Composite Video - A/C Couple and Clamp or Dual Supply

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Hi,

I've been doing a lot of research regarding composite video, and as far as I can tell, a better design approach would be to use a dual power supply such that incoming video can avoid all of the problems associated with A/C coupling and clamping.

I'm just looking to switch incoming composite video (no on-board DAC), and the AD8184 looks interesting. It's a little unnerving not to see the coupling caps in the reference design, but in theory this should be better. Of course it means a more expensive power supply, but I'm not sure that I really care.

For reference, all of the video sources should be on the same ground, of course how they are actually implemented is a different story.

Do any of you video engineers think that using the AD8184 (with the AD8009 follower) solely would be a good approach, or should I a/c couple the inputs and put a sync clamp (like a MAX4090) as a previous stage?

I'm especially interested in knowing if there are any regulations that require A/C coupling (like CE/UL/etc).

Every time I think I understand this, I find contradictory application notes.

FWIW, I've read just about everything maxim has put out on the subject, and their ICs seem targeted to the single supply/AC couple designs. Analog seems to take the other route, and I lack the practical experience or test equipment to *know* which approach is better.

Thank you.

James

Last Edited: Mon. Sep 9, 2019 - 10:10 AM
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Greetings -

I am one of several here with substantial video experience. I have done large video switches and then the DC restore circuit on each channel becomes a big cost (and power consumption) issue. I don't think there is anything "quality" wise with respect to single supply AC coupled vs dual supply DC coupled. Dual supply does give more headroom if you have over-amplitude signal, but that situation should be rare, at best.

I am not aware of any CE, etc, "requirement" for AC coupling. However, if you need to pass CE ESD susceptibility, you will need to transient-protect your video inputs and that is hard because you need something with low capacitance so it does not kill the system bandwidth. There are transient protectors out there (same kind as used for RS485/USB, etc often work well).

Jim

Jim Wagner Oregon Research Electronics, Consulting Div. Tangent, OR, USA http://www.orelectronics.net

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In light of that information, would it be necessary (or advisable) to ac couple and clamp/buffer the input to the dual supply mux?

FWIW, analog's app engineer was a little vague, so I appreciate the advice.

Also, the CE/UL/etc part was in reference to a maxim note that suggested there were government regulations requiring you to ac couple the input. http://www.maxim-ic.com/appnotes.cfm/an_pk/3303/

Maxim's own docs can't seem to decide whether video output should be ac coupled, or input. (see above, and http://www.maxim-ic.com/appnotes.cfm/an_pk/3768).

the MAX9503 with 'directdrive' does look interesting.

Thanks,

James

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I think AD8184 should do the job just fine. If you switch 75 ohm 1Vpp video (NTSC or PAL), there should not be too much DC offset.

It should be easier to make dual power supply than to cope with all the aspects of AC coupling. Even if you would use AC coupling, you don't have to use clamping, but things may require to settle for a while.

Depending on what are your switching requirements, many people have used 74HC4066 successfully. With appropriate input and output buffering maybe.

For clamping, not much than a 4148 diode and suitable clamp voltage is needed, and a bleeder resistor.

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Since ka7ehk (Jim) thinks that +/- supplies don't seem to matter (advice greatly appreciated), I was now leaning towards something like:

AC Couple -> Max4090 -> AD8188 -> MAX9503

That should handle all of the level shifting for a pretty low price with a single supply. Besides, the AD8188 and MAX9503 just look neat.

I just wish I could replace the MAX4090 with something else. That is way too small a package for me to hand solder (SC70 is all that's available on digikey). Reminds me of hand soldering 4 resistor arrays in a 2mmx1mm package under a microscope. utter disaster. Of course the QSOP on the MAX9503 doesn't look fun either.

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I have also used ordinary CMOS analog switches for video. xxx4051s if I recall, correctly. You DO run into a potential bottle neck with large switching arrays due to the on-resistance of these switches and the open circuit capacitance of the others that are bussed together on the output side. I think the largest practical was 32x1.

These were run with dual supply in order to avoid big input coupling caps, clamping circuits, and the potential shift in baseline as the coupling caps charged or discharged after switching.

The switches were always output buffered. First, you need 2X gain to drive 75 ohms with a 75 ohm source impedance and you need some way to isolate the high-Z bussed switch line at the output of all the switches. A 2X non-inverting video op-amp does a great job.

Jim

Jim Wagner Oregon Research Electronics, Consulting Div. Tangent, OR, USA http://www.orelectronics.net

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