RGB->Composite Encoder Project (Basically, an op amp question) - Page 1
Forum section: electronics / projects

author: ArtemisGoldfish
author_rank: Regular Contributor
timestamp: June 17, 2010, 06:56:34 am
content: Okay, I'm going to be working on a circuit that converts RGB+Hsync+Vsync from a video source (In this case, an Atari 1040ST) to an NTSC composite signal. Since the chip that originally did this conversion in units with composite output is EOL and just plain difficult to find these days (the MC1377), I'm going to base the circuit on the Analog AD725.Just as a forewarning, I'm not an engineer, just a plain old technician, so I know just enough to get me into trouble.Anyways, attached is the reference demo design from Analog. I'm basically going to strip off the parts I don't need (the PAL oscillator, the S-Video output components, the VGA output.) I've been advised by someone who built up a similar circuit to put buffering on the composite output, so I selected an op amp from Ti for this purpose, and made sure to get one with sufficient bandwidth (I picked one with 38MHz unity gain bandwidth. Seems overkill-ish, but I'd rather have too much than not enough. Plus, I couldn't remember the exact number for bandwidth of a composite NTSC video signal, I think it's 4MHz.)ANYWAYS, I'm going to be running the op amp off a single 5V supply. I'm thinking I should basically take the circuit they have, put the op amp buffer between the 220 uF cap and 75 Ohm resistor. The circuit should be (correct me if I'm wrong) capacitive coupling input to the non-inverting input of the op amp, along with two resistors to bias the input (could I just put two equal value resistors like 1k and 1k, since I won't be swinging too close to either rail?) I'll connect the non-inverting input of the op amp to the output, and then couple the signal out of the op amp with another 220 uF cap, and that'll go into the impedance-matching 75 Ohm resistor.Any thoughts on this? I value some professional opinions. Thanks.

author: ArtemisGoldfish
author_rank: Regular Contributor
timestamp: June 17, 2010, 10:46:46 pm
content: I seem to only make threads that flop over and die. Can ANYONE give me any hints?

author: jahonen
author_rank: Super Contributor
timestamp: June 18, 2010, 06:36:45 am
content: If you want to add buffer to this circuit, then you must put it just after the AD725. Otherwise, the signal will have also negative voltages and you would need bipolar power supply for the opamp. But anyway, long time ago I built something like this using AD725, and it worked just fine just as-is without buffering. AD725 is designed to drive the double-terminated 75 ohm line directly.Regards,Janne

author: marianoapp
author_rank: Regular Contributor
timestamp: June 18, 2010, 04:39:03 pm
content: Quote from: ArtemisGoldfish on June 17, 2010, 06:56:34 amthe output should be connected to the inverting input. You could also use a specialized buffer ic likethis one[check the application circuit in the first page, is pretty much the same you were describing]

author: ArtemisGoldfish
author_rank: Regular Contributor
timestamp: June 19, 2010, 01:17:42 am
content: Hell, I'll be working for an engineer at CrystalFontz in a few days, maybe I can just ask him. Anyways, I don't see why this circuit (attached) wouldn't work. I know you shouldn't put a whole lot of stock in simulation, but it simulates exactly as I would expect it to.

author: NiHaoMike
author_rank: Super Contributor
timestamp: June 20, 2010, 03:10:42 am
content: There are boxes that convert VGA (RGB+syncs) to composite/s-video for recording purposes. Now that VCRs are far less common, you might be able to pick up a used converter for cheap.

author: ArtemisGoldfish
author_rank: Regular Contributor
timestamp: June 21, 2010, 12:15:15 am
content: Quote from: NiHaoMike on June 20, 2010, 03:10:42 amThat's probably a good idea, now that you mention it. I'll probably do that, but I still want to build up this circuit, just for the education experience.I've been advised that my op-amp circuit is alright, I just needed to find an amp with proper noise performance up at the frequencies composite video has to handle, so I had to order a better amp (namely, an AD811.)