What is Normal Variation for the Kinetis MCU ADC Calibration Results

[dakutaga]

We are using an MKE18F512VLH16 MCU in our application. We have noticed variation in the ADC calibration results which results in measurement variation in the voltage of sensor inputs. We performed a test where we performed around 3500 calibrations on 4 different MCUs without powering down or resetting the MCUs to determine the range of values we get for calibration registers ADCx_CLPS, ADCx_CLP3, CLP2, ADCx_CLP1, ADCx_CLP0, ADCx_CLPX, and ADCx_CLP9. The data is contained in 4 tabs in the attached Excel file.

Questions:

1) The software checks each of the calibration register vales according the Sec. 4.3 of AN5314 ADC Calibration on Kinetis E+ Microcontrollers so all of them are in range. We see what appear to be "outlier" calibration register values. Is the amount of variation we are seeing normal?

2) We have tried to implement the median calibration method described in the last paragraph of 16-bit SAR ADC calibration. We have several questions about this?

   a) Is this method intended for the Kinetis E Series 12-bit ADC calibration?

   b) The method appears to describe determining the median value of each register independently, so the resulting calibration would not necessarily be a coherant set of calibration values. Is this what is intended?

   c) The median calibration method did not achieve much improvement in the ADC calibration variation. It only reduced the probability of observing the "outlier" calibrations but did not eliminate them. Is there any further guidance the NXP could provide.

Thanks,

Doran Akutagawa

[AndyS]

We have done some more testing with median (and mean-about-median) calibrations. Not much has improved.

However, we noticed that the auto-calibration seems to impact more registers than CLP0-3, and CLPS/X/9?

Should any other ADC registers be subjected to the "median" treatment? I'm wondering about the _xOFS, _G, and _UG registers.

[FelipeGarcia]

Hi Doran,

Sorry for late reply.

I checked you calibration results and as you mentioned your values seems to be in range, the outlier values you are getting are normal as long as your results are in range.

Kinetis KE18x uses a 12-bit SAR ADC, you can use the 16-bit SAR ADC calibration if you keep in mind the that the ADC resolution is different.

The median method is intended to reduce the variation by executing the calibration many times as you described so your statement is correct.

Please take the following recommendations:

1). Enable hardware average and set average number to 32.

2). No parallel calibration of ADCs because they will disturb each other.

3). For VREFH pin on PCB, use 3 bypass capacitance in the range: 1uF, 100nF and 1nF and place them as close as

     possible to the VREFH pin.

I hope this helps.

Best regards,

Felipe

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