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 Posted: Jul 24, 2012 - 02:31 PM |
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Joined: Jul 18, 2009
Posts: 106
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Hello,
Attached is my video circuit. I have recently added a splitter to it. Prior to this worked very well with the LT6552CS8#PBF-ND IC2, and then the max7456. From there, out to my rca header. No I have printed a board with the LT6206CMS8#PBF video splitter chip on it and copied their suggested schematic as sown in the data sheet.
I also use jumpers called Video_select1 and Video_select2 so I have the ability to totally bypass the max7456. So what I get is this:
With the max7456 bypassed Video_select1 (1&2 connected)+ Video_select2 (1&2 connected) the video feed looks great, and both video outputs seem to function. When I integrate the Max7456 (3&2 connected)+ Video_select2 (3&2 connected) the image is all crazy white/black/sometimes there, sometimes not. See the video of it on you tube.
http://www.youtube.com/watch?v=6uy_yJUi2as
So I thought it was the max7456, since the video splitter circuit seemed to prove itself without it, but when I lifted the far side of R34 it works fine though the max7456. So what this seems to be is a combo of the max7456 and the video splitter....?
1) Is there something I am missing here in my schematic?
2) Do I even need the op-amp splitter or can I just T the traces of to the 2 video headers?
I have tried in the past to split video using cheap cord splitters but often the screens flash black on me.
Thanks in advance for any help.
JL |
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Posted: Jul 24, 2012 - 04:17 PM |
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Joined: Nov 22, 2002
Posts: 12196
Location: Tangent, OR, USA
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A few video op-amps can drive double-loads. That is. two 75 ohm resistors that each drive a separate load. In that case, the op-amp sees a net 75 ohms. Most video op-amps are designed for 100 ohms or more.
When you CAN do that, the op-amp IS the splitter.
You have designed your video outputs incorrectly. Most CCTV video devices are 75 ohms. You only need a 75 ohm resistor from the op-amp output to the load (usually with a coupling cap to remove DC) and a larger resistor as a DC bias path. Your 75 ohm resistor to ground significantly reduces the output amplitude and is highly undesirable.
There may well be other issues, but hat stands out.
Given that circuit with the op-amps, I do not see the need for an IC video splitter.
Jim |
_________________
Jim Wagner
Oregon Research Electronics, Consulting Div.
Tangent, OR, USA
"The only thing standing between us and victory is defeat" P.G.Wodhouse in Wooster & Jeeves series
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Posted: Jul 24, 2012 - 05:04 PM |
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Joined: Jul 18, 2009
Posts: 106
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Hello Jim,
Thanks again for your help with my video issues. I revised the current sch. that uses the video op amp ic and scratched out the 75 ohm R that I think you indicated should go away. Is this right?
Taking it another step and eliminating the 2nd op-amp altogether which is even better, I deleted R34 from the max7456 output and ran it directly into R37 and R36 just by splitting the traces. From the other side of these two resistors they go directly to my RCA jacks. Is this right? I like this option even more! Less parts = less break downs.
Thanks,
JL |
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Posted: Jul 24, 2012 - 06:06 PM |
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Joined: Nov 22, 2002
Posts: 12196
Location: Tangent, OR, USA
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It appears that the MAX7456 can only drive ONE standard video load. All of the test circuits show a load of 150 ohms.
So, if you need to drive two loads, then you need an op-amp that WILL drive two loads, OR you need two op-amps. An LT6206 is nothing more than a dual video op-amp that CAN be used as a splitter, but is not dedicated to that purpose. Somewhere, you are going to need either two op-amps ore one with enough drive current to drive two loads.
Two op-amps might be easier.
Jim |
_________________
Jim Wagner
Oregon Research Electronics, Consulting Div.
Tangent, OR, USA
"The only thing standing between us and victory is defeat" P.G.Wodhouse in Wooster & Jeeves series
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Posted: Jul 24, 2012 - 07:58 PM |
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Joined: Jul 18, 2009
Posts: 106
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Thanks Jim - does this mean that the original design without the extra 2 R75ohm resistors is my ticket (shown in the previous pic labelled Revised1).
Thanks,
JL |
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Posted: Jul 24, 2012 - 08:09 PM |
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Joined: Jul 18, 2009
Posts: 106
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Hello again. Just looking through the MAX7456 data sheet, and on page 20 is reads as follows:
Video-Driver Output
The MAX7456 includes a video-driver output with a
gain of 2. The driver has a maximum of 2.4VP-P output
swing and a 6MHz large signal bandwidth (≤ 0.2dB
attenuation). The driver output is capable of driving two 150Ω standard video loads.
Does this mean it is ok do go for the schematic as shown in previous picture labelled REVISED2 - NO SPLITTER.jpg?
Thanks,
JL |
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Posted: Jul 24, 2012 - 08:50 PM |
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Joined: Nov 22, 2002
Posts: 12196
Location: Tangent, OR, USA
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I missed that statement.
Yes, Connect TWO 75 ohm resistors to the output of the OSD. Each goes to an output jack.
Jim |
_________________
Jim Wagner
Oregon Research Electronics, Consulting Div.
Tangent, OR, USA
"The only thing standing between us and victory is defeat" P.G.Wodhouse in Wooster & Jeeves series
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