Be a bit clearer man......1680, do you mean Roland 1680 or what?
Also, you need to decide what you need the mixer for. If it's simple inputs into the PT system, then there are ways around this. But if you need multiple ins and outs, auxes for efx and inserts for dynamics, then an analogue mixer is a great way to work. A digital mixer will also help, but you need to be even clearer about how to operate this with the system you have......you don't want 2 stages of conversion, plus a lot of the dig mixer's functions will be repeated in the PT system, so make sure you know what you need from each.
With PT setups, many people use a control surface and a straight forward outs to ins rack, ADAT, Lightpipe etc.. systems, maybe a good 8in card and a control surface, that type of ****.