Ground loop isolation of a USB-powered device
Laptops are a real traps for young players when you hook them up to an analog circuit.

A slightly simplified schematic of a laptop supply shows the totally counter-intuitive capacitors that are straight across the transformer. C1,C2 form a voltage divider that floats the laptop at 115V (when you are in a grounded-neutral mains system). These are the fat blue disc capacitors when you open up a laptop psu. (They are there so that RF energy can flow back out to the low impedance mains to get rid of it)







simulate this circuit &ndash; Schematic created using CircuitLab

If you connect the laptops ports to anything grounded, 75 microamps will flow through the ground/microphone/usb/data wire. There will be 115V available to force it to flow, if the connection happens to be a high impedance and poorly protected IC input.(Goodnight Nurse), or you get incurable hum in audio circuits (ground-loop)



You should read up about ground loops, it is a major (THE major?) issue building recording studios.


Star Grounding: You need a single defined ground point. All grounds radiate outwards from that and never loop back. You need to join the USB shields at the USB connectors with a wide strap (braid,foil). If you can Join the signal/power grounds similarly. Ideally you will choose a point in the ADC and DAC modules where the digital meats the analog part, and join them from those points, but at the usb connectors is a good start. 


If you can't get into the modules you can cut up two usb cables and join the shields and grounds near the device ends, but leave the data wires separate. Join this point to physical ground.


Differential analog is how professional audio deals with this. Once you can detect microvolt levels there is no other way.