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VREF+ not working as expected

Question asked by gage.aaron on Sep 3, 2014
Latest reply on Sep 3, 2014 by gage.aaron
Greetings all --

I have a custom board built around the STM32F303VC, which has separate pins for VDDA and VREF+.  In the interests of keeping the voltage reference quiet, a 3.0V regulator is attached to VREF+, and VDDA is powered at 3.3V.

I am monitoring some input voltages that are scaled to 2.0V through resistor dividers.  I have verified all of the voltages and they are where they should be.

The problem is this: all of my input voltages are reported about 10% lower than they should be.  If I didn't know better, I would say that my reference voltage is still effectively 3.3V even though I know it is 3.0V.

I would expect, given an input voltage of 2.0V and a reference voltage of 3.0V, sampling 12 bits, I should get (2.0/3.0)*4095 = 2730 counts (or so).  Instead, I am getting about 2510.

I have confirmed that all of the inputs are single-ended.  The sample time is 7.5 cycles.  I am scanning several channels with DMA every 40ms (triggered by a timer).

I cannot find anything that needs to be enabled in the firmware to use the VREF+ -- it looks like it should be automatic.  Is there something else I should be doing to get the number of counts I expect?  Is sampling from voltage dividers (with a series resistance ranging from 6K-133K Ohms) a problem?  Any undocumented errata?

Thanks.

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