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From: "Edward Rawde" <invalid@invalid.invalid>
Newsgroups: sci.electronics.design
Subject: Re: signal leads that pick up less ambient noise?
Date: Wed, 26 Feb 2025 10:59:47 -0500
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"Bill Sloman" <bill.sloman@ieee.org> wrote in message news:vpmnih$2gl7g$1@dont-email.me...
> On 26/02/2025 4:55 pm, Edward Rawde wrote:
>> "Bill Sloman" <bill.sloman@ieee.org> wrote in message news:vpm0qm$29gbe$1@dont-email.me...
>>> On 26/02/2025 4:39 am, Christopher Howard wrote:
>>>> Bill Sloman <bill.sloman@ieee.org> writes:
>>>>
>>>>> On 25/02/2025 4:46 am, Christopher Howard wrote:
>>>>>>> Google for coaxial feed through capacitors.
>>>>> Capacitors and resistors don't ring. Adding inductance can introduce
>>>>> ringing. but enough resistance can make the resonant circuit
>>>>> critically damped and the voltages and current will decay
>>>>> monotonically.
>>>>
>>>> So, when you use a coaxial feed through capacitors on your Faraday cage,
>>>> do you add a resistor right after the capacitor, to reduce/eliminate
>>>> ringing?
>>>
>>> The whole point about coaxial connectors is that the distributed capacitance and inductance gives you a R50R transmission line.
>>> The only way to get "ringing" out of that is to fail to terminate the transmission line with  it's characteristic impedance. In
>>> practice it is hard to do it perfectly and you do tend to get low level reflections, but they die out fast,
>>>
>>>> Or are you just trying that all your inputs on the board have
>>>> resistors before whatever op amps or other components that they feed
>>>> into?
>>>
>>> The message is rather more complicated than that. The later editions of Ralph Morrison's book do go into that in more detail 
>>> than
>>> the earlier editions.
>>
>> The sixth edition only mentions the "feed-through" capacitor in one paragraph on page 65.
>> The fifth edition does not mention them at all as far as I can tell.
>
> Feed-through capacitors seem only to be used in RF electronics,

So how is it that when I Google feedthrough capacitor use I get this:

"Feedthrough capacitors are used to filter out electromagnetic interference (EMI) and radio frequency interference (RFI) in 
electronic devices.
They are used in a wide range of applications, including medical equipment, power supplies, and communication systems."

Ok that was generated by AI, and it's true that in the last few days I've had at least one blatantly wrong response.
So I tend to ignore the AI generated responses but in this case I'd say it's reasonable.

> and Ralph Morrison's book initially concentrated on regular industrial electronics. Later editions did move on to higher frequency 
> applications.
>
> I never used a feed-though capacitor anywhere in  the work I did - if we need to put a fast signal through a conducting bulk-head 
> we used coax feed-throughs.

The OP does not need coax feed throughs.
Simply using a ground plane may be sufficient,

> Feedthrough capacitors are relatively exotic devices.

I don't see anything exotic about a capacitor to ground at a metal enclosure boundary.
I can imagine a road sign at the boundary.
Noise and RF follow ground.
Low frequency signals keep straight ahead.

If the OP needs filtering at all then a capacitor to ground at the enclosure boundary is likely more than sufficient.
Going to the trouble of putting it at the enclosure boundary is also possibly not necessary in this case.
Put RC filtering on the board as needed.
Feedthrough capacitors are becoming more expensive due to not being used as extensively as they were in the past.

>
> Your example of their application seems to be a case where an RF specialist went in for a bit of over-kill.

Actually this is just a case of you making yet another completely wrong assumption.

>
> -- 
> Bill Sloman, Sydney
>
>