It's funny you mention that TI chip, I just found out about it myself a few days ago. If TI says it's a drop-in then it should indeed be fine to use instead of the LT1675.Seikenfreak wrote: Edit: Oh yea, almost forgot. When I saw you post about the LT1675, I did a search for it and one of the first things Google had was a datasheet or something about a direct replacement. It does say on the datasheet bullet points "DROP-IN UPGRADE TO LT1675"
Texas Instruments OPA3875 - Triple 2:1 High-Speed Video Multiplexer
Which I was then able to find available at Mouser, where as the LT1675 was not.
I didn't read into the details because I'm sure it wouldn't make sense to me, but perhaps this is a suitable alternative if the LT1675 is harder to come by?
That said, I'm coming to the unfortunate realization that despite my prototype being a success, the layout of the board as it is right now will most likely not do the job. I'm having issues getting a clean OSD overlay and I don't think it can be bodged into working and still meet the goal of being a cleaner, simpler, better alternative to the 4PDT switch. I guess when it comes to analogue signals, board layout is very important.
The external RGB looks very nice and crisp just as it should be, but the multiplexed OSD only looks ok when the external RGB blanking is on but the external console RGB off. If the external blanking is off, the OSD shows up as intensely white and very bloomy. As if the RGB levels are just just maxed out. When external blanking is on and the external RGB is active (that is, console RGB video is on the screen), the OSD is faded, washed out with colours shifted orange. If there's interest, I'll try to get some video demonstrating what's going on.
I tried hooking up my oscilloscope but frankly, I'm rather inexperienced with it and I wasn't really able to get any useful insight looking at the waveforms once I was able to get them to show up. Adding additional bypass caps to the LT1675's power inputs improved things somewhat, but I think the issue is just too much stray capacitance from the ground plane and the two other ICs being too close to the LT1675's I/O. In other words, I think I just packed everything too tightly and this batch of boards is most likely a write off. Live and learn I guess.
I'm going to do some tests to see if I can find what exactly is the culprit. I'll make a board with just the LT1675, another with just the negative voltage supply and another with just the OR gate. By isolating each I can see if the interference is eliminated. If none of that does it, it may mean the ground plane will also need to be moved away from the LT1675. This was mentioned in the TI OPA3875 datasheet's design recommendations but not the LT1675's. Thanks Linear.
I've sent a tech support request to Linear Technology and hopefully they'll get back to me and help me sort things out better. If they don't, I'll post on TI's forums haha. Once I have a better understanding of the problem, I'll redesign the layout, add some of the improvements that have been recommended by viletim and Voultar and get another batch made. I'll try to keep the size small and the components will likely remain unchanged, but a few more caps and resistors might be needed. I'll be out a hundred something dollars, but hey I want this to be a success and I want to share something with people who share my enjoyment in good old analogue RGB video. Hopefully those that have expressed interest in this project will still want them once I've got the layout worked out.
Sorry if I let people down; believe me, nobody's more disappointed than I am.