Value for a DC rail shunt cap? - diyAudio
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Old 25th February 2015, 01:33 AM   #1
andyr is offline andyr  Australia
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Default Value for a DC rail shunt cap?

AIUI, it is "good practice" to put a cap from DC rail to ground, right next to any opamp or transistor/JFET.

Is there a "rule of thumb" for deciding what value this cap should be - say, related to the voltage on the DC rail ... and/or the current going through the device?

I'm currently using 2x 22uF Wima MKS4 for this purpose, on a +24v rail and these take up a fair bit of real-estate ... so I was wondering if 1x 22uF Wima would be fine - or even less? This is for JFET pairs & quads passing either 10 or 20ma in total.


Thanks,

Andy

Last edited by andyr; 25th February 2015 at 01:37 AM.
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Old 27th February 2015, 10:16 AM   #2
AndrewT is offline AndrewT  Scotland
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What opamp have you got?
Did you download the datasheet for your opamp?
Did you read the datasheet for your opamp?
What did the manufacturer state in the datasheet for local decoupling? They may refer to it as "bypassing", but that to me means something quite different from local decoupling.
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Old 27th February 2015, 11:18 AM   #3
andyr is offline andyr  Australia
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Quote:
Originally Posted by AndrewT View Post
What opamp have you got?
Did you download the datasheet for your opamp?
Did you read the datasheet for your opamp?
What did the manufacturer state in the datasheet for local decoupling? They may refer to it as "bypassing", but that to me means something quite different from local decoupling.
Actually, Andrew, as I said ... I am using JFETs - not opamps. 2SK170s, to be precise - and their datasheet makes no mention of "local decoupling/bypassing" (that I can find).

Andy
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Old 27th February 2015, 11:57 AM   #4
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You did say opamps...

2x 22uF seems overkill to me. I would also think with the size of those wima things that they have a fair amount of inductance.
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Old 27th February 2015, 03:38 PM   #5
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by andyr View Post
........... it is "good practice" to put a cap from DC rail to ground, right next to any opamp or transistor/JFET................
Quote:
Originally Posted by andyr View Post
Actually, Andrew, as I said ... I am using JFETs - not opamps. .......................
you said "opamp"

I am not the only one that noticed what you posted.
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Old 27th February 2015, 07:47 PM   #6
andyr is offline andyr  Australia
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Quote:
Originally Posted by richie00boy View Post

2x 22uF seems overkill to me. I would also think with the size of those wima things that they have a fair amount of inductance.
Thanks. I posted my Qu also on Tweaker's Asylum where someone suggested 10uF was the norm - so that's what I'll use instead.

I wasn't aware of the potential inductance issue ... are you saying size --> inductance? In which case:
* a 10uF Wima is much smaller, so that's good.
* but a 1uF 'boutique' cap which is typically used for coupling duties is physically much larger ... so will these large caps have inductance, too?


Regards,

Andy
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Old 28th February 2015, 04:54 PM   #7
DF96 is offline DF96  England
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All caps have inductance. As a rough guess, assume the same inductance as a piece of wire of the same length as the cap plus its leads. There is no need for 'boutique' caps for decoupling (well, actually, there is almost no need for boutique caps anywhere).

Decoupling cap value and type depends on context: analogue audio, digital, RF? Likely signal rise time in current terms? Maximum tolerated supply rail voltage excursion? Supply rail inductance and resistance?

Some locations need a lowish value electrolytic. Others need a highish value electrolytic. Others need a physically small but highish value ceramic cap, which would be bad as a coupling cap but perfect for decoupling. The only 'rule of thumb' I know is that there isn't a rule of thumb.
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