Digital PC Scope (Part 2)

After I completed the PCB and build it in the case, I started the calibrating process. I had a little trouble and didn't really understand why. I fiddled a bit more with the trim potmeters and finally calibration succeeded. But it didn't go as planned actually. Then I hook it up to my function generator and saw that it actually did some very strange stuff. I used my other oscilloscope to find out what was wrong, but I could only find correct signals on the PCB. I'll try to explain to you what I saw by showing you three pictures from the software. The input is a pure sine wave:

As long as I stay below the 0 Volt line everything looks OK.

When I try to center the image by shifting its Y position a bit up this is what happens:



You can see that part of the positive signal is actually shown as negative values. Very strange. I measured the whole analog circuit and up to the A/D converter it all looked good. I looked on the support desk of Velleman and found a post with the same problem. There it turned out the AD converter chip was broken. Well I couldn't find the same chip easily so I decided to contact Velleman about it. I'll keep you posted on the progress. I don't think I can solve this problem myself since I don't have spare parts and I have no idea what happens in the digital domain.