What are the CMRR impacts on negative-connect unit gain buffer

[mirrobbs]

Hello.
Can anyone give a explanation about the CMRR impacts on negative-connect unit-gain buffer (using single-end opamp)?

Thanks in advance.

regards,
Mirro

 

[adi0803199]

Can you post the schematic of the circuit so as to better understand your question.Is this the circuit that you are referring to ,

 

[mirrobbs]

Yes, exactly. My question is will the CMRR of opamp affect performance of buffer?
And if two channels are implemented, how can we evaluate the CMRR regarding the differential signal between them?
Below shows the schematic；


Thanks in adv.
Regard
Mirro

 

[kenambo]

Hi,
Yes of course, The CMRR will affect op-Amp especially in Buffer configuration

In unity buffer configuration, if CMRR is small then the difference between the input and output gets increase...

Higher the CMRR good buffering action of OP-AMP

and in the 2 buffer configuration the CMRR should be measured independently because you are giving independent inputs.. and you get same CMRR in theoretical...
But after the extraction.. you may get different results...

Thanks

 

[mirrobbs]

Hi,
...In unity buffer configuration, if CMRR is small then the difference between the input and output gets increase...

and in the 2 buffer configuration the CMRR should be measured independently because you are giving independent inputs.. and you get same CMRR in theoretical...
But after the extraction.. you may get different results...

Thanks, Kenambo.

I can understand the 1st point you put forward intuitively that 'In unity buffer configuration, if CMRR is small then the difference between the input and output gets increase. '

And as for the two-channel implementation, you can assume the inputs are Vinp and Vinn of some kind of differential signal. So how should we evaluate the CMRR? If you can formulate the problem, I think that's better to completely undertand.

Thanks in adv.
regard,
Mirro

 

[adi0803199]

If the both the op-amps used are the same,then the CMRR of the op-amps can be calculated using Datasheet.If we are giving a differential signal,then the difference between the Vinn and Vinp will be the same if the CMRR of two op-amps are the same.Even though the output will now be a less lower than the input(Vinn or Vinp) at the op-amp input terminal,the difference is still the same.This is the advantage of using the two buffers.Hope this helps.

 

[kenambo]

Hi mirro,

nice to see your response... and for the two channel implementation.. i tell my point of view only.. and its my assumption...

In buffer configuration i think we cant determine CMRR.... You have to use open loop configuration to determine the gain and CMRR...

Since its a buffer, The input is always common... and the gain is unity.. so we can't differentiate differential gain and common mode gain..

So an Op-Amp should be checked open loop or closed loop(it should not be a buffer) for CMRR and differential gain..

it's my assumption only..
Thanks...