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ADC calibration register behavior on Kinetis K22

Question asked by 力 関 on Mar 2, 2018
Latest reply on Mar 7, 2018 by 力 関


I'm facing on a problem, that is, it's difficult to satisfy the settling time required described in section 3.2.4 in this document, since the output impedance (RIN in the document) of a sensor device I'm using was pretty higher than I'd expected. I know it's our hardware design problem and the best way is to reduce the impedance using op-amp or something like that, but I'm still looking for a workaround to give a relief to products that we'd already have shipped.


While investigating this problem, I found an interesting symptom.

input signalwhile capturing

The left figure shows the input signal, and the right is also an input signal to K22 while ADC is running.


More interestingly,

signal w/o calibrationsignal when all cal registers filled with zeros

The left figure shows the signal when all values on calibration registers were default value (ADCx_CLP0 = 32, ADCx_CLP1 = 64, ADCx_CLP2 = 128...). It clearly shows that the charging current to S/H capacitor depends on the values written to ADC calibration register, and finally, I found I could minimize charging current by writing zeros to all calibration register except ADCx_PG/MG remains 0x8000 (right figure).



Could you let me know how K22 utilizes values written to calibration registers?


Thanks in advance,

Seki, Tsutomu