coupling cap gnd reference

[electrolust]

Voltage is differential.

So if I have a single supply op amp design and apply a bias to the input signal, as far as the op amp is concerned it still has +/- supply and the signal reference is at GND.  (modulo variation in bias level due to circuit design)

With a coupling cap, that isn't connected to any reference level (signal in, signal out), how is it that DC voltage is blocked?  How can the cap know what is earth/true GND vs what is signal GND?

[bson]

It sits in a closed circuit, and at w=0 the voltage drop across it is the bias voltage.  Therefore, the output sits at the input minus the bias voltage.  (If you view it purely as a black box.)  It's worth noting also that it settles at whatever voltage results in no current, which depends on the rest of the circuit, it's just in amplifier and filter circuits this tends to be either 0V or some other specific bias point for that circuit.

[electrolust]

It sits in a closed circuit

ahh, of course