Hi Ralf,
When using laptops and external circuitry there's always a whole lot of noise generated by the conflicting coupling of the laptop's power source and your external source. You might find that using the external power supply but powering the laptop from the battery also helps. I'd suggest that first you make sure that your laptop and circuitry are powered from the same electric line, then you can connect your external digital ground to the AIGround via a 10K resistor, to protect the DAQ board. This will help, but I believe the best way will be to use a low pass filter between your amplifier and your Analog Input, that way you'll be able to get rid of most of the high frequency noise.
In general, if you have an external digital circuit powered by an e
xternal power source and you're using a laptop I recommend you use opto-couplers, powered by the 5V of the DAQ card to get rid of any possibility of noise.