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Accuracy of factory programmed VREFINT_Factory_CONV

Question asked by mclean.mark on Oct 9, 2013
I'm using the STM32L151 for a project which needs some reasonably precise (~0.1%) voltage measurements.  I'm measuring the micro's 3V3 supply by converting the internal reference and using the factory programmed VREFINT_Factory_CONV value, which I believe is set at the factory by converting the reference with Vcc=3.000V.

I see errors as large as 20mV in my 3.3V supply measurement on some boards, some boards are spot-on.  Any suggestions as to why I get such a large error?  I would have hoped that the error would have been of the order of the total unadjusted ADC accuracy which is about 5 bits (=4mV).

Everything is at room temperature.  I'm using the longest ADC sample time.

This is the maths:
f32_3V3 = 3.0f * (float)(*(uint16_t*)VREFINT_Factory_CONV) / (float)ADC_Value;