Both USB & DC jack plugged in: which does Arduino use?

Using Arduino IDE 1.x
1

I need a stable 5V reference for analogRead, so I want to power the board through the DC jack, but I also need to record data serially through the USB cable at the same time. Ideally if they are both plugged in, I can use USB for data while the board draws power from the AC adapter. Is this actually what would happen?

2

If the voltage on the DC jack is above 6.6V then the power is taken from the DC jack.

3

Got it! Thanks

4

If Vin is >6.6V, the barrel is used.
Vin is after the reverse polarity protection diode, so Vbarrel really needs to be > ~ 7.3V.