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TMS320F280039-Q1: the ADC ref

Jenney Ni
Intellectual 1040 points

Part Number: TMS320F280039-Q1

Hi all,

The customer choose the 1.65V (which can only choose 1.65V or 2.5V) as the ADC internal ref voltage. If the input voltage of ADC is 1.65V, the result of ADC sample is 4096, right? But customers just get the 1024. What is the reason?

Regards,

Jenney

  • 0
    TI__Mastermind 25633 points

    Hi Jenney,

    There may be a slight misunderstanding here.

    The key point is that the internal reference voltage (VREFHI) and the ADC conversion range are not the same when using the internal reference mode.

    When the internal reference uses VREFHI at 1.65, a full scale 3.3V input range is supported:

    • VREFHI = 1.65V
    • ADC Conversion Range = 0V to 3.3V (0-4095)

    VS

    • VREFHI = 2.5V
    • ADC Conversion Range = 0V to 2.5V (0-4095)

    This means the ADC can actually measure input voltages up to 3.3V, even though the reference voltage is only 1.65V. The ADC achieves this by internally dividing the input signal by 2 before conversion.

    Correct Expectation

    To achieve a full-scale reading of 4095:

    • With the 3.3V range selected (VREFHI = 1.65V), the input voltage must be 3.3V, not 1.65V
    • The 1.65V reference voltage is used internally for the conversion algorithm but does not represent the maximum measurable input voltage

    Best Regards,

    Zackary Fleenor

  • 0 in reply to Fleenor
    TI__Intellectual 1040 points

    Hi Zackary,

    Ok, i got :When the internal reference uses VREFHI at 1.65, a full scale 3.3V input range is supported:

    • VREFHI = 1.65V
    • ADC Conversion Range = 0V to 3.3V (0-4095)

     But why the result of ADC sample is 1023, when the input voltage of ADC is 1.65V? We expect  the result of ADC sample is 2047?

    Regards,

    Jenney

  • 0 in reply to Jenney Ni
    TI__Mastermind 25633 points

    Hi Jenney,

    That is a good question, how are they verifying the input voltage of 1.65V? If they are applying multimeter probes to measure this, it will cause interference with the input circuitry. If you scale the input voltage from 0 to 3.3V do you only see results scaling 0-2048?

    Best Regards,

    Zackary Fleenor