What is this "drive circuit" for CCD sensor referenced to?
It means that Vos is referenced to the ground pin which usually doesn't need to be defined. The Vdmk signal is not referenced to ground, it's referenced to the OS signal (which I think is also Vos). This could also mean that both are a sum of some kind. Either way the Vos/OS signal goes from 1.5 to 3.5V