I was told the Vref voltage is more stable when the Arduino is plugged in to a wall wart vs. the USB cable (I'm doing some very sensitive analog readings). However, I need to read serial data in so I need the USB cable plugged in. When both are plugged in, is there any chance the board defaults to the wall wart for power but sends data over USB? That would be ideal but I'm not sure how to confirm if that's happening.
I was told the Vref voltage is more stable when the Arduino is plugged in to a wall wart vs. the USB cable
No not necessarily.
When both are plugged in, is there any chance the board defaults to the wall wart for power but sends data over USB?
Yes that is what will happen.
No not necessarily.
I posted about this on my blog, and someone commented as such:
http://www.dustynrobots.com/news/digital-signal-processing-and-filtering-motor-current-sensing/#comments
I haven't tested it yet though, in process.
Yes that is what will happen.
Can I ask how you know this? I believe it, I'd just like to be able to troubleshoot better on my own in the future. I didn't see anything in the schematic that jumped out at me to say this would be so.
The stability of the Vref depends on the stability of the supply. What exactly are your concerns?
I didn't see anything in the schematic that jumped out at me to say this would be so.
Well I did. It is a voltage comparator outputting into some FETs.
The stability of the Vref depends on the stability of the supply. What exactly are your concerns?
Got it. I'm trying to do current sensing on a robotic arm system through the analog in pins, and getting noisy output no matter what kind of RC filter I use. The blog commenter suggested that some of the noise could be from Vref itself, so I want to know how to minimize this if it exists (a well place capacitor?) or at least quantify the noise so I can more or less subtract it from the current signal.
Well I did. It is a voltage comparator outputting into some FETs.
Oh okay. Mechanical engineer here, still learning to make sense of the schematics.
and getting noisy output no matter what kind of RC filter I use
given that:-
Mechanical engineer here, still learning to make sense of the schematics.
I would suggest you look at a better filter both for the motor and for the power supply.
A good read is:-
http://www.thebox.myzen.co.uk/Tutorial/De-coupling.html