Clamping DC Biased Composite Video Signal Back to ground - Page 1
Forum section: electronics / projects

author: syntax333
author_rank: Regular Contributor
timestamp: March 26, 2019, 12:28:54 am
content: Hi, as the title suggests I want to clamp the DC Biased Composite Video Signal back to its original position.I have tried to clamp the signal by extracting sync information and drive a switch which connected to ground according to sync info.The DC bias which I added is approximately 5V. After trying clamp the signal back to ground DC bias drops to 2V. I want the signal at its original position which is 0V.Are there any suggestions? Thank you.

author: xavier60
author_rank: Super Contributor
timestamp: March 26, 2019, 01:15:29 am
content: I have a circuit that will clamp the sync tip to 1V, you want to clamp to 0V?Can you show us your circuitry?

author: syntax333
author_rank: Regular Contributor
timestamp: March 26, 2019, 02:57:45 pm
content: The circuit I used is shown below. I want to clamp to 0V.

author: xavier60
author_rank: Super Contributor
timestamp: March 26, 2019, 03:29:06 pm
content: The CSOUT pulses are positive going. They should not be inverted to drive the 4066.DC restored video signal doesn't tolerate much loading.EDIT: I was wrong about the CSOUT polarity.

author: dzseki
author_rank: Frequent Contributor
timestamp: March 26, 2019, 03:32:35 pm
content: You can use a DC coupled opamp (Au=1 or 2 depending on what output termination you want ), but with DC bias applied to the inverting pin, therefore subtracting that bias from the input signal.

author: xavier60
author_rank: Super Contributor
timestamp: March 26, 2019, 03:49:59 pm
content: I think I'm wrong about the CSOUT pulses. So maybe there is a loading problem.

author: xavier60
author_rank: Super Contributor
timestamp: March 26, 2019, 04:03:31 pm
content: If there is 5V bias on the input, that would apply the wrong polarity to C3.

author: syntax333
author_rank: Regular Contributor
timestamp: March 26, 2019, 04:15:17 pm
content: Circuit works fine when I give a ref voltage instead of ground in order to shift a CVBS signal to ref Voltage therefore I don't think that CSOUT logic is wrong.At the moment no load is connected to Video_out.I will invert the polarity of C3 you are right.Also I might apply negative voltage instead of ground to get rid of remaining 2V.

author: xavier60
author_rank: Super Contributor
timestamp: March 26, 2019, 04:19:31 pm
content: If C3 has the wrong polarity applied to it, it will likely to be leaking current.

author: Benta
author_rank: Super Contributor
timestamp: March 26, 2019, 05:20:39 pm
content: The circuit is fine, but you need to watch the polarity of C3. You can get around this by using 2 x 220 uF electrolytic back-to-back, then you have a bipolar cap.The output needs buffering. I can highly recommend the TI LM6171, it's the best video amp I've worked with.I'd place R1 after C3 and change it to 75 ohm. Then you'll never have to think about DC biasing again.

author: syntax333
author_rank: Regular Contributor
timestamp: March 27, 2019, 07:08:03 pm
content: circuit doesn't seem to work for biased CVBS signal. LM1881 does not trigger as it suppose to. Is this because bias?

author: xavier60
author_rank: Super Contributor
timestamp: March 27, 2019, 07:21:00 pm
content: The bias shouldn't matter because C1 would block the DC. Can you show us the input video waveform? What is the p-p and polarity?

author: syntax333
author_rank: Regular Contributor
timestamp: March 27, 2019, 07:56:51 pm
content: Since my CVBS signal is biased 5V and it is 2Vpp overall voltage is approximately 5-7V. I just checked the datasheet of LM1881 which states that for Vcc=5V Input can 3Vpp. So I AC coupled the input and buffered then send to lm1881. Now lm1881 triggers correctly.However, as a buffer I use pnp follower configuration, output of my buffer seems noisy, this causes lm1881 to trigger wrong sometimes.I want to use dedicated buffer (opamp) for cvbs.I have only "MAX457CPA+"or "LMH6704MA" as a buffer. I have never used one before which one do you suggest to use?Unfortunately, I don't have LM6171 at the moment.

author: xavier60
author_rank: Super Contributor
timestamp: March 27, 2019, 08:02:59 pm
content: I don't know, I have only ever used discrete circuitry for amplifying, sync separating and clamping.

author: syntax333
author_rank: Regular Contributor
timestamp: March 27, 2019, 08:07:50 pm
content: Your circuit seems interesting I have never tried to extract the sync information with discrete circuitry.

author: xavier60
author_rank: Super Contributor
timestamp: March 27, 2019, 08:13:43 pm
content: Sync separation is easy so long as not too much low frequency content has been lost from the video signal.The sync tips are clamped at 1V. It cant be set to 0V.

author: xavier60
author_rank: Super Contributor
timestamp: March 27, 2019, 08:25:10 pm
content: Maybe a small MOSFET could be used for clamping. Something like a 2n7002 or lower rated to keep the capacitance low.

author: IconicPCB
author_rank: Super Contributor
timestamp: March 27, 2019, 10:18:39 pm
content: A simple one transistor clamp.One side of capacitor is clamped to ground the other ( input ) floats at an offset.

author: xavier60
author_rank: Super Contributor
timestamp: March 27, 2019, 10:27:46 pm
content: I tried unidirectional clamping years ago. I found that it didn't cope well with scenes that contained large brightness difference between top and bottom.Extra: A good test of the performance of sync and clamp circuits is to put an undersized capacitor in series with the input video cable like 47uF. This causes large fluctuations in sync tip level with various video content.

author: syntax333
author_rank: Regular Contributor
timestamp: March 28, 2019, 10:26:52 pm
content: I manage to solve the problem. It was broken decoupling cap at sync separator LM1881. After I replaced it, LM1881 triggers as it suppose to.Which results clamping the CVBS to ground.