> By "-14.5dB" do they mean a LOSS of 14.5dB
> The recommended input buffer looks very strange to me. It has a gain of about 0.25, so you need a 24Vpp input signal!?!
Yes, yes.
Remember we are coming from many-volt analog systems to under-5V digital comparators. When honking a long telephone line through an urban area, you need real power at significant voltage. But the 1-inch path from buffer-out to the gizmo inside the ADC doesn't need overpowering, and low-volt digitals work more-perfectly on analog inputs than high-volt comparators.
The -12dB/-14dB pad (or loss-buffer) brings our +24dBm/600Ω line-level peaks down to the 0.7VRMS (or so) levels that the actual ADC can swallow.
You want a buffer, not just a passive pad, because the input of the ADC is full of switching-crap and depends on a VERY low impedance drive to keep the analog input clean. If all sources drove 600Ω with grace and aplomb, a 220-150-220Ω U-pad might be OK; since modern practice dictates 10K inputs, we need an active buffer.