CMRR at different closed loop gain

[Junus2012]

Dear friends,

I am searching for some instrumentation amplifier IC, I have noticed from data sheets they show the CMRR at different closed loop gain,

It is strange for me because in all of the op-amp design CMRR is supposed to be one value and should be the same for every value of closed loop gain or open

Thank you

 

[frankrose]

Datasheets are written by different engineers from different countries, not everybody is thinking on the same way, sometimes they make mistakes too when they write the datasheet (in many cases not the designers write the datasheets).
I don't think CMRR should be the same next to any condition, definitely it depends on the circuit/feedback network around it. Datasheets should contain the worst case open-loop CMRR I think, but it is not bad if there are more information, like what happens with different closed loop gain values, with 5% tolerance devices, etc.

 

[Junus2012]

Thank you Franjk for your reply,

Regardless the change from data sheet to other, and assuming perfect feedback matching,

Do you think the CMRR is closed loop gain dependent ?

In my circuit I tested it under different closed loop gain and the result about the same

 

[frankrose]

Feedback of fully differential amplifier should improve the CMRR I guess (assuming perfect resistor matching...).
But even without feedback the fully diff. OPAmps have great CMRR usually. This is why you didn't see too much difference I think.
Link the datasheet rather where did you see the swept gain vs CMRR.

 

[vivekroy]

Dear friends,

I am searching for some instrumentation amplifier IC, I have noticed from data sheets they show the CMRR at different closed loop gain,

It is strange for me because in all of the op-amp design CMRR is supposed to be one value and should be the same for every value of closed loop gain or open

Thank you

Does the closed loop gain change with your feedback network? If it does (for example due to loading), then theoretically, CMRR by definition changes.

 

[FvM]

The question is about instrumentation amplifiers. They are usually comprised of a programmable gain first stage and a differential-to-single-ended second stage. Bothe have different input referred CMRR, resulting in different total CMRR for different gain settings of the first stage. In so far it's normal operation.