I'm designing a device that will receive and digitize analog video signals. In analog video, it's recommended to capacitively couple inputs. A typical input might look like this:
In this case the signal is on connector pin 2, then there is a termination resistor, and a capacitor connects the signal to the rest of the receiver. DC current returns via connector pin 1 and the video cable.
Such a configuration is recommended by this Maxim app note (page 2) as well as various other places. But none of them seem to discuss grounding. It seems as though this app note (and others I have found) are trying to isolate the two devices, while also assuming they have a common reference!
The problem is that there's no obvious reference or return path back to the source for the signal that passes through the capacitor. If the receiver (or the source, or both) were battery-powered and not connected to anything else, the whole electrical potential could float and follow the signal waveform and the ADC would see nothing.
It would be possible to connect the signal return (pin 1 of the connector) to the receiver's circuit ground, but this could create a ground loop, would defeat isolation among the connected devices, and would expose the receiver's ground to EMI/ESD coming through the connector. It seems better to keep the cable isolated from the receiver's circuit ground.
So I think it might be necessary to include capacitor C2, as shown here:
Otherwise the AC signal passing through C1 might have to go all the way through the power mains, or some other undesired route, to return to the source. So my question is whether capacitor C2 should be included in the circuit (and if so, what value it should have; I guess it should be large relative to C1), or if it is not necessary for some reason that I have missed, or if something else should be done instead.