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Terry

MAX4311

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I have been trying to do away with the need for a -5v supply when using an LM6574 by using a MAX4311. After wasting 2 chips trying to find out why my circuit was not working I have now found a fix. But what is going on ?

I built the circuit as per the data sheet both with feedback resistors and without but all I got was a black screen. By chance I then built it again forgetting the 75R resistors on the inputs and it worked, but too bright as you would expect. As soon as I added the 75R resistor the picture went black, why ?

The fix I have working for me is to add a 0.1uF cap between the 75R resistor and the MAX4311.

Any ideas why ?

Thanks, Terry

Edited by Terry

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Because if your video source is AC-coupled, some of the incoming video signal will be negative. As your circuit doesn't have a negative supply it will clamp and lose the negative part, mostly the syncs, and only the positive part will go through. Maybe by some magic combination with your video source the lack of 75Ohm termination doesn't draw enough on the video line to bring it down enough to negative levels.

What you need to do is bias the MAX4311's inputs. To do it really properly you'd need to add a capacitor to AC-couple the video as a first thing before entering your circuit. That way you'd be sure it would be working well, regardless of whether the video source is AC-coupled or not. Without it, it would work fine with an AC-coupled source, but not necessarily with a DC-coupled one. Then, bias each input with a voltage divider, making sure the parallel equivalent resistance is close to 75Ohm so it will also serve as termination. I use 620Ohm to +5V and 82Ohm to ground - that's a 0.6V bias and 73Ohm input resistance with good results, now a higher bias might be better, I haven't done many tests but it could be required for certain sources if their signal goes lower than -0.6V.

Then, as the bias will be reflected on the output, you should again add a AC-coupling cap to be sure any video receiver would work fine, not only thoser that include the cap on their side. And of course the 75Ohm series res.

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And of course the 75Ohm series res

OK, it works great with the output and the feedback tied together and no series resistance but if I add the two 500R resistors as per the data sheet I get no output.

Do you use these ?

Terry

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if I add the two 500R resistors as per the data sheet I get no output.

That seems to suggest that the 2X gain is driving the output to a rail. But with the bias resistor values suggested by Kilrah, it seems odd that it is the culprit. Post a schematic of what you are working with. I think that will tell a better story.

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This is what I was working from.

RF and RG are both 500R and the series RT is 75R. The input now is biased as Kilrah sugested.

post-6-1196892795_thumb.jpg

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What I meant was that it would be good if you posted a schematic of *your* video circuit. That is, a redrawn schematic with the bias resistors, all component values, power supply voltages, MAX4311 pin connections (including SHDN), etc. That way we can tell what is going on.

Given your report, something seems to be incorporated incorrectly. A second set of eyes may be the best cure.

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I've personally taken a chance (yeah it's bad) and shorted OUT and FB for a gain of 1, without adding the 75Ohm res. It was originally a lack of knowledge when I did it about 4 years ago, but on the other hand I've never had a problem with it ;)

Indeed the 0.6V bias, doubled to 1.2V by the 2x gain, should have no reason to throw the output into the + rail...

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I've personally taken a chance (yeah it's bad) and shorted OUT and FB for a gain of 1, without adding the 75Ohm res.

In that case, the input bias resistor values might need to be altered for use with the 2X gain configuration.

The goal is to move the DC point so that the entire video signal will fit within the OpAmp's output swing. So, with your existing values, what DC voltage do you measure on the *selected* video input mux pin, as well as the output? Do this with no video on the input.

Also, if you have a scope, look at the video output and check for a railed or clipped video signal.

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I wouldn't think so, 0.6V bias plus 1V signal (being pessimistic there as there's part of the 1V being negative) * 2 = 3.6V... if he powers the MAX with 5V I wouldn't see a problem. On the other hand, if he reduces bias he risks clipping the input again.

The problem I could see would be a DC component in the source signal... but if there was one he wouldn't have had to bias the MAX in the first place :unsure:

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In that case, the input bias resistor values might need to be altered for use with the 2X gain configuration.

Yes, yes, yes, what a twit I am, I will adjust the input resistors and report back :rolleyes:

Terry

Edited by Terry

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OK I did it, all I had to do was add a 560R resistor from the input to 5v and it works great. :P

Shame they dont show it on the data sheet :(

post-6-1196958621_thumb.jpg

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OK I did it, all I had to do was add a 560R resistor from the input to 5v and it works great.

Ahh, you must have missed the bias information Kilrah posted earlier. Your 560/75 derived DC bias is essentially what he recommended. The input cap value you choose (not shown on your schematic) should ensure that you have good AC coupling down to 5Hz.

Shame they don't show it on the data sheet

The reason the data sheet does not mention the bias resistor trickery is because they are not used in DC coupled designs. However, these sorts of things sometimes show up on an App note.

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OK I did it, all I had to do was add a 560R resistor from the input to 5v and it works great.

Ahh, you must have missed the bias information Kilrah posted earlier.

No, what I ment was before that.

First I used the circuit from the data sheet that did not work.

Next I used Kilrahs bias values but they did not work if I had the resistors on the output.

Last I changed the values of the bias resistors so it would work with the resistors on the output.

Now I am happy :D

The input cap value you choose (not shown on your schematic) should ensure that you have good AC coupling down to 5Hz.

I did not show it as it is part of the output circuit of my receiver and not the input circuit of the 4311.

Terry

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