LM6172 for differential video - Page 1
Forum section: electronics / beginners

author: dirtyice
author_rank: Newbie
timestamp: May 30, 2025, 12:21:44 pm
content: Hi all, I'm new to PCB designing and had need to make analog video over CAT5, so learned about differential video from here:https://www.audiodesignguide.com/AVcat5/Since could not get components described, I went with LM6172 (thx chat gpt) and made schematics, ordered pcb and parts, soldered, but it won't workBanging head against the wall, chat gpt no help really, I know I have missed something :/dif_video.png(57.75 kB. 1245x403 - viewed 78 times.)

author: jwet
author_rank: Frequent Contributor
timestamp: May 30, 2025, 05:21:22 pm
content: You don't say where you're seeing a problem, without test equipment, all you can do is an end to end test. You also don't say what your supply is, it should be +-5v so your differential signal can swing around ground. You can do twp video with a single supply but not with this part- its needs 5.5v minimum and you'd have to have a pseudo ground at Vcc/2 to work as your baseline.The SE to DE converter schematic is not right. You can likely salvage your board with some bodges. Keep all the added wires very short and direct. I also didn't see any bypass caps in the circuit. You should have a .01/2.2uF at both supply pins to ground. See figure 7-9. Keep this all tight.First, on the SE to DE converter, there s a pretty good circuit in the data sheet at figure 7-8. This is a classic circuit. The top path has a gain of +1 and the bottom has a gain of -1. This example assumes that its driving two 75 ohm coaxes that are terminated at the far end with 75 ohm. CAT-5 is usually assumed to be about 120-150 ohms. The change would be to the two 37 ohm back terminations on the on the outputs- make these half of 120 ohms or 60 each (56 and 68 are standard values), 68 might be best.On the DE to SE converter, your SE to DE converter looks more like this circuit and you might have luck carving up that board a bit to get to the right circuit. I found a link to a good app note/circuit below-https://www.analog.com/en/resources/analog-dialogue/raqs/raq-issue-145.htmlThe input z of these is 1k. You probably want to terminate each incoming line at 68 ohms- just an R from the two video inputs to ground. On the output, add a 75 ohm back termination before you go back to coax SE world. You'll be driving a 75 ohm load.Finally, the path through both SE to DE and DE to SE will attenuate the signals due to the terminations and back terminations. These each take 1/2 the signal. So if we assume the first input is 1v RS-170 type input, the first termination will knock it down to 1/2 for each driver. The amps have gains of 1 and a back termination and termination at the far end so the signal on the CAT5 will be 1/4 level on the wire and at the far end. You might want to make the SE to DE have a gain of 2 to claw back some of this. Use the rules for standard inverting and noninverting amps. For the inverter you'd a 2k FB and a 1K input. For the non inverter, you have two 1k since the gain is 1+rf/rgnd. At the DE to SE, you could also take a gain of two to get the signal back to RS170. The attached app note explains some of this. Luckily your amp has loads of gain BW so you can take gain. Don't get carried away, you'll soon run into the rails with much gain.PS- first boards never work- luckily they're cheap and quick these days.PSPS- There are little transformer baluns that can be used to drive Video on TWP- BNC on one side and terminals on the other- no power. They're used in security video and available on Amazon. Video quality isn't stellar but money savers for CCTV security type video.Good luck-

author: Benta
author_rank: Super Contributor
timestamp: May 30, 2025, 09:28:23 pm
content: Several bad ones here:What's the DC level of your video? If it's ground, you'll need a negative supply for the amps. Or AC couple and bias the input signals.Start from there.And PLEASE draw a schematic using opamp symbols, not DIP-8 packages. Unreadable.Good choice of opamp, BTW.

author: dirtyice
author_rank: Newbie
timestamp: June 02, 2025, 12:40:07 pm
content: Thank you for feedback, like I said, I'm new. Trying to understand all info provided hereVoltage is plain 5V BEC, what I have, so now need to see how to improve this partWill add cap, but rest I need to grasp what's that and how it works properly, tried to find working schematics, but had no success for LM6172

author: Benta
author_rank: Super Contributor
timestamp: June 02, 2025, 08:47:34 pm
content: What EDA are you using?Selecting a generic symbol for a dual opamp and assigning the part nummber later is easier and more flexible.

author: jwet
author_rank: Frequent Contributor
timestamp: June 04, 2025, 04:25:57 am
content: If you're constrained on voltage- look at the AD8031, not quite as fast as your first pick but plenty fast enough for most stuff with low gains. Also cheap and very lower power. Amazing parts.Have fun.

author: dirtyice
author_rank: Newbie
timestamp: June 06, 2025, 09:33:13 am
content: Thank you for input, took me a while to do some correctionsI'm using EasyEDA for making these schemas and still learning what's possibleNow SE to DE should be correct, I hope I understood all correct. Made +/-5V source as well.DE to SE is still work to do, not sure if I can make using existing opamp, may need to switch to AD8031 (looking before on it, but JLPCB did not have stock when started. My needs are aprox 50m for CAT5