# Ac coupling capacitor - HDMI

#### Danieledp

Joined Oct 22, 2021
7
I'm studying the AC coupling capacitor for high frequency video signals and in every schematic they are always using a 0.1uf value at the source.
I've looked around to find some proper document explaining what will happen (with tests) if an higher/lower capacitance is used in the circuit.
Obviously higher capacitance means lower impedance for lower frequencies but wouldn't that improve the signal overall?

Thanks

#### nsaspook

Joined Aug 27, 2009
13,265
There was this meeting in 1971 that set the 0.1uf standard. Let me see if I can find the document.

Yes, using 01.uf is a 'rule of thumb' type standard that usually works. If experience or circuit requirements dictate something else, then you use something else.

#### Danieledp

Joined Oct 22, 2021
7
So hypothesizing, the usual 0.1uf capacitor for videosignals are mostly 0402 size.
Lowering the size (0201) will improve the impedance for higher frequencies
What will happen to the HDMI output signal, hypothetically, if we place a 10uf 0201 for ac coupling?
Shouldn't it distort the videosignal with higher capacitance overspecs?
OR simply increase the signal quality because the lower lowfrequency impedance?
So only the impedance plays the fundamental role (0402 0.1uf vs 0201 10uf)
Ergo increasing capacitance given the same capacitor quality (x7r c0g etc) could only improve the signal?

#### nsaspook

Joined Aug 27, 2009
13,265
So hypothesizing, the usual 0.1uf capacitor for videosignals are mostly 0402 size.
Lowering the size (0201) will improve the impedance for higher frequencies
What will happen to the HDMI output signal, hypothetically, if we place a 10uf 0201 for ac coupling?
Shouldn't it distort the videosignal with higher capacitance overspecs?
OR simply increase the signal quality because the lower lowfrequency impedance?
So only the impedance plays the fundamental role (0402 0.1uf vs 0201 10uf)
Ergo increasing capacitance given the same capacitor quality (x7r c0g etc) could only improve the signal?
Think logically without just throwing out ideas. Explain why you think something should happen.

#### Danieledp

Joined Oct 22, 2021
7
Think logically without just throwing out ideas. Explain why you think something should happen.
That's why I'm here, to find answers...
(anyway thanks)

#### Danieledp

Joined Oct 22, 2021
7
doublepost

#### nsaspook

Joined Aug 27, 2009
13,265
That's why I'm here, to find answers...
We are not AI programs. Is it asking too much for you to express how you came to your conclusions about size and values.

#### Danieledp

Joined Oct 22, 2021
7
We are not AI programs. Is it asking too much for you to express how you came to your conclusions about size and values.
No, no, those aren't conclusions at all. I was just hypothesizing of what could happen.
It's known the effect of impedance and capacitance between higher and lower frequencies.
Just what would happen in the AC coupled circuit as it is in HDMI or Displayport if we increase or reduce capacitance specifically.

Looking for some paper explaining about this but couldn't find anything online, atleast as far as i've found

#### MrChips

Joined Oct 2, 2009
30,795
So hypothesizing, the usual 0.1uf capacitor for videosignals are mostly 0402 size.
Lowering the size (0201) will improve the impedance for higher frequencies
What will happen to the HDMI output signal, hypothetically, if we place a 10uf 0201 for ac coupling?
Shouldn't it distort the videosignal with higher capacitance overspecs?
OR simply increase the signal quality because the lower lowfrequency impedance?
So only the impedance plays the fundamental role (0402 0.1uf vs 0201 10uf)
Ergo increasing capacitance given the same capacitor quality (x7r c0g etc) could only improve the signal?
One problem is, what is the definition of "could only improve the signal"?
RCL filters alter the frequency response of the signal path. What does one need to do to "improve the signal"?

#### nsaspook

Joined Aug 27, 2009
13,265
No, no, those aren't conclusions at all. I was just hypothesizing of what could happen.
It's known the effect of impedance and capacitance between higher and lower frequencies.
Just what would happen in the AC coupled circuit as it is in HDMI or Displayport if we increase or reduce capacitance specifically.

Looking for some paper explaining about this but couldn't find anything online, atleast as far as i've found
It depends on the nature of the capacitance. If it's a pure (ideal) capacitance (no such thing but it can be pretty close if chosen correctly), then adding more would increase the coupling but it's the nature of diminishing returns. If 99.8 percent of signal is coupled across the desired range of frequencies with a 0.1uf then adding a 10uf might be an improvement of 0.01 percent. But adding the extra non-ideal capacitance might decrease coupling due to parasitic effects.

You need to increase your search foo. Google: how to select AC coupling capacitors
First few results:

Last edited:

#### Danieledp

Joined Oct 22, 2021
7
One problem is, what is the definition of "could only improve the signal"?
RCL filters alter the frequency response of the signal path. What does one need to do to "improve the signal"?
Allowing more signal to be transmitted without any "bottleneck" (speculation, theorycrafting)
OR improve the signal impedance by selecting the capacitor specifically tailored for the desired AC Coupled frequency.

It depends on the nature of the capacitance. If it's a pure (ideal) capacitance (no such thing but it can be pretty close if chosen correctly), then adding more would increase the coupling but it's the nature of diminishing returns. If 99.8 percent of signal is coupled across the desired range of frequencies with a 0.1uf then adding a 10uf might be an improvement of 0.01 percent. But adding the extra non-ideal capacitance might decrease coupling due to parasitic effects.

You need to increase your search foo. Google: how to select AC coupling capacitors
First few results: