Commented Jun 14, 2010 at 16:41
  • \$\begingroup\$ The air gap is between the two antennas. Just two antennas inductively coupled. The devices have their grounds connected together, this is why it is important no current flows across transformer. If there is no current across the transformer there is no current that can flow this common ground between devices causing issues (there is no ground loop return path). \$\endgroup\$
    – Callum
    Commented Jun 14, 2010 at 16:47
  • \$\begingroup\$ you could possibly use an opto-isolator as opposed to isolating with a transformer as well. \$\endgroup\$
    – Mark
    Commented Jun 14, 2010 at 20:32

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