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Internal Vref for adc on K28

Question asked by jason larson on May 15, 2018
Latest reply on May 15, 2018 by xiangjun.rong

I'm working with adc reading and trying to decide if I should use the internal our external Vref for my results. During my testing I see that the internal vref seems to give me more stable readings but I see a couple issues. If anyone can explain them or has run across them please fill me in


1) Even though the Reference manual says the internal vref is 1.2v after I did some measuring it appears to be at 1.4v not 1.2v is this possible? Where do I set this voltage? I see I can change it by 0.5mv with a register but the max deviation from 1.2v is 30mv using this reg so not sure why I see 1.4v as the reference.

2) For some reason the first time I make the adc reading using internal vref I see a much higher readings than all the following readings if done with in 200ms. i.e my first sample of 16 readings averaged value was 3228 but when I run it again right after getting the 16 bit average I see 2998 and this only changes by 1 or 2 no matter how many more samples I take. Sure I can just ignore the first sample but want to know what causes it. If I leave things the same and just switch to using external vref and I don't see this error on the first reading.