Issues with analogRead when using Vin on Arduino

[SolarTrap]

Hello,

I am having hardware specific problems with analogRead when I am supplying power via the Vin pin on an Arduino MKR1000 (

https://www.arduino.cc/en/Main/ArduinoMKR1000)

— It’s an Atmel SAMD21.

Attached is a very simple voltage divider circuit

that should always return a value around 2047 when I am using 12 bit ADC resolution of pin A2. This is indeed the case when I am supplying power via USB - either directly to the computer or USB charger device.

But when I am using the Vin pin via an external source

then the ADC values are 1954 - 1890 —> around 100 values lower! I have no explanation for this and the Vcc of 3.3 is not changing if I use either way of supplying power.

(In case you are wondering: I am sending the ADC values via Wifi to the computer in both power supply methods)

What could be the source of this?

Thanks
Markus

 

[IRstuff]

That works out to an 80 mV error, which could be bad grounding in your circuit

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert!

https://www.youtube.com/watch?v=BKorP55Aqvg

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[SolarTrap]

You are right, that seems to be the issue: If I supply the GND from my power supply to the GND of the USB connector then it's all good - but: If I tie the GND from the USB to the GND pin of the board than I am back at my wrong results. How does that make sense?
Two different GNDs?

 

[SolarTrap]

Correction: it looks like I am not connecting to USB GND directly

I am connected to the USB shield which has a ferrite bead (BLM18PG471SN1D) in line with it.

 

[mudandsnow]

Not sure if this applies but it might help.

EEVblog #441 - How To Track Down Common Mode Noise

https://www.youtube.com/watch?v=BFLZm4LbzQU

I find this guy pretty entertaining, sometimes I listen to random videos of his while I work.