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VREFINT Measurement Issue in STM32L

Question asked by tidhar.gil on Jan 11, 2014
Latest reply on Jan 12, 2014 by tidhar.gil
I am trying to implement indirect voltage measurement in a battery operated application.
Our MCU supports VREF+ and VREF+ is derived from the battery voltage.
Therefore - VREFINT must be used to extract VREF+.
The trouble is I keep getting a wrong digital value when measuring VREFINT via ADC (I get a digitized value of around 600 for the first measurement and from the second measure and forth I receive a value around 1400).
Thus the VREFINT I get is ~1.12V (when neglecting the first measurement).
VREF+ is stable at 3.3V during the measurements.
The ADC is configured to a sampling time of 384 cycles 
The code for indirect VREF+ measurement is based on AN3964 (see below).
Please be so kind to share your thoughts as to why the ADC measurement deviates.
Thank you very much

void Indirect_Input_BatterySampling( void )
    static  U16       val_vrefint;   //for vref int (1.2 volt via channel17
    static  U16       val_bat_mon;         // bat volt in units via adc
    static  U32       v_bat;               // bat in volt 
    static  U32       vref_plus;

//    For Indirect VREF+ calculation

    ADC_TempSensorVrefintCmd(ENABLE);                                        // ENABLE VREFINT

    val_vrefint = Adc_GetAdcValue ( ADC_Channel_17 ); // Measured value of VREFINT via ADC.

    val_bat_mon = Adc_GetAdcValue (ADC_BATTERY ); 

    vref_plus = (vrefcal*3*10000) / val_vrefint;
    v_bat = ((val_bat_mon * vref_plus) / 4096) * 2;