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.

 

[ronsimpson]

Between pins (1,2,3 and pins 4,5,6) there is a small capacitor. High frequency noise can pass from input to output. Using two resistors (usually of equal value) helps reduce the amount of current that passes over the isolation gap.

The capacitor is small. At 500mhz the capacitor can pass too much current.

Because SMT resistors are very small, I use two 0.1watt resistors in placed of one 0.25 watt resistor.

 

[eladta]

why only a high frequency noise can pass from input to output ?
also, what is a common noise ?

 

[ronsimpson]

A capacitor at 60hz passes a very small amount of current. A capacitor at 60mhz will pass 1,000,000 times more current.

Generally there are two types if noise, differential or common mode. or a combination of both.

 

[eladta]

just got back from work..
Thanks for your reply, i will go over the document and see if i got it right.. Thanks again