Common Mode Rejection (CMR)

Thread Starter

magnet

Joined Apr 25, 2005
16
[attachmentid=639]


Please refer to the attached diagram

I am in need of help in calculating the change in decibels of the CMR if impedance falls by a factor of 500.

Thanks
 

hgmjr

Joined Jan 28, 2005
9,029
I may be wrong but the schematic you have posted looks a bit odd.

The two resistors labeled Re are in parallel and both ends of these two resistors are tied to ground. That pretty much means they have nothing to contribute to the circuit.

Also the amplifier looks like it might be an opamp. If that is the case, then it would be helpful to indicate which input is the negative one and which is the positive one.


hgmjr
 

Thread Starter

magnet

Joined Apr 25, 2005
16
Originally posted by hgmjr@May 12 2005, 09:31 PM
I may be wrong but the schematic you have posted looks a bit odd.

The two resistors labeled Re are in parallel and both ends of these two resistors are tied to ground. That pretty much means they have nothing to contribute to the circuit.

Also the amplifier looks like it might be an opamp. If that is the case, then it would be helpful to indicate which input is the negative one and which is the positive one.
hgmjr
[post=7656]Quoted post[/post]​
Sorry, please refer to the new attachment.
[attachmentid=642]
 

Brandon

Joined Dec 14, 2004
306
Originally posted by mozikluv@May 16 2005, 10:23 PM
hi

where is the feedback network?
[post=7744]Quoted post[/post]​
There is no need for a feedback network. This is just a typical difference amplifier application but done with an op-amp. If you remember, the op-amp's input is 2 Darlingtons connected in via differential input.

Telling you how to find the CMR is some technical data that got pushed out by other crap. Maybe if you attempted to redraw it into a format like what ever you covered in class for CMR.
 

Thread Starter

magnet

Joined Apr 25, 2005
16
Originally posted by Brandon@May 18 2005, 07:07 PM
There is no need for a feedback network. This is just a typical difference amplifier application but done with an op-amp. If you remember, the op-amp's input is 2 Darlingtons connected in via differential input.

Telling you how to find the CMR is some technical data that got pushed out by other crap. Maybe if you attempted to redraw it into a format like what ever you covered in class for CMR.
[post=7786]Quoted post[/post]​
This question is on a distance learning course and is only covered by a description within the unit.

I you know how to help or where I may find information on this, especially where calculations are involved, it would be greatly appreciated.

Thanks
Magnet
 

beenthere

Joined Apr 20, 2004
15,819
Hi,

Your CMR will always be infinite with a perfect device - considering the op amp input as a differece amp. Without Rs, this would be the case for your example - both Re's are identical, so an common voltage applied to the inputs will leave the differential input balanced.

The voltage drop across Rs will unbalance the voltages felt by the input resistors, and cause a shift in the output. If you think about it, lowering Rs by 500 ohms will decrease the voltage across it, and thus improve CMR.
 

Brandon

Joined Dec 14, 2004
306
Originally posted by magnet@May 19 2005, 01:57 PM
This question is on a distance learning course and is only covered by a description within the unit.

I you know how to help or where I may find information on this, especially where calculations are involved, it would be greatly appreciated.

Thanks
Magnet
[post=7811]Quoted post[/post]​
Mm.....

Everything I have is for the transistor level and without knowing Betas/gm of the xtrs I couldn't give u my equasions to solve for it.

You could possible take a different approach to solving this tho. CMR is just the diff mode gane / common mode gain. You can simulate both of these situations and divide the resulting gains and get an approximate answer. Just put a differential input, say 5v and 0v, and then a common mode input, 4v and 4v or so.
 

Thread Starter

magnet

Joined Apr 25, 2005
16
Originally posted by Brandon@May 20 2005, 04:15 PM
Mm.....

Everything I have is for the transistor level and without knowing Betas/gm of the xtrs I couldn't give u my equasions to solve for it.

You could possible take a different approach to solving this tho. CMR is just the diff mode gane / common mode gain. You can simulate both of these situations and divide the resulting gains and get an approximate answer. Just put a differential input, say 5v and 0v, and then a common mode input, 4v and 4v or so.
[post=7833]Quoted post[/post]​
This question is a bit puzzling because all references about CMR equations are to do with the Voltage. The hint within the question was to add minimal resistor values!!
 
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