USB 6009 OEM - ground problem?

[Michelix]

I got some problems using these boards.

I connected them through the 34 pin connector to a pcb board, so there's no direct usb cable inserted in the NI board, but the usb cable is connected to the PCB.

I use a laptop to acquire signals, this laptop power supply has no ground. If I use this laptop on batteries everything is fine, if i use it with the power supply connected the usb 6009 stops working very often (the led goes off or stay steadly lit). The strange is that if I connect the power line ground to the pcb everything is fine as well.

I tried to connect the NI board directly with a standard usb cable and everything is ok also with power supply.

I think I have some problems with ground and/or shielding.

Anyone can help?

Thank you very much!

[AdnanZ]

Have a look at this:

My Laptop is Reading a Floating Voltage from a Grounded Source in Differential Mode 

Also, be sure to have a look at Table 1 in the following article:

Field Wiring and Noise Considerations for Analog Signals