Measuring AC voltage with multimeter

[Google+SucksBalls]

My electronics knowledge is a bit rusty, so I'm double checking here.. I'm having some trouble measuring voltage with arduino: the readouts kept fluctuating. So I figured out something is wrong with my aref/power supply. So I measured the voltage of the usb, which I'm using to supply the arduino and it is 4.6v steady. But when I switch to AC voltage measure, I should expect near 0V, right? It's reading 0.3V - does that mean the usb power is going up and down for 0.3V? I don't have oscilloscope to double check it..

thanks,
Žiga

[PSR B1257]

does that mean the usb power is going up and down for 0.3V?

Not necessarily. It could be even worse. The multimeter is calibrated for a sinusoidal wave and can only measure those correctly. With any other wave form (unless you know the type) it can pretty much display everything.

Nevertheless, if it reads 300 or whatever mV, there is something seriously wrong the supply rail.

[Rolo]

My experiance with arduino and ADC is not to use your computer`s USB as powersupply. It fine for the digital I/O but the ADC needs a better power source. Try powering it external with a simple linear power supply and will improve a lot.

[Google+SucksBalls]

Thanks for answers!

It was a blown capacitor on psu of usb hub.