I am designing a hand-held device whose power and signals come from a cable. The device circuit will be +5 volt and powered through the cable. The cable (a re-purposed HDMI cable) has multiple twisted shielded pairs available. The device will be on a 2-sided pcb. I plan to create ground plane on the solder side of the pcb and connect that to the cable shield, which in turn will be connected to earth ground at the opposite end. The digital circuit commons WILL NOT be tied to this ground plane. The idea of the ground plane is simply to provide some EMI noise immunity. Is electrically connecting a ground plane to the shield a good idea or not? Thanks in advance!
This may be reasonable or not, depending on the environment.
First, you should have reasons to suspect EMI may be a problem, otherwise I wouldn't bother.
Second, you should understand where the source of EMI noise is. If the source is near the device (or the device itself), grounding a back plane via the cable sounds like a good thing to do, provided the device is hand-held and cannot have its own proper grounding.
On the other hand, if the device is operated in a relatively calm (EMI-vise) environment, whereas the cable goes e.g. through a plant with heavy machinery, your ground connection will do more harm than good. The cable shield will effectively act as antenna, picking up all the noise from the plant and delivering it to your device, which would be much better off without it. In this case, you should still ground the cable shield, but not connect it to the device.