HiFiBerry Digi+ Pro is a high-quality S/PDIF output for the Raspberry Pi. It uses the I2S sound port that connects directly to the CPU without the need for an additional USB conversion.
You will need a distribution with an up-to-date kernel (at least summer 2016). The drivers are included in the latest versions of most common distributions.
The board can be equipped with an additional BNC connector. Note that you have to solder this by yourself.