Electronic – How to condition supply voltage for project

In general you can assume that as long as the ripple is not too bad on a supply then it shouldn't cause an issue for functionality of an IC. In this case the datasheet gives a ripple of 100mV peak-to-peak, BUT you seem to imply that you're taking this voltage and further regulating it down. Passing a signal with a relatively small ripple and putting it through a linear regulator like a 7803 or an LDO will significantly reduce the ripple and cut back on switching noise passed through to the final circuit. So a ripple which is less than 5% of the expected voltage like this, being passed through a second line of regulation, should be fine for 95% of projects, even one using analog components like an ADC. Just make sure to have input and output filtering capacitors on the linear regulator, and not too much (2200μF is the limit for the power supply you gave) (maybe 47uF and 4.7uF (or 220uF and 22uF if you want it really smooth)).

Because the ripple is so small this supply would probably be fine for digital operations if you were using a 5V compatible chip and you weren't doing Analog work, but I digress.

So looking at the datasheet you provided, the operating frequency of the power supply is 132kHz. The chances that the noise from that is going to manage to produce harmonics in the 2.4GHz spectrum is pretty limited, especially at such small power outputs. Where you really have to start worrying about generated EMI is when you have isolating switching supplies, because these will pass power through the magnetic field, and the interface leaks EMI on most supplies I've used. Generally the higher-power the switching power supply the more noise it generates. I've had a 100Watt isolating switching power supply which put so much noise out I had 30V of noise on the frame of my robot, but that's way beyond the scope of this. I think you'll be fine with your usage!

Hope that helps!

You have a couple of easy options here: feed in an unregulated voltage, like the 12v, to the Arduino's Vin pin (on the Arduino headers), and let the Arduino regulate it, or feed in 5v to the 5v pin. In either case, of course, you should connect the ground pin to your PSU's ground rail. From the Arduino schematic, you can see that Vin is only used to feed the 5v regulator and the comparator for the USB/Vin switch, so if you power the Arduino over the 5v rail, you should not connect anything to the USB port.

Assuming you have a fairly stable and clean 5v source, feeding the 5v is the more efficient option.

You are correct