Decoupling Cap across the postive and negative pins

[mudihan]

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]

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]

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]

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]

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

 

[AndrewT]

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]

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?