The analog outputs are impedance-balanced, not fully differential.This means that the tip carries the full signal with an impedance of 51 ohms.The ring simply carries an impedance of 51 ohms, no signal. That particular conductor is not left "hanging", but is referenced to signal ground, and should be referenced or grounded on the receiving end. This is in contrast to a fully differential system that would assign half the signal voltage out of phase with one another to the tip and the ring. The impedance would remain 51 ohms on each conductor. The result is that both systems have almost identical noise rejection characteristics.