VGA switch design - Page 1
Forum section: electronics / projects

author: thomasbean
author_rank: Newbie
timestamp: February 27, 2022, 09:18:50 am
content: I am using the TS5v522c (Data sheet below) to switch VGA signals from a PC that are going onto a VGA video "bus", the video is passing through the fine but when the enable is set to turn the output off or the power is removed from the chip - some minor component of the video signal still bleeds through. I'm thinking this is a parasitic effect- the circuit is currently built on a breadboard with the chip on a SMT to DIP adapter, the VGA connectors are screw-terminal breakout boards which connect to the breadboard using 15cm unscreened wires.I'm wondering if I could cute the above issue if I laid out a double-sided PCB for this making one side a ground plane, completely unetched and connected to PSU GND, the other side having traces from the VGA connectors to the SMT-DIP adapter, I would space the traces on the PCB with ground plane between all of them and leave all unused copper unetched and connected to supply GND ?I haven't brought any VGA grounds into the circuit, I'm wondering what should I do with them, connect them to the ground plane?The ultimate goal for this project is to create a VGA video bus, where only one PC at a time outputs VGA onto the bus and therefore it is essential that the IC fully switches off the signal.https://www.ti.com/lit/ds/symlink/ts5v522c.pdf#page=5I unfortunately do not have access to an oscilloscope or schematic layout software.Thanks

author: jwet
author_rank: Frequent Contributor
timestamp: February 28, 2022, 07:03:17 pm
content: That's a neat part! I suspect that your feedthrough is caused by grounding and parasitics on your breadboard lash up and would be fixed with a careful layout. Just a few pF of parastics at these frequencies can put a lot of signal into 75 ohms and give you fits. The TI part is really just an analog T switch with about 3 ohm on resistance and 8 pF being shunted into 3 ohm in the off state reading between the lines in the data sheet. They list 83 dB of off isolation which shouldn't be visible at all- -40dB is probably the threshold. I suspect the crosstalk/leakage is going around your part.VGA is kind of an imprecise term, depending on resolution, it can be from 8 MHz to several hundred Mhz of BW. This part is listed to 380 Mhz! with good diff gain and phase specs.Maxim makes some passive switches like these and some active switches too. They also have some good app notes- probably old ones, they got out of most analog video around 2005. There is a company called Extron that makes equipment like this and you might want to look at the app notes for applying their stuff. They used a lot of Maxim stuff and I called on them in LA as an FAE 20 years ago.A couple of other notes-1. Grounding this stuff can be tricky, a lot of pro video gear had switches so you could disable the termination R at the destination of AC couple it. When you AC couple, you generally need to do a baseline restore which a lot of pro stuff does. This is especially important in composite systems with H/V embedded but can apply to RGB + Syncs like VGA. Look at app notes.2. I guess your "BUS" should be terminated at 75 ohms but make sure you don't double terminate. You might want to have a simple RGB buffer at each output but maybe this ruins the beauty of the BUS. Think about this some.3. Bussed systems are nice for small numbers but you get into a problem with lots of guys hanging on the bus. You have one transmitter on and everyone else is kind of tristated with it just contributing a little off isolation leakage to the bus. Maxim used to call this "all hostile crosstalk" and is something that limits the performance of wideband analog busses. (and analog cable TV in a different way) At some point you get forced to use real cross points- Maxim and others made some of these. The 456 was an 8x8 Xpoint but I'm sure there are better and cheaper parts these days.4. This can be done- IBM or Dell (can't remember which) used something like this on servers in the early 2000's maybe later. They called it a "distributed KVM" that allowed bussing mice, keyboards and video among a bunch of rack gear to one monitor and keyboard, mouse. I don't recall exactly how it worked but you might be able to look up old patents- they're expired but could give you an idea of what the architecture was.I think a good layout with these parts should work. You might want to see if TI has an eval board that you can copy from. Good luck.