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.
The modulation of light from the LED is used for ambient light rejection. By only detecting light modulated at a particular frequency, light coming from other sources are rejected. This reduces noise in the signal received by the photodiode using digital synchronous detection.
Using synchronous detectors both boards use frequency modulation to reject signal that are not the same as the frequency used to drive the LED. While CN0312 has a fixed frequency at 500kHz, CN0363 is able to change the excitation frequency via its software.
If you would like to learn more about synchronous detectors please visit this link>> Synchronous Detectors
In addition to this, the LED is being driven by AD8618 for CN0312 which forms a simple constant current source. the current source is being defined by the resistor divider that generates certain bias voltage derived from Vref.
For CN0363, the AD52021(digital potentiometer) was used to vary the voltage reference for AD8615 which forms a simple constant current source.
Thanks for the answer. So this means that even if we don't cover our system to protect it from random lights from outside it is good to go or we still need to properly cover it in a box or something?
Thanks for the answer. Do you mean 5KHz for CN0312? Also there is option to change the frequency of modulation in GUI for CN0312. Now, if I change the frequency through GUI will the synchronous detector adapt to that particular frequency or 5KHz is the only option for CN0312 case?