what kind of power supply are you using with the arduino? i had noise trouble because of a switching power supply. it's better to get a regular one.

you're problem might be something else though. the regularity in the oscillation is odd.

There are debouncing chips like this: http://uk.farnell.com/on-semiconductor/mc14490pg/ic-4000-cmos-4490-dip16-15v/dp/9664947?whydiditmatch=rel_3&matchedProduct=386595

But I'm not sure that is going to help you here.

thanks for answers. dtr: I have just tried with external power source - 9v batery, but nothing has changed. maybe the problem is in shared ground with usb interface? Luke Woodbury: good idea to use external ic, but unfortunately this chip is for hex operations, so it works only with binary numbers..

How about using a capacitor ?

Check this article : http://www.thebox.myzen.co.uk/Tutorial/De-coupling.html

i found that capasitor, works like slide object, and it doesnt works for my situation. i need accurate readings...

Cwiek: "but unfortunately this chip is for hex operations, so it works only with binary numbers.. "

A hex contact debouncer means there a 6 contact debouncers in 1 IC package. It has nothing to do with binary numbers. In other words, you could use it to debounce the contacts of 6 switches or 6 relays.

Your issue has nothing to do with debouncing. I haven't seen you mention any switches or contacts opening/closing.

Where is the signal coming from that you are looking at with an analog input?

I would try isolating that using an optoisolator..

You have power supply noise (aka ripple) or some kind of ground loop.

If you can avoid using the USB connection, try doing that. Many PCs (even big name brands) use the cheapest, crappiest power supplies on the planet.

I recently measured 150mv p-p of noise coming from a brand new Acer computer.

A small capacitor between power & ground may help.

finally i have used ADuM4160 + external 9v bat, and it works super great! thanks for help :) adam