If you're already planning on getting a DR680 for your recording rig (or already have), you can use that for doing the A/D+recording, and just transfer your files to your pc. You need to be doing real-time transfers anyway for analog conversions, and adding a step to transfer the recorded files to the pc doesn't take much extra time (and will save you money if you already have recording gear, or plan to get it).
Otherwise, there are probably a lot of options, though I'm not too familiar with all of them. One option is to get a used Sound Devices USBPre version 1 or 1.5 from ebay. You can probably get one for $125-175. I knew some tapers who used them for laptop recording back in the day. They are/were pretty nice from what I gather, and I'd imagine that any Sound Devices product would have a pretty good A/D.
You could also go all out for the newer USBPre 2 which can be used in the field as a standalone A/D or mic preamp+A/D, but they are far more expensive.