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Old 18th May 2015, 08:50 AM   #1
johnm is offline johnm  United Kingdom
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Default Op-amp decoupling - best practice?

Hi folks.

I was wondering if there's any definitive rule on which approach is best in regards to op-amp bypasses: a single 100nF cap across the + and - pins, OR a 100nF cap from + to ground, and another from - to ground?

I have read so many conflicting opinions on this over the past month I don't know what to think. Some think mounting the cap across the pins does nothing. Others believe coupling to ground can inject noise into the op-amp. A few differing opinions I've read recently include:

From Rod Elliot:

Coupling Capacitors

"High speed opamps must have good bypassing. Most of the time, this will be between the power supplies, avoiding the earth (ground) circuit completely. A normal opamp has no knowledge of earth, ground planes or anything else earth related. It is only interested in the voltages present at its two inputs, and when used in linear mode will attempt to make them the same voltage."

From DaDa (Quad modification web forum & shop):

Quad 34 and DaDa's new LME op-amp upgrades

"My experiences with decoupling of LME's are the opposite, the oscillation will not go away with ceramics between the plus and the minus. Only when decoupled both supplies (from + & - to ground) the oscillation is cured. The oscillation of this kind is a LF oscillation, a loud, in most cases 100Hz, based noise is there. Although I believe you, my next remark will contradict this, I doubt if you really can hear the placement of a decoupling cap."

And from Douglass Self:

http://www.eetimes.com/document.asp?doc_id=1278963

"The essential requirement is that the positive and negative rails should be decoupled with a 100 nF capacitor between them, at a distance of not more than a few millimeters from the op-amp; normally one such capacitor is fitted per package as close to it as possible.

It is not necessary, and often not desirable, to have two capacitors going to ground; every capacitor between a supply rail and ground carries the risk of injecting rail noise into the ground."

In my case I'm using a Quad 34 (which has a virtual ground) which has had most of the coupling caps removed, and different op-amps tried. OPA134s 'sound' very natural/organic to me, and don't require any decoupling caps to operate well in this circuit. Trying LME49710/20s - a superior op-amp, on paper - and it oscillated. So following DaDa's advice I bypassed from + to ground, and - to ground on one of the op-amps which cured the oscillation hum. But I was left with the impression they weren't performing at their best - sounded a little too 'lean' and boring?! I had to remove them after a week as I wasn't enjoying the music. I was wondering if the Quad 34s virtual ground in its PSU supply was injecting a little noise into the LMEs via the op-amp decoupling caps perhaps?

Any thoughts about this would be welcome to this 'non expert'

Cheers,

John.

Last edited by johnm; 19th May 2015 at 11:53 AM.
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Old 19th May 2015, 10:55 AM   #2
johnm is offline johnm  United Kingdom
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118 views and no replies? :/
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Old 19th May 2015, 11:38 AM   #3
Mooly is offline Mooly  United Kingdom
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118 views and a million and one possible answers.

Opamps typically need to see a low impedance between the supply pins which in practice means a small cap of say 10uf. Should it be low ESR ? You need to know your enemy... if there is a lot of spiky hf on the rails (a switching supply maybe) then a low ESR cap may cause ringing and such like. Decoupling from rail to ground can actually feed noise on the rails into the ground scheme which again can cause further problems.

So each case is different.
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Last edited by Mooly; 19th May 2015 at 12:01 PM. Reason: 'below' should have read 'be low'
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Old Yesterday, 02:55 AM   #4
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Some famous hi-end, expensive manufacturer use 100nF MKS/MKT decoupling for op-amp in their circuits.
Stay away low ESR caps.
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Old Yesterday, 04:47 AM   #5
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Quote:
Originally Posted by johnm View Post
Hi folks.

I was wondering if there's any definitive rule on which approach is best in regards to op-amp bypasses: a single 100nF cap across the + and - pins, OR a 100nF cap from + to ground, and another from - to ground?
I think 99.999% have caps at the supply pins down to ground if the load is connected to ground.
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