usb 6501: 200 Hz noise

Hi

I use the NI USB 6501 device to send trigger signals to a EEG recording system (Nicolet C64 amplifier). As soon as I connect the NI-device to the EEG system (analog Input channel 1), I get very strong noise in the EEG signal at precisely 200 Hz.

Any ideas on what the source of this 200 Hz noise might be?