Hello,
I will try to explain the issue, hopefully without missing out information

Scenario:
A device (IR sensor) which includes analog and digital circuits is powered by a 24V DC power supply.
The digital and analog grounds are shortened, and wired via 10M + 1u to device's case at a single physical point.
Some time ago all the electrical equipment in the room had to be taken out, due to painting been done.

Issue:
Since the equipment has been reconnected some of the sensors show noises ,from an unclear source, at the
analog channel. These noises appear only when powering up the default device in one room, but not in the next-door room.
Several test done didn't change the result, including using a different power supply, removing all other electrical equipment
at the same power line and removing chassis connection.
The earthing grounding line of the building is being checked every once in a while with no issues found.

At the link below is a zipped folder with two videos of an oscilloscope filming, viewing two points at the device - a dc-motor's feedback digital signal
at the top, and the IR Sensor's measurement point at the bottom. One video includes the noise appearing at the 2nd signal,
and the other without (which is taken in the near-by room).

Mod: Zipped link, removed pending Moderation

I will appreciate any ideas of figuring out what is the source of this noise and how do remove it.

Thanks,
Amitai

 

I'm not sure if posting a link to download video files is a good idea. Most of us would be wary of downloading files from unknown sources. I strongly suggest you post your videos in YouTube, and then link them here using the link video (shown as an icon ) tool.

 

Thanks for the remark,
here's the video with no interference:

and with interference:

Pay attention that the volts/div rate in the oscilloscope is different in both videos.

 

Using the same settings as the second video (081528), can you raise channel's 2 trigger voltage? This to see at what frequency those "traveling waves" are manifesting themselves.

 

I already don't obtain the device - these are devices in production process,
of which I took an advantage of the error appearing to record it.

That's something that adds to the difficulty of troubleshooting..
The noise doesn't appear in all devices, only in one room, and malfunctioning device can't be kept for long...

I'll add to that the information that there are three types of such sensors with similar arrangement, which don't have the issue.

 

The info you're giving is quite limited, but it's also understandable if you do not want to disclose it in more detail. But the first thing I would do to approach the problem is to restart your system gradually by enabling things one by one, until the problem manifests itself. For example, last year I had a similar problem with an analog signal that was traced back to a lack of ground connection between a step motor and its driver. The driver works through a switch mode current supply to the motor, and that's what was causing a horrible EMI emission that affected the rest of the circuitry.

 

Thanks,
I guess it is somehow different, since the issue is only in one location, and therefor it is some
earthing loop issue?

If needed I can provide the PCBs boards, I simply don't think the issue is sitting there.