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TMS320F28069: ADC non-linearity problem

Part Number: TMS320F28069

Hello.

We have a product with TMS320F28069 MCU.

ADCINA6 used as analog input, ADC using internal reference and supplied with filtered (pi-filter) VDDA. Ripple on VDDA is around 16 mV p2p. 

As I understand from the datasheet we can expect non-linearity around plus minus 9-10 LSB that coming from DNL and INL. Offset and gain errors don't cause this kind of behavior if I am not mistaken.

I attached graph of ADC output gathered by communication from the unit vs input measured with NI-DAQ 6008. As you can see on the graph there is areas where we have some sort of plateaus with error around 50 LSB.

I think that the ripple on ADC less likely can cause this kind of behavior. Can you please give me some suggestion what can cause this kind of behavior of ADC.

Thank you.

Best regards, 

Nikita Zapesotskiy