Hello,
Thank you for contacting National Instruments.
The shielding is tied to your laptop's ground. If your laptop's power supply has a grounding pin (3 prong plug), you can tie the shielding to a common power system ground by plugging your laptop in.
By grounding the SCB-68, you would also be grounding the your laptop.
Both of these methods would tie the measurement system to a common ground.
I hope this helps,
Sean C.
Applications Engineer
National Instruments