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Unable to select 3V external reference (internal 3.3V is used)

Pelle Windestam
Intellectual 620 points

I am using an LM4F232H5QD board and for my AD conversion I want to use the external 3V reference. In my initialization code for the AD converters I use ADCReferenceSet(ADC0_BASE, ADC_REF_EXT_3V); to make it use the ext. reference. I also use 12-bit resolution. When connecting a 2.16V source to the ADC channel I read 2680 from ADC0. I would expect to read 2.16/3.0 * 4095 = 2948. Since 2680 = 2.16/3.3 * 4095 it appears that my change of reference isn't working. Could there be something wrong in my initialization code which causes these problems? I checked the registers in the debugger and I can see that the 3V external reference is set.