plugging in digital I/O HDMI cable gives increased noise in headstage inputs

[mvdm]

On our v2.1 acquisition board with RHD2132 headstages, plugging in a HDMI cable into a digital I/O input port on the board causes an increase in the noise level in the data coming from the headstage (eyeballing it, mostly 60Hz line noise):

HDMI cable unplugged

HDMI cable plugged in

This is with the HDMI cable ground connected to the ground on the acquisition board. (Without this the noise really becomes terrible.) The sine wave is a 77uV 16Hz test signal from our signal generator.

Is this a known issue? Any recommended fixes?

[jvoigts]

This could be a lot of things. It looks like you're picking up more noise on the ground whenever the HDMI is connected - what is the I/O board connected to? Maybe it would help to ground the aq. board to your table / the signal tester board directly, or make sure that whatever is connected to the I/O board is properly grounded. Also check if your aq. board is properly grounded to the table or room.

[mvdm]

Thanks for the suggestions! I tried many different ground connection permutations without any improvement. I figured if the HDMI cable is not connected to anything except the acquisition board on one end, the only way it could (should) mess with the neural signals is through HDMI ground. So connecting the HDMI ground to system ground should eliminate the issue, and indeed doing that is a big improvement over not grounding the HDMI cable. But so far no luck in reducing the noise back to no-HDMI levels (connecting the (battery-powered) signal generator to system ground, connecting system ground to room (power outlet) ground, and various permutations). Trying a different room and acquisition computer ground next...