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LAUNCHXL-F28379D: Current Limitation Recommendation for ADC Input

Ryan Pyrch
Prodigy 30 points

Part Number: LAUNCHXL-F28379D

Hi Expert,

I am looking for a recommendation for Signal Conditioning required to read the voltage off of a sensor that outputs from 0-12VDC. I am voltage dividing it down to 0-3V using a 3K resistor and 1K resistor voltage divider. Do I need an additional resistor between the output of the voltage divider and the input of the microcontroller ADC? A 30K resistor would limit current to 100uA, preventing any potential damage to the microcontroller. 

Does the microcontroller need this protection on its ADC input?

What is the minimum current the microcontroller needs to be able to read ADC inputs? With the 30K resistor, as shown, current would be limited to ~17uA.

Thank you for the help,

Ryan

  • 0
    TI__Mastermind 41330 points

    Hi Ryan,

    Short answer is that the R1/R2 voltage divider network will work and it is not recommended to include the 30K series resistor.  You would need a shunt cap as well (in order of a few hundred pFs for transient filtering along with R1/R2 impedance).  The input network, as seen by the F28379D ADC would be any resistor, cap that would be outside of the chip, in this case the voltage divider network, any shunt caps as well as the source impedance of the 12V supply.  All these will affect signal settling of the sampling capacitor in the ADC module.  You would want to keep the impedance as low as possible (hence not recommending to place the 30K series resistor).  A good discussion of how the external impedance affects conversion can be found in the TRM under section "Choosing an Acquisition Window Duration".

    And yes, it is recommended to put input protection (like reverse biased diode or TVS diode).

    There are several application notes as well (the long answer) that you can refer to on signal conditioning:

    Regards,

    Joseph

  • 0 in reply to Joseph Casuga
    Prodigy 30 points

    Hi Joseph,

    I'm looking through those application notes you shared now. Where would I find the TRM? 

    Based on your short answer and my preliminary reading, this is my updated circuit.

    In your experience, would you be able to confirm this is the optimal value for the shunt capacitor? I saw this used multiple times in the application note examples.

    The diode will cutoff any voltage over 3V.

    Will this approach work for all of the ADC inputs on the board? 

    Thanks again for your help,

    Ryan

  • 0 in reply to Ryan Pyrch
    TI__Mastermind 41330 points

    Hi Ryan,

    In general, the approach is ok but all depends on what accuracy you need to get.  There is a good discussion on this topic in the TRM section that i mentioned above for the settling time and resolution and the input network plays a big part.  The Technical Reference Manual is available in ti.com on this link (https://www.ti.com/lit/pdf/spruhm8).

    Regards,

    Joseph