AC coupling caps

Ricky · 2023-04-24T17:51:12+00:00

Typically, an AC coupling cap is driven from a low impedance source and only the load needs to be factored into the frequency equation. But when the cap is on the input, is it typical to factor in the expected source impedance, or assume a worse case of low driving impedance? I have a design with 600 ohm differential audio inputs and I keep seeing a different response on the input and output. I finally realized this was because my input simulation used a low impedance drive, but the output included the output impedance and the load impedance. Duh! So, do I need to beef up the input caps to handle a low impedance driver? Or, I guess I'm asking, what is typically expected of the frequency response at the input? Is it typically for a rated driving impedance or for all source impedances? Not much point in asking my customer. He has indicated that the various test gear this will be connected to is all over the map on this. That's one reason why setting a dBm level on the test gear output does not give consistent voltages measured at the interface. I don't expect he will know much more about the user equipment this will be connected to. The 47 uF input caps required for worse case are a bit on the large size and the voltage rating is limited. I'd prefer to use 33 uF caps if practical. I guess this caught me by surprise. I thought I had completed this design, but I moved the input caps to a slightly different point in the circuit, and things changed a lot.

AC coupling caps

Apr 24, 2023 Last reply: 3 years ago 45 Replies

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Phil Allison 3 years ago

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** Who's definition is that ? Yours?

The word " mismatch" implies there is a flaw or error involved in the combination. Which is not the case here.

...... Phil

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John Larkin 3 years ago

To most engineers, it means that the source and load impedances are not the same as the transmission line impedance. It's not a moral judgement.

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Phil Allison 3 years ago

** Which IS usually a design flaw.

** Just a colossal misnomer from clueless types like JL and Tabby.

....... Phil

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John Larkin 3 years ago

Not in an RF or fast pulse system.

Audio is your world, but not everyone's.

Google for TDR

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Phil Allison 3 years ago

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** Google " usually ".

** So JL is in fact deaf. Thought so.

** Yawnnnnn.....

Goggle " misnomer " .

..... Phil

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John Larkin 3 years ago

At my age I've lost some HF gain, but my hearing is very sensitive otherwise. I can hear birds chirping and kettles boiling before most other people.

Not interested? TDR is fascinating. You could use it on even your RCA-connectorized cables. Start a business to sell TDR-tested RCA cables for big bucks. Critics will rave over the difference in the microgranularity or the soundstaging or something.

The world does not end at 20 KHz.

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