Common Mode feedback circuit

[m_kuty]

Hello everyone

I trying to design Fully differential Amplifier and I am stock in Common Mode feedback circuit. what should be the gain of Common Mode feedback circuit , unity, positive or negative dB.

 

[KlausST]

Hi,

There are a lot of threads about the same topic.
Did you try the links in the box below "similar threads"?

Klaus

 

[m_kuty]

Thank you for reply,
Yes I looked to them. but I didn't Find my answer.
The problem is Tran analysis goes in wrong way. I tried to simulated as the circuit (attachment) and the result. I don't not when are the errors

 

[KlausST]

Hi,

From your signal waveform it seems as you didn´t set V_COM at all.

--> Show us your circuit, with your voltages not a datasheet circuit.

***
I personally don´t like the phrase "feedback" in this case. V_COM is an input to set the common mode voltage of the output signals.
It should be somewhere between the supply voltages, but not close to them.
I see no true "feedback".

All this should be explained in the datasheet.

***
Often one uses a differential amp to feed a (high speed) ADC. If so, then often the ADC has a V_COM output. Connect it to the amplifier_V_COM input.

Klaus