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Old 11-06-2019, 07:16 AM   #1
Y. Signal
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Default What are the best caps for AC coupling a video signal?

Hello.

I'm currently using 6.3V 330F Panasonic FC capacitors for AC coupling a 75 ohm impedance video signal coming from a THS7374. They work fine, but I like to improve my designs so I'm considering using better ones.

I wonder if polymer capacitors would be better for my application. If so, I'd like to know what parameters I should look for to make a good choice.
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Old 11-06-2019, 09:48 AM   #2
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Default Re: What are the best caps for AC coupling a video signal?

"Improve" ?

Left out too many details to make heads or tails of your query.

Normally large electrolytics are used to decouple. The way I read your query you either overdesigned or made a serious design error...

Schematic would be good.
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Old 11-06-2019, 10:09 AM   #3
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Default Re: What are the best caps for AC coupling a video signal?

your blocking DC while allowing as much AC signal as possible,
so as long as the cap has the bandwidth, the lower the esr the less losses you will incure.
having said that, video signals dont have to travel far so minor losses arent important.
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Old 11-06-2019, 01:44 PM   #4
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Default Re: What are the best caps for AC coupling a video signal?

Thank you. My reason to replace aluminum electrolytics with polymers is reliability, endurance and lower ESR.

My circuit uses the example in the THS7374 datasheet for AC coupled output (page 25): http://www.ti.com/lit/ds/symlink/ths7374.pdf (330F capacitors polarity is wrong in this schematic).

Last edited by Y. Signal; 11-06-2019 at 01:46 PM..
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Old 11-06-2019, 02:53 PM   #5
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Default Re: What are the best caps for AC coupling a video signal?

Polymer capacitors are used not for signal coupling because they have high dielectric absorption and non-linear behavior.
You will find your video is distorted because of the polymer capacitors. This is also the case in audio use of polymer caps for blocking and AC signal coupling.
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Old 11-06-2019, 05:32 PM   #6
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Default Re: What are the best caps for AC coupling a video signal?

Thank you for the information. Then I'll continue using Panasonic FC.
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