Calibration procedure

To start the procedure, perform Configuration Example and Initialization with included extra channel, which can be re-used after this procedure is complete:

/* Scenario: Third channel was selected to be used in the calibration 
    * procedure. */

cy_stc_sar2_channel_config_t pass_0_saradc_0_sar_0_channel_2_config = 
{
    .channelHwEnable = true,
    .triggerSelection = CY_SAR2_TRIGGER_OFF,
    .channelPriority = 0U,
    .preenptionType = CY_SAR2_PREEMPTION_ABORT_RESUME,
    .isGroupEnd = true,
    .pinAddress = 0U,
    .portAddress = 0U,
    .extMuxEnable = false,
    .extMuxSelect = 0U,
    .preconditionMode = CY_SAR2_PRECONDITION_MODE_OFF,
    .overlapDiagMode = CY_SAR2_OVERLAP_DIAG_MODE_OFF,
    .sampleTime = 120U,
    .postProcessingMode = CY_SAR2_POST_PROCESSING_MODE_NONE,
    .resultAlignment = CY_SAR2_RESULT_ALIGNMENT_RIGHT,
    .signExtention = CY_SAR2_SIGN_EXTENTION_UNSIGNED,
    .averageCount = 1U,
    .rightShift = 0U,
    .positiveReload = 0U,
    .negativeReload = 0U,
    .rangeDetectionMode = CY_SAR2_RANGE_DETECTION_MODE_INSIDE_RANGE,
    .rangeDetectionLoThreshold = 0U,
    .rangeDetectionHiThreshold = 4095U,
};

The ADC has an offset adjustment function to compensate for offset error.

It is possible to select code from +127 to -128 in a dec. for analog calibration.
The offset adjustment step is a quarter of 1LSb.
Offset = max( 0, min( 4095, floor( VIN/VREFH x 4096 + OFST/4 ) ) )

Note

Enable reference buffer mode using Cy_SAR2_SetReferenceBufferMode function.

Following code snippet can be used:

/* Scenario: SAR2 block 0 with it's clock source is configured, channel 2 is
 * not used. */
/* Calibration initial setting. */
cy_stc_sar2_analog_calibration_conifg_t calibrationConfig =
{
    .offset = 127ul,/* Offset Calibration start setting */
    .gain   = 15ul,/* Gain Calibration start setting */
};

/*******************************************************************************
* Function Name: getAdcValue
********************************************************************************
*
* Issues software trigger for the channel 2, waits for SAR2 block to complete
* it's calculation and returns calculated value.
*
* \return
* Measurement value
*
*******************************************************************************/
uint16_t getAdcValue(void)
{
    /* Trigger ADC. */
    Cy_SAR2_Channel_SoftwareTrigger(PASS0_SAR0, 2U);

    /* Wait for group of single channel to complete. */
    while (CY_SAR2_INT_GRP_DONE != Cy_SAR2_Channel_GetInterruptStatus(PASS0_SAR0, 2U));
    Cy_SAR2_Channel_ClearInterrupt(PASS0_SAR0, 2U, CY_SAR2_INT_GRP_DONE);

    /* Get the result(s) */
    return (Cy_SAR2_Channel_GetResult(PASS0_SAR0, 2U, NULL));
}

/*******************************************************************************
* Function Name: offset_Calibration
********************************************************************************
*
* Performs SAR block offset calibration
*
* \return
* Boolean value if calibration was successful.
*
*******************************************************************************/
bool offset_Calibration(void)
{
    /* Start ADC conversion and get the ADC value */
    int16_t result = getAdcValue();
    int16_t offset_VREFH = 0U;
    int16_t offset_VREFL = 0U;
    uint32_t value = 0U;

    pass_0_saradc_0_sar_0_channel_2_config.pinAddress = CY_SAR2_PIN_ADDRESS_VREF_L;
    Cy_SAR2_SetAnalogCalibrationValue(PASS0_SAR0, &calibrationConfig);

    /* Initialize channel. */
    Cy_SAR2_Channel_Init(PASS0_SAR0, 2U, &pass_0_saradc_0_sar_0_channel_2_config);
    if(result > 0)
    {
        /* Decrement the Value of Offset Compensation by "1" */
        for(offset_VREFL = 127; offset_VREFL >= -128; offset_VREFL -= 1)
        {
            /* Set "offset_VREFL" to offset register value */
            calibrationConfig.offset = offset_VREFL;
            Cy_SAR2_SetAnalogCalibrationValue(PASS0_SAR0, &calibrationConfig);

            /* Get ADC conversion value. */
            result = getAdcValue();

