I'm hoping someone can offer an informed opinion on whether my recently purchased Uno R3 (my first Arduino) has a defect that could be causing problems. I've attached a picture that shows a close-up of the edge of the PCB right below the USB input. The far right 'notch' has what appears to be an embedded piece of copper that spans the full thickness of the board. (it was hard to capture with the camera; it is much more clearly 'shiny and copper/metallic-looking' when examining with a 3x loupe; also its discernable outline appears rectangular when viewed from the bottom.)
On a project involving digital calipers and very low current (10-20 uAmps) I'm getting correct circuit voltages and perfectly stable gauge readout when using external power; but with USB power (or USB + external), the gauge goes haywire and just keeps jumping further and further into negative territory. This happens on two different computers and with different USB cables. (voltages from the USB cables and out from the Uno do measure correctly, though; and the code is accurately capturing these negative values in the serial monitor.)
I'm not familiar with PCB details and whether this could be bridging layers that it shouldn't (or maybe its position means it's of no consequence and I can move on); but since I'm just using simple circuits and code that others have used successfully (and since I've already spent days researching, rechecking, trying adjustments, and no amount of capacitor combinations has any effect), I'd like to at least cross this off the list if it's not an issue.
I first noticed this when I was doing some continuity checks on the USB input pins, and to my eyes at least, it looks ragged and rather 'non-standard', so hence my concern. I'm certainly no electronics wiz, so I'm fully prepared to be sent to the back of the 'electron-challenged' bus and move on to a different section of the forum if it's a non issue.
Any assistance will be appreciated.