# Common mode rejection

Discussion in 'Homework Help' started by magnet, Nov 8, 2005.

1. ### magnet Thread Starter Member

Apr 25, 2005
16
1
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

2. ### magnet Thread Starter Member

Apr 25, 2005
16
1
If the sections had individual CMRvalues of say 110db, 150db, 165db, 120db & 175db. Can anybody help in the calculation for the overall CMR value??

3. ### hgmjr Moderator

Jan 28, 2005
9,030
214
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

4. ### n9352527 AAC Fanatic!

Oct 14, 2005
1,198
4
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.

5. ### Dave Retired Moderator

Nov 17, 2003
6,960
145
Try this section from the All About Circuits on-line textbook.

Dave