[MKW01] ADC strange behavior in differential mode

[giancarlozanuz]

Hello,

I'm having some issues when using the ADC of the MKW01Z128 in differential mode. In this mode, when I put any voltage between pins PTE18-PTE19 the samples oscilates as you can see in the picture bellow. The same behavior don't occur when in single mode for the same pins individually:

For this test I used a voltage divisor around 975mV, and I'm using a voltage reference of 1.5V as VREFH.

I'm using the ADC in One-Time-Trigger mode, with resolution of 16bits:

#if FSL_FEATURE_ADC16_HAS_CALIBRATION
    adc16_calibration_param_t userCalConfig;
#endif /* FSL_FEATURE_ADC16_HAS_CALIBRATION */

    adc16_converter_config_t userConvConfig;
    userConvConfig.lowPowerEnable = false;
    userConvConfig.clkDividerMode = kAdc16ClkDividerOf8;
    userConvConfig.longSampleTimeEnable = true;
    userConvConfig.resolution = kAdc16ResolutionBitOf16;        /* 16bits */
    userConvConfig.clkSrc = kAdc16ClkSrcOfAsynClk;
    userConvConfig.asyncClkEnable = true;
    userConvConfig.highSpeedEnable = false;
    userConvConfig.longSampleCycleMode = kAdc16LongSampleCycleOf24;
    userConvConfig.hwTriggerEnable = false;
    userConvConfig.refVoltSrc = kAdc16RefVoltSrcOfVref;
    userConvConfig.continuousConvEnable = false;
#if FSL_FEATURE_ADC16_HAS_DMA
    userConvConfig.dmaEnable = false;
#endif /* FSL_FEATURE_ADC16_HAS_DMA */

    if(kStatus_ADC16_Success != ADC16_DRV_Init(ADC16_0_IDX, &userConvConfig))
        return false;

    /* Execute the auto-calibration */
#if FSL_FEATURE_ADC16_HAS_CALIBRATION
    if (kStatus_ADC16_Success != ADC16_DRV_GetAutoCalibrationParam(ADC16_0_IDX, &userCalConfig))
        return false;
    ADC16_DRV_SetCalibrationParam(ADC16_0_IDX, &userCalConfig);
#endif /* FSL_FEATURE_ADC16_HAS_CALIBRATION */

What else can it be? Thanks.

[giancarlozanuz]

I've found something about this, but I'm still confused. This is the simplified test circuit that is produting that odd oscillating graph:

But when I connect the same GND to ADC- channel, it works:

You can see below the comparison graph of both circuits, for 1V on PTE18-PTE19:

But since I'm working with the ADC in differential mode, why do I need to connect the common GND on the negative input of the ADC?

[jorge_a_vazquez · Accepted solution]

Hi Giancarlo

As it is mentioned in the datasheet, your ADC input voltage has to be Min VREFL and Max VREFH, this voltage is the one that is taken as the reference to sample the ADC input, so these voltages must have the same ground reference.

Best regards