            /* Does the ADC conversion value equal to 0? */
            if(result == 0)
            {
                break;
            }

            /* If offset has reached lower bound. */
            if(offset_VREFL == -128)
            {
                /* Report error. */
                return false;
            }
        }
    }
    else
    {
        /* End of Offset Adjustment with Abnormality. */
        return false;
    }

    /* Set VREFH Voltage to Input */
    pass_0_saradc_0_sar_0_channel_2_config.pinAddress = CY_SAR2_PIN_ADDRESS_VREF_H;

    /* Set Offset Compensation to "-128" and init channel. */
    calibrationConfig.offset = -128;
    Cy_SAR2_SetAnalogCalibrationValue(PASS0_SAR0, &calibrationConfig);
    Cy_SAR2_Channel_Init(PASS0_SAR0, 2u, &pass_0_saradc_0_sar_0_channel_2_config);

    /* Start ADC Conversion and get the A/D value. */
    result = getAdcValue();

    /* Is the ADC Conversion value lower than 0xFFF */
    if(result < 0xFFF)
    {
        /* Increment the Value of Offset Compensation by "1". */
        for(offset_VREFH = -128; offset_VREFH <= 127; offset_VREFH += 1)
        {
            /* Set "offset_VREFH" to offset register value. */
            calibrationConfig.offset = offset_VREFH;
            Cy_SAR2_SetAnalogCalibrationValue(PASS0_SAR0, &calibrationConfig);

            /* Get ADC Conversion value. */
            result = getAdcValue();

            /* Does the ADC Conversion value Equal to "4095"? */
            if(result == 0xFFF)
            {
                break;
            }

            /* Does the value of offset compensation equal to "127"? */
            if(offset_VREFH == 127)
            {
                /* Offset adjustment failed - report error. */
                return false;
            }
        }
    }
    else
    {
        /* Offset adjustment failed - report error. */
        return false;
    }

    /* Does "(floor((offset_VREFH+offset_VREFL)/2) + 2)" greater than "125" ?*/
    value = (floor((offset_VREFH+offset_VREFL)/2U) + 2U);
    if(value > 125U)
    {
        /* Offset adjustment failed - report error. */
        return false;
    }

    /* Set the value of offset compensation to (floor((offset_VREFH+offset_VREFL)/2) + 2) */
    calibrationConfig.offset = (floor((offset_VREFH+offset_VREFL)/2U) + 2U);
    
    return true;
}

After that, the Gain should also be calibrated as well. The ADC has a gain adjustment function to compensate for gain error. It is possible to set code from +15 to -15 dec. The gain adjustment step is a quarter of 1LSb.
Gain = max(0, min(4095, floor((4096 - GAIN)/VREFH x (VIN - VREFH/2) + 2048)))

/* Scenario: SAR2 block 0 is left after offset calibration intact, global
 * structure is still available with offset calibration results. */
/*******************************************************************************
* Function Name: gain_Calibration
********************************************************************************
*
* Performs SAR block gain calibration
*
* \return
* Boolean value if calibration was successful.
*
*******************************************************************************/
bool gain_Calibration(void)
{
    int16_t gain;
    int16_t result = 0U;
    /* Set Gain Correction setting to "15". */
    calibrationConfig.gain = 15;

    /* Set VREFL Voltage to Input and init the channel. */
    pass_0_saradc_0_sar_0_channel_2_config.pinAddress = CY_SAR2_PIN_ADDRESS_VREF_L;
    Cy_SAR2_Channel_Init(PASS0_SAR0, 2u, &pass_0_saradc_0_sar_0_channel_2_config);

    /* Apply the initial calibration settings. */
    Cy_SAR2_SetAnalogCalibrationValue(PASS0_SAR0, &calibrationConfig);

    /* Start ADC value Conversion */
    result = getAdcValue();

    if(result > 0)
    {
         for(gain = 15; gain >=-14; gain -= 1)
         {
            /* Set gain to register */
            calibrationConfig.gain = gain;
            Cy_SAR2_SetAnalogCalibrationValue(PASS0_SAR0, &calibrationConfig);

            result = getAdcValue();

            if(result == 0)
            {
                /* Set gain compensation to "gain - 1" */
                calibrationConfig.gain = gain - 1;
                Cy_SAR2_SetAnalogCalibrationValue(PASS0_SAR0, &calibrationConfig);

                break;
            }

            if(gain == -14)
            {
                /* Gain adjustment failed - report error. */
                return false;
            }
         }
    }

    return true;
}

Once done, the calibrationConfig global structure contains calibration data for SAR2 block 0. Calibration is advised to be done as frequent as possible.