Fully diff to Single ended

Hi all,
I was wondering...
Can I connect one of the outputs of a fully differential opamp to ground and use it as a single ended opamp?

If I short one of the inputs to ground I set it as a reference, so at the input this will work. It should work pretty much the same at the outputs. My only issue is with the common mode. Will it be destinctly defined if I short one of the outputs?

Am I missing something all together?
Any thoughts or coimments are welcome.

Thank you!

No you cannot. Fully differential need to be very symetrical: each half of the opamp must have aproximatelly the same current as the other. By connecting one of the outputs to ground you will be changing that side's current significantly which will cause some of your transistors to enter triode region.

However you can use a Fully differential opamp as a Single-Ended in another way: just use one of the outputs and ignore the others. The outputs are symetrical, if one output's voltage increases the other decreases by the same amount. If you use one of the ouputs you'll still have your signal but with half the gain.

This only works in open-loop. If you try to make an inverting or non-inverting amplifier you'll probably fail.

Thanks for the info.
How can I "ignore" an output? I can`t just leave it hanging mid-air.
Even if I did, the other output is capacitvly loaded by the next stage and the two branches will not be identical.
I actually need this for a summing amp. So I need it closed-loop anyway.

Thanks.

How can I "ignore" an output? I can`t just leave it hanging mid-air.

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Yes you can only if you use it in open-loop configuration. Both sides of the opamp will have the same DC operating point and opposite variations to input signals. When you apply a variation in the input signal both outputs will change by the same measure but opposite to each other. However this is only true for input signals with the same DC voltage. Different DC input voltages will unbalance the circuit.

Even if I did, the other output is capacitvly loaded by the next stage and the two branches will not be identical.

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Capacitive loads do not change DC operating point so they will not change the circuit's symmetry. You'll only have problems with the capacitive load if it's impedance is smaller than the circuits output impedance.

I actually need this for a summing amp. So I need it closed-loop anyway.

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In that case you should use a single-ended opamp anyway. There is no point in trying to adapt one circuit to do a thing it wasn't designed for. Is there any special reason why you want to use the fully differential for your summing amp?

Is there any special reason why you want to use the fully differential for your summing amp?

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Sure there is, it`s just not a good one
I have a fully diff amp I`m already using in the project, and it`s a great design and I wanted to reuse it if possible, but I`ll just have to go back to the drawing board and create another single ended design.

Thanks!

If you are using a fully differential circuit you also would need to use a CMFB circuit. Otherwise there is no problem.