Decoupling Cap across the postive and negative pins - diyAudio
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Old 16th April 2009, 02:39 AM   #1
mudihan is offline mudihan  United States
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Default Decoupling Cap across the postive and negative pins

This is regarding op amp bypassing. I have seen people putting a 0.1uf film/ceramic cap across the positive and negative power supply pins, instead of one cap to the ground for each of the pins. Why? And are there any advantages/disadvantages doing the former instead of the latter (besides saving a bit of money and slightly easier implementation)? Also, is there any point doing both?
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Old 16th April 2009, 09:28 AM   #2
Atilla is offline Atilla  Norway
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http://www.designers-guide.org/Design/bypassing.pdf

This link could give you a little more detailed info about where and how caps are being placed and the desired effect on the circuit in mind.
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Old 16th April 2009, 11:02 AM   #3
AndrewT is online now AndrewT  Scotland
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Hi,
some opamps have one cap from V+ to V- and a second cap from Ground to one Vsupply. The data is usually in the datasheet.

It depends very much on how the internal connections are run inside the IC.
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Old 16th April 2009, 06:26 PM   #4
mudihan is offline mudihan  United States
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Quote:
Originally posted by AndrewT
Hi,
some opamps have one cap from V+ to V- and a second cap from Ground to one Vsupply. The data is usually in the datasheet.

It depends very much on how the internal connections are run inside the IC.

Thanks. I thought that it's another kind of general approach.
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Old 17th April 2009, 11:46 AM   #5
Atilla is offline Atilla  Norway
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Which opamps have a direct path from positive to negative supply, so that they'll require a bypass directly rail-to-rail?
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Old 17th April 2009, 12:29 PM   #6
AndrewT is online now AndrewT  Scotland
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If I understand your question correctly then I'd suggest that all opamps have this direct path since none have a ground with the 8pins normally available.
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Old 17th April 2009, 12:50 PM   #7
Atilla is offline Atilla  Norway
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Hm, I didnt word this correctly at all. Of course, most opamps don't even have a ground pin.

I mean that in the usual Class AB amplification, current flows go from one rail to ground, at a time, which is the path you'd want to bypass, thus having bypass caps from each rail to ground, to assist you with short high-current demand. In such an application, how does a bypass between the rails help, I'm not sure I'm getting it right?
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