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.

 

It's impossible to tell without seeing more of hte circuit that this is a part of.

 

this is 99% of the circuit. there is just a connector connected to the resistors from the left

 

The the circuit doesn't do anything because there is not current flowing through the diode. This thing is intended to be part of a larger circuit.

But as it stands, I can see no reason for putting the resistors on either side of the diode versus both on one side. I certainly don't see any CMR benefit.

 

maybe this can help u help me
refer to pages 12, 13

 

I don't have the bandwidth to download large files. Perhaps later when I am in the city I can take a look at it. Or, you could extract the relevant pages and post just those.

 

sorry but i can't do that since it's a PDF protected document.
waiting for your reply when you get back to the city..

 

sorry but i can't do that since it's a PDF protected document.
waiting for your reply when you get back to the city..

It's not "back" to the city. I'm at home right now!

 

my bad..

 

It's not "back" to the city. I'm at home right now!

Hope I got it correct

 

nice how did u do that ?

 

nice. how did u do that ?

 

According to my tool it was not protected. So I just extracted the pages

 

Hope I got it correct

Sure looks like it. And thanks for including page 14, which is the key.

They are looking at a more complicated model that included the effect of capacitive coupling between the LED's anode and cathode to the output side ground. Thus, you no longer have a strictly series circuit. The goal is to pick resistor values that will equalize these impedances. Read the section very carefully and try to walk through the analysis and the math alongside them as you go.

 

The goal is to pick resistor values that will equalize these impedances.

can you elaborate on that ?

 

Have you carefully read what the datasheet has to say on those pages? If not, please do. Then ask specific questions about the points that are tripping you up and we will try to provide specific answers, if we can.