Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.


Issues with analogRead when using Vin on Arduino

Issues with analogRead when using Vin on Arduino


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


RE: Issues with analogRead when using Vin on Arduino

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?

RE: Issues with analogRead when using Vin on Arduino

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.

RE: Issues with analogRead when using Vin on Arduino

Not sure if this applies but it might help.

EEVblog #441 - How To Track Down Common Mode Noise

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

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close