# Common mode rejection

#### magnet

Joined Apr 25, 2005
16
Does anybody have any links or information on CMR.

If I have a transmission system with 5 section all having a CMR, how can I calculate the overall CMR??

Thanks

#### magnet

Joined Apr 25, 2005
16
Originally posted by magnet@Nov 8 2005, 10:09 PM
Does anybody have any links or information on CMR.

If I have a transmission system with 5 section all having a CMR, how can I calculate the overall CMR??

Thanks
[post=11535]Quoted post[/post]​
If the sections had individual CMRvalues of say 110db, 150db, 165db, 120db & 175db. Can anybody help in the calculation for the overall CMR value??

#### hgmjr

Joined Jan 28, 2005
9,027
Originally posted by magnet@Nov 16 2005, 06:04 PM
If the sections had individual CMRvalues of say 110db, 150db, 165db, 120db & 175db. Can anybody help in the calculation for the overall CMR value??
[post=11698]Quoted post[/post]​
Hi magnet,

CMR or perhaps more accurately CMRR (Common Mode Rejection Ratio) is generally associated with a differential amplifier. Differential amplifiers are most often used to convert a differential signal source into a single ended signal. Once the signal is single-ended, CMRR is no longer a consideration. I can not immediately imagine a scenario that would involve multiple differential amplifier stages.

All that being said, is your question a hypothetical one or have you a specific circuit in mind?

hgmjr

#### n9352527

Joined Oct 14, 2005
1,198
Originally posted by magnet@Nov 8 2005, 11:09 PM
Does anybody have any links or information on CMR.

If I have a transmission system with 5 section all having a CMR, how can I calculate the overall CMR??

Thanks
[post=11535]Quoted post[/post]​
For a common signal at the first section, the CMRR at the last section would be the sum of all the sections CMRR. For a common signal at the second section, the CMRR at the last section would be the sum of all the sections CMRR bar the first section.

It is just another way of stating the ratio of the differential gain and the common signal gain between two points. After passing each section it is 'rejected' by a certain dB value. If you work in gain instead of dB then you can see that the amplified common signal is attenuated by a certain value. Two attenuations in series are equal to the first att. multiplied by the second att. If you work in dB then it is the sum of the first and the second attenuation in dB.

#### Dave

Joined Nov 17, 2003
6,969
Try this section from the All About Circuits on-line textbook.

Dave