DC Clamping Circuit, Feedback Wanted! - Page 1
Forum section: electronics / projects

author: Ryeno
author_rank: Newbie
timestamp: June 10, 2022, 07:31:47 am
content: I want to inject RGB into a Panasonic monitor. The problem is DC Bias. Most monitors expect an AC coupled signal but this monitor needs a DC coupled signal. IRE0 must be 0V.https://www.maximintegrated.com/en/design/technical-documents/tutorials/3/3303.htmlI tried using a TH7316 amp. It’s internal DC restore worked to clamp the signal but it has a 140mv level shift hence the output is above 0V. I looked online and eventually I found this DC restore circuit:http://elm-chan.org/works/yuv2rgb/rw/rgb2yuv.pngI was going to build but it seems unnecessary. I don’t want to install a LM1881 sync separator nor a multiplexer and I might need a -5V charge pump. There must be a simpler way.I’m pretty sure that because my video signals don’t contain sync pulses, I can use an active diode clamp to clamp the porch to 0V. So I built that circuit and it works but it was showing some distortion near the end of the porch. I tested the circuit without connecting the load and there was no distortion so I believe my issue is needing a buffer?Thanks.

author: Terry Bites
author_rank: Super Contributor
timestamp: June 10, 2022, 02:32:51 pm
content: micon? jungle?D1-D3 etc what are they for?If it works and the video looks ok then leave it as is.The circuit will take a while to settle while C1 charges and this may be problematic. The RGB signals may end up with slightly different timings.When the diode is reverse biased the opmp goes into saturation, this will increase your recovery time. This willl be especially noticeable with a steam powered opamp like the 324.Just about any small signal diode will work. You probably need to control the output impedance with a series 75R resistor. This usually means that you need 6dB of gain to make up the luma level.The THS7373 does all the work and more in one TSSOP-14 package from TI Its still current, works at HD. Less than $1

author: TimFox
author_rank: Super Contributor
timestamp: June 10, 2022, 03:49:24 pm
content: For C1 to C3, you may use NP0/C0G ceramic, but X7R should be avoided, and Y5P and Z5U are right out.Film capacitors, preferably polypropylene are as good as C0G ceramic, but will be physically larger.Polyester (Mylar) film capacitors may be good enough, and are not quite as large as polypropylene.The film capacitors available as surface-mount are similar to polyester in their properties: see data sheets.

author: Benta
author_rank: Super Contributor
timestamp: June 10, 2022, 10:41:54 pm
content: Your design fills my head with questions.First, your inputs are SCART RGB signals. I've never seen those in the US before.Second, you have  (which is CVBS) on your schematic, which would be the natural place to extract DC.Also, the positions of D4...D6 are not obvious.Comments:The LM324 is a very bad choice as opamp here. It's far too slow.

author: strawberry
author_rank: Super Contributor
timestamp: June 12, 2022, 08:23:56 am
content: comparators (LM339...)are faster than most OPTVS diode

author: strawberry
author_rank: Super Contributor
timestamp: June 12, 2022, 05:02:27 pm
content: input current will load source, divider, ... . JFETs are usually poorly selected (larger input offset) but input resistance in Mohm Gohm rangethey make CMOS versions as well LM339LV (VCC = 5.5Vmax) delay 0.6ussome are push-pull but mostly SOT-23distortion: diode reverse recovery maybe

author: strawberry
author_rank: Super Contributor
timestamp: June 12, 2022, 07:27:32 pm
content: not rail-rail 324 output can switch between 0.05..3.5V @5V