The input channels on the Saleae devices are not isolated, and share a common ground. The devices are also not isolated from the USB connection to the PC.
The new devices have a ground lead for each signal. When signal integrity is a concern -- such as when working with high speed digital or analog in the presence of cross-talk sources, connect a ground for each signal. Connect noisy digital inputs to leads as far as possible from leads connected to sensitive analog measurements.
At least 1 ground connection is required in order for the device to capture signals from the device under test, and additional connections are optional.
trello ID: 560ec68e67e13443ab8ee81a