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

Hi,

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.

Regards,

Umer

  • Hi Umer,

    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.

    Regards,

    Nikko

  • Hi Umer,

    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

    Regards,

    Glenn

  • Hi Umer, 

    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.

    Cheers,

    Erbe

  • Hi Nikko,

    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?

    Regards,

    Umer

  • Hi Glenn,

    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?

    Regards,

    Umer

  • Hi Erbe,

    Thanks for your input.

    Regards,

    Umer

  • Hi Umer,

    Synchronous detection makes the system's noise performance better when compared to dc measurements. For the CN0363, this further increases when you use frequencies which aren't close to the frequency of other light sources.

    Yes, the system will still have good performance in the presence of random ambient light. However, covering it with a box will much more strongly ensure that possible random light sources close to the modulation frequency will have negligible effect to the measurement.

    By the way, in what environment and application are you using the boards?

    Regards,

    Nikko

  • Hi Umer,

    Apologies, it should be 5kHz for CN0312. Looking at their circuit notes, only the CN0363 has the variable frequency for the modulation.

    CN0363 implements digital synchronous detection using an FPGA rather than implementing it in the hardware.

    Regards,
    Glenn

  • Hi Nikko,

    Thanks for reply. I am using CN0312 as a reference system for Soil Nutrients Analysis, Nitrate and Phosphate concentrations mainly. The samples develop from light yellow to reddish colour as concentration increases. So, I am studying the absorption and comparing them with some custom designed Colorimeter Chambers as well as some Colour Sensors. I also bought CN0363 but haven't tested it yet.

    Regards,

    Umer

  • Hi Glenn,

    I am aware that in CN0363, Synchronous detection takes place in FPGA rather than in hardware but are you sure that in CN0312 we can't vary frequency because in software it allows. Also in circuit diagram the synchronous detector part has LED Clock input on both switches so if we vary the frequency isn't it suppose to work?

    Regards,

    Umer