[MKW01] ADC strange behavior in differential mode

giancarlozanuz — Wed, 11 Apr 2018 13:29:34 GMT

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.

Re: [MKW01] ADC strange behavior in differential mode

giancarlozanuz — Tue, 17 Apr 2018 12:54:51 GMT

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?

Re: [MKW01] ADC strange behavior in differential mode

jorge_a_vazquez — Wed, 18 Apr 2018 21:49:14 GMT

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

topic Re: [MKW01] ADC strange behavior in differential mode in Kinetis Microcontrollers

[MKW01] ADC strange behavior in differential mode

Re: [MKW01] ADC strange behavior in differential mode

Re: [MKW01] ADC strange behavior in differential mode