Internal Vref for adc on K28

jasonlarson — Tue, 15 May 2018 16:28:30 GMT

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.

Re: Internal Vref for adc on K28

xiangjun_rong — Wed, 16 May 2018 05:45:11 GMT

Hi, Jason,

First of all, from hardware perspective, you have to connect 100nF with +/-25% tolerance capacitor to the Vref_OUT pin if you use internal 1.2V voltage reference.

Anyway, I think it is out of the range that the Vref_out pin voltage is 1.4V, ,pls test it with a digital meter.

Xiangjun Rong

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Internal Vref for adc on K28

Re: Internal Vref for adc on K28