Help with a CMR issue

[eladta]

Hi all,
As u can see in the attached circuit there is a resistor in the cathode side and one in the anode side.
The question is why not to put one equivalent resistor equals to the series sum of them (when the led is on lets assume it's a short) ?
I know the answer has something to do with CMR but i didn't get it.
Thanks.

 

[shab_eda]

If you are trying to get rid of common mode signal, you need to put similar impedances (resistance ) for both the inputs. This way, at the input of the photo diode you cancel out the common mode.

 

[eladta]

If you are trying to get rid of common mode signal, you need to put similar impedances (resistance ) for both the inputs. This way, at the input of the photo diode you cancel out the common mode.

i understand that this combination of resistors cancal the common noise but i don't understand why..

 

[shab_eda]

The common mode noise gets cancelled because the input is differential not because of the resistors. The resistors are added to limit diode current.

 

[crutschow]

I see no reason for two resistors. The CMR is provided by the opto coupler, not the resistors. One equivalent resistor should work fine.