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MC56F84452 ADC Accuracy

Question asked by Douglas Lynn on Apr 11, 2016
Latest reply on Feb 1, 2018 by xiangjun.rong

Last week I observed an interesting behavior in the 12 bit ADC of the MC56F84452.  One of the pins that was being measured by the 12 bit ADC was reading a 2.24V signal about 150mV low.  My initial thought was that the input was being sampled too fast even though there was very little impedence on the input.  So I set the sampling time to the slowest possible time.  This did not help.  So I made the sample and conversion time faster.  This made the sample more accurate.  So I made the sample time slower and left the conversion time at the faster level.  Accuracy was the same.  Slowed the conversion time down and accuracy dropped again.  Any ideas on why a faster conversion time would result in a better more accuracy result.  The accuracy on the other pins stayed about the same which were good to begin with.

 

After the sample time, could there be internal leakage on this pin during the conversion time?

 

This is pin 28 (56F84452VLH) which is GPIOB3/ANB3/VREFLB/CMPC_M0.

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