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To be or not to be single

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TechnoGilles

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I'm talking about power supply of course !

I have and op-amp specified for ±2.5V to ±17V.

Am I right to say that if I have a power rail of 12V and a reference rail of 6V, I can use the part and it would be equivalent as ±6V ?

If so, how would I decouple it with bypass capacitors ? Between 12V and GND only ? Or between 12V-6V and also between 6V-GND ?

Thanks !
 
I forgot to mention that I understand I would have to use proper AC coupling where needed in that case...
 
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The op amp is oblivious to the actual power supply, so this part can be used on a single supply from 5V to 34V. Yes you are right in saying that this is the same as using the part as +/- 6V. For decoupling, put a cap from 12V to 0V and one from 6V to 0V. Do not put one from 12V to 6V, or this will couple noise from the 12V into the reference. Ultimately this depends on what your input signal looks like. Does this go below 0V? If so, you need a power supply that goes below 0V so the op amp can recognise signals lower than 0V too.
 
The op amp is oblivious to the actual power supply, so this part can be used on a single supply from 5V to 34V. Yes you are right in saying that this is the same as using the part as +/- 6V. For decoupling, put a cap from 12V to 0V and one from 6V to 0V. Do not put one from 12V to 6V, or this will couple noise from the 12V into the reference. Ultimately this depends on what your input signal looks like. Does this go below 0V? If so, you need a power supply that goes below 0V so the op amp can recognise signals lower than 0V too.

The signal will be AC coupled and referenced to 6V. So no negative voltage w/r to the op amp.

I had the feeling that it would be ok from the op amp point of view. It's really the decoupling thing that got (and still gets) me confused !

So if it's the same for the op amp, why is is different for the decoupling ? Is it purely because decoupling should always be w/r to ground ?
 
Just imagine decoupling as providing a low ac impedance from one point to another. If you have noise on your 12V line and put a decoupling cap to ground, you are providing a short circuit (for the noise) from the 12V down to ground. You can decouple to the 6V if you want, as long as the 6V is beefy enough to withstand signals being injected into it via the decoupling cap. I would not recommend it, especially as you are then asking the 6V to be rock solid as a reference for your signal. This is why decoupling is normally done down to 0V, because the 0V line normally has a low enough impedance to be able to absorb anything that the decoupling cap shunts down to it. The purists (and there are plenty on forums like this) would argue that you should never assume the 0V is true 0V. It will always have some noise on it, but (hopefully) the op amp will have a lot of noise rejection inside it (called power supply rejection ratio or PSRR) to remove this. By definition, the signal input has none, so you are always best decoupling to 0V.
 
Have a look at the opamp 'sticky' at the top of the General Electronics Chat forum.

https://www.electro-tech-online.com/threads/basic-opamp-circuits.35258/

The diagrams are indeed interesting, thanks. But my question about the decoupling caps kinda remains...

The caps shown in the diagrams are located at unusual places. Normally, we find them near the op-amp supply rails. What Audioguru seem to suggest is that they should be placed near the non-inverting input of the op-amp. Am I interpreting the diagrams correctly ?

If so, what about dual or quad op-amp packages : should the bypass capacitors be repeated for each of them instead of once for the package ?
 
The caps shown in the diagrams are located at unusual places. Normally, we find them near the op-amp supply rails. What Audioguru seem to suggest is that they should be placed near the non-inverting input of the op-amp. Am I interpreting the diagrams correctly ?
The caps ARE located near the supply rails, since the resistive voltage dividers are being used to establish the supply rails. It's a coincidence that the dividers are drawn near the op-amp inputs.
 
The caps ARE located near the supply rails.

Ok, so for the case of quad op-amps in the same chip (e.g. DIP14), I understand that there woule be one cap from 12V to GND near the chip's V+. But for the 6V reference which is used to feed the non-inverting input of the 4 op-amps, should it be decoupled once for the chip or should it be decoupled for each individual op-amp ?
 
One decoupling cap should suffice. You just want to make sure that any noise on the 6V line gets smoothed out by the cap (or shunted to ground if this is easier to visualise), so your 6V is nice and flat and not bouncing around
 
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