Typically, a soundcard *is* simply one or more converters. However, you can buy digital cards from companies like RME that only do digital transfers...like these http://www.rme-audio.com/english/digi96/index.htm
You'll usually see cards like this advertised as digital I/O cards, not soundcards.
If the soundcard is going to playback or record any analog signals, it will of course have to have converters.