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Old 12th April 2010, 08:45 AM   #11
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For 4562 local decoupling should be 1 cap between +and- and 1cap from - to ground.
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Old 12th April 2010, 09:06 AM   #12
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Based on what Naz?
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Old 12th April 2010, 09:07 AM   #13
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Quote:
Originally Posted by SoNic_real_one View Post
Low ESR caps on power lines can make more damage than good... The rails inductance coupled with the capacitor needs some damping - usually the "normal" ESR is enough for that. If you go with too lower ESR you will get ringing and other artifacts on the rails.
As for the ideea that caps between rails and ground will "dump" noise there - that is nonsense perpetuated by urban legends.
By the way an OpApm is build, the currents will flow thru the ground back to the PS - on each alternance to the other rail. And that's the current you need to bypass!
Not between the rails, that will ADD more current to bypass at the PS on the opposite rail! Bigger loops instead of short ones.
Best post so far.....
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Old 12th April 2010, 12:47 PM   #14
marce is offline marce  United Kingdom
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Have a look around the web at decoupling, bypass capacitors and EMC, I think you'll find Naz is correct. Based on thousands of designs and work done on this subject.
The Urban legend is a big base for a lot of EMC engineering in electronics!
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Old 13th April 2010, 12:55 AM   #15
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Seems pretty non standard form what I have seen. Do you have a link to show me please?
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Old 13th April 2010, 01:24 AM   #16
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Quote:
Originally Posted by SoNic_real_one View Post
Low ESR caps on power lines can make more damage than good... The rails inductance coupled with the capacitor needs some damping - usually the "normal" ESR is enough for that. If you go with too lower ESR you will get ringing and other artifacts on the rails.
Urban legendary nonsense. Ringing only occurs when there's something to set it off- do bells ring in the absence of being struck? But its true that low ESR caps can do more harm than good... see below.

Quote:
As for the ideea that caps between rails and ground will "dump" noise there - that is nonsense perpetuated by urban legends.
You're quite welcome to your own opinion. However I've simmed this in LTSpice so the effect is a real one. Ignore it at your peril

Quote:
By the way an OpApm is build, the currents will flow thru the ground back to the PS - on each alternance to the other rail. And that's the current you need to bypass!
As I've set out on another thread where this came up, there are cases where the current from the opamp does not return to ground. When feeding another opamp for example, or when the opamp is feeding into a chipamp.

Quote:
Not between the rails, that will ADD more current to bypass at the PS on the opposite rail! Bigger loops instead of short ones.
Do please explain this further as I'm not clear where you're coming from.
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Old 13th April 2010, 01:33 AM   #17
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Another interesting post. Thanks

As I understand it decoupling is basically the process of isolating one stage of an amplifier or circuit from another. This is traditionally done by a locally placed cap from rail to ground. Its supposed to stop noise entering the next stage right?

How does a cap from +VE to -VE have the same effect in bi-polar situations?

Also, if your LTspice sims prove that the traditional method dumps noise on the GND (which its supposed to do no?), then where does the noise go if the cap is between +/- rails?
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Last edited by mikesnowdon; 13th April 2010 at 01:35 AM.
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Old 13th April 2010, 01:41 AM   #18
dimitri is offline dimitri  United States
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cap between +and- and cap from - to ground came from:
http://www.analog.com/static/importe...ets/AD8099.pdf
also you might have a look:
http://focus.ti.com/lit/an/sloa069/sloa069.pdf
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Old 13th April 2010, 01:43 AM   #19
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More importantly....

Has anyone compared the 2 techniques on the same opamp in the same circuit, and could comment on the change in sound for better or worse? If you have I'd very much like know how it affected the sound quality.

Thanks in advance
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Old 13th April 2010, 01:44 AM   #20
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Nice one Dimitri

EDIT: That second link is superb! I will read it properly tomorrow. Bit tired now so off to bed. G'night all!
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Last edited by mikesnowdon; 13th April 2010 at 01:47 AM.
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