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Purpose of Modulation frequency of 5KHz on CN0312 and CN0363 Evaluation Borads


I am using CN0312 Evaluation board to measure the absorption characteristics of different samples.

My question is why is there a need of modulation frequency for the LED i.e. 5KHZ or any other frequency on CN0312 and also CN0363.

The reason why i am confused is because there is no option for changing duty cycle in the software which could have allowed us to change the LED's brightness.

So, if we are not changing LED's brightness then what is the purpose of just modulation  for this specific application and why didn't you used just normal non-modulated dc source for this purpose?

Does it have anything to do with the Photodiode used in the board? Because I couldn't find anything conclusive.

Thanks in advance.