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-   -   Decoupling Cap across the postive and negative pins (http://www.diyaudio.com/forums/chip-amps/142373-decoupling-cap-across-postive-negative-pins.html)

mudihan 16th April 2009 02:39 AM

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:)?

Atilla 16th April 2009 09:28 AM

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.

AndrewT 16th April 2009 11:02 AM

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.

mudihan 16th April 2009 06:26 PM

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.

Atilla 17th April 2009 11:46 AM

Which opamps have a direct path from positive to negative supply, so that they'll require a bypass directly rail-to-rail?

AndrewT 17th April 2009 12:29 PM

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.

Atilla 17th April 2009 12:50 PM

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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