RM57L843: 10-bit ADC1 Group 1 ch0 offset / dc gain problem with a 10kohm variable resistor, Vref = 3.3V

Hello Everyone!

Observing the voltage on the variable resistor with a properly calibrated multimeter gives different value, as the one detected by the AD in the controller, and it is not a constant offset, neither seems to be a DC-gain error. I've made some measurements:

What can cause the difference and how could I solve it or minimize it down below 0.01? Tried it with and without the calibration functions, even tried to set it to a 12-bit ADC, neither of them helped. Thanks in advance for your reply.

I include the code, too:

/* USER CODE BEGIN (0) */
/* USER CODE END */

/* Include Files */

#include "HL_sys_common.h"

/* USER CODE BEGIN (1) */

#include "HL_esm.h"
#include "HL_adc.h"
#include "HL_gio.h"



adcData_t adc_data[1];
void wait(uint32 time);

/* USER CODE END */

/* USER CODE BEGIN (2) */
/* USER CODE END */

int main(void)
{
/* USER CODE BEGIN (3) */
       uint32 ch_count=0;
        uint32 id    =0;
        uint32 value =0;
        float voltage = 0, valuefloat = 0;

        /* initialize gio     */
        gioInit();
        gioSetDirection(gioPORTB, 1);

        /* initialize sci/sci-lin  : even parity , 2 stop bits */
        sciInit();

        /* initialize ADC  */
        /* Group1 -> Channel 0 and 1                        */
        /* HW trigger trigger source as GIOB  Pin 0         */
        adcInit();

        adcCalibration(adcREG1);
        adcMidPointCalibration(adcREG1);

        /* start adc conversion */
        adcStartConversion(adcREG1,adcGROUP1);

        while(1) /* ... continue forever */
        {
            /* trigger using gio port b, pin 0  */
            gioSetBit(gioPORTB, 0, 1);

            /* ... wait and read the conversion count */
            while((adcIsConversionComplete(adcREG1,adcGROUP1))==0);
            ch_count = adcGetData(adcREG1, adcGROUP1,&adc_data[0]);

            ch_count = ch_count;
       

            id    = adc_data[0].id;
            value = adc_data[0].value;



            gioSetBit(gioPORTB, 0, 0);



            voltage = ((float)value / 1024) * 3.3;



            wait(0xFFFFF);
        };

/* USER CODE END */

    return 0;
}


/* USER CODE BEGIN (4) */

void wait(uint32 time)
{
    while(time){time--;};
}
/* USER CODE END */