Say a DAC that accepts 110ohm digital input (i.e. from transport) - what is the typical input impedance? Is it 110ohm or something very high? Thanks.
110 ohm in the frequency band of interest. It could very well be very low or very high at DC, when it has an input transformer or DC blocking capacitors.
It will be 110 ohms differential. Its a high speed interface so the load has to match the source to prevent reflections (this is difference between high speed / RF signals and low speed like audio (where the signal wavelength is much greater than the cable length). I suspect these digital signals have bandwidth down to DC so it will be a flat 110 ohms across all frequencies - however some digital encodings are DC-free and thus can be transformer-coupled as mentioned.
110 ohms is the characteristic impedance of many twisted-pair cables.
110 ohms is the characteristic impedance of many twisted-pair cables.
S/PDIF and AES3 (AES/EBU) signals are often AC coupled, either with capacitors or a transformer or both.