[D21] ADC external reference and gain

Discussions around product based on ARM Cortex M0+ core.

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[D21] ADC external reference and gain

Tue Jun 07, 2016 10:50 am

I'd like to use the internal ADC of SAM D21 MCUs. After reading the datasheet, I have two doubts.

I often have connected external ADC reference pins directly to analog positive supply rail (Vdda) to get the full conversion scale 0-Vdda.
The range of external VREF for SAM D21 is [1.0V - Vdda-0.6V]. Does this mean I can't connect directly VREFA to VDDA?
If Vdda=3.3V, the widest conversion range is 0-2.7V. Is it right?

Maybe I can select internal reference VDDA/2 and configure a gain of 0.5X. In this case, I will be able to convert analog signals in the range 0-3.3V, right?

I read about the programmable gain, but I couldn't understand one thing. Suppose I have the highest reference voltage (2.7V external reference) and I configure the programmable gain to 0.5X (that is an attenuation, not a gain).
This means I could convert signals in the range 0-5.4V.
Is it possible to apply 0-5.4V analog signals to analog inputs pins in this scenario?
The problem here is the absolute maximum rating of input voltage (VPIN) that is GND-0.3V to VDD+0.3V. 5.4V is higher than VDD+0.3V=3.6V.

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