Will kADC16_ReferenceVoltageSourceValt allow me to use the 1.2 volt reference of my MK80FN256VDC for the ADC reference voltage?

[fkostedt77]

Will kADC16_ReferenceVoltageSourceValt in the ADC16_Init allow me to use the 1.2 volt reference of my MK80FN256VDC for the ADC reference voltage?  Presently the VREFH pin is tied to VDDA and I would like the ability to usse the 1.2 volt reference for the ADC without changing the board layout.  I have based my external circuitry on the 1.2 volt VREF_OUT and want to optimize my ADC resolution by using 1.2 volts as my VREFH for the ADC.

[Robin_Shen]

Hi Fred,

If you want to use the 1.2 volt reference of MK80FN256VDC for the ADC reference voltage, please enable the VREF module(read the "Chapter 39 VREF: Voltage Reference Driver" of MCUXpresso SDK API Reference Manual_MK80F25615).

A 100 nF capacitor must always be connected between VREF_OUT and VSSA if the VREF is being used.

Best Regards,

Robin

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[fkostedt77]

Hello Robin,

Thank you for your thorough information about setting up VREF for use.  I have found that I have VREF set up correctly.  I have also found that if I set:  adcUserConfig.referenceVoltageSource = kADC16_ReferenceVoltageSourceValt;

I switch to using VREF for the ADC reference.  My confusion lies in the fact that a compiler definition: BOARD_ADC_USE_ALT_VREF is not getting defined and it was forcing my code to skip the Valt setting.  I am curious where this should get defined.  I would have thought it would be in the board.h file but the code for my FRDM-K82F board does not have it defined.  I would have thought the K82 and K80 would allow use of this compiler switch.

While I have your ear can you also help me determine why I have a 4.4 MHz sawtooth signal on my VREF output on my FRDM-K82F board.  The maximum and minimum voltage of the sawtooth goes from 1.8 to 1.2 volts.  On my target board I have a steady 1.2 volts and it uses the same setup for VREF.

Best Regards,

Fred