Mike,
Okay, let me see if I can explain this. The Quad Capture is a four channel USB audio interface, with two channels of analog in and outputs and two channels of digital ins and out.
The primary purpose for this device would be to connect to your analog devices, i.e. your guitar or microphone using the analog inputs, and using outboard devices to send digital audio to and from the Quad Compare via the digital outputs. Those might be effects devices, or a keyboard with a digital output.

To me, the idea of buying a separate A/D converter to take the analog output of the Casio and convert it to digital and then feed it into the Quad Capture, and then having to use the internal D/A converter to listen to it, is a bit crazy. The 'Sending Coals to Newcastle' sort of thing.

A more affordable way, in my opinion, would be to get a small Behringer Mixer, the UB802 is a good one for this, run your guitar or the output of your stomp/effects/DI box to the input of the mixer, and then run the analog outputs from the Casio to another set of inputs on the mixer, and finally, run the outputs from the mixer to the Input 1 and 2 of the Quad Capture.

However, if you really want a good A/D converter, I happen to have a Lavry Engineering first generation DA10 and AD10, which are the D/A converter and the A/D converter in the LavryBlack line, for sale. If you want to buy them, I'll see to it that they're gone over and meet factory specs before shipping. I'd like $1,000 for the pair, and they will come with the factory rack mount kit as well.
Here's a pair for sale on ebay, just to give you an idea.
http://www.ebay.com/itm/Lavry-AD-10-and-...=item4d0c6d6884

The only reason I'm selling mine is that I don't use them any more, I have the replacement AD11 and DA11 in my rack, a LavryBlue 4496-21, a LavryGold Mk III and a LavryGold DA924. The AD10 and DA10 are absolutely great devices.

Gary

I'm blessed watching God do what He does best. I've had a few rough years, and I'm still not back to where I want to be, but I'm on the way and things are looking far better now than what they were!