Export master stereo track using external A/d converter?

I am using Cubase 8.5 Pro and i installed my Focusrite Isa220 A/D stereo converter as External FX in Cubase and i setup it on my soundcard like a Master Clock!
When i want to export the final audio i use tha Isa220 A/D like insert in the Master Track in console of Cubase. Is this the right method to real time export my final master through the ISA A/D converters?
or i have to open stereo channel in Cubase and to manual record the stereo spdif digital of my ISA220 A/D converter?