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Strange touchpad behavior

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Shirakaba:
Hello all wise EE-people.

I am using a laptop, a Sony Vaio VPCSA, which is equipped with a touchpad detected by X.org as an AlpsPS/2 ALPS DualPoint TouchPad. The machine is running Linux. When running it at work, with PSU connected to mains, the touchpad behaves very strange. The mouse pointer movement is jumpy and sometimes it seems to think that my single finger are two fingers, thus acting as the mouse wheel. When running the machine attached to the docking station, I use an external touchpad connected to USB, and every now and then it freaks out too. When the laptop is running on batteries and not connected to the wired network, there are no problems whatsoever.

At home I use the very same setup, with another docking station, an external touchpad of the same brand as the one at work, PSU connected to mains and connected to a wired network, and I rarely ever experence any of the problems I have a work.

It could be worth mentioning that the PSU for the laptop has connection to mains earth and the negative rail connected to the laptop seems to be mains earth referenced. The wall sockets are earthed in both locations.  Just for fun, I measured the potential difference between myself as I am sitting now, and the VGA socket shield, measuring 0.9 VAC, 50 Hz.

Is there anything that could be worth measuring and what could be a possible cure for this?


Regards,
Shirakaba

daveshah:
If you have a scope, try measuring the output noise of the PSU. Sometimes a noisy power supply can cause this.

Shirakaba:
Sounds like a good start. Too bad my scope is at home and my problems at work, but I guess it could give me some kind of idea how the PSU is behaving.

senso:
Lack of earthed sockets caused the same in my old laptop, either crappy wiring at work, or crappy PSU.

daveshah:
In general, SMPSs will have capacitors going from live/neutral and possibly earth to the output as part of the EMC filter. This may be causing any mains noise at work to pass through to the output and cause this behaviour.

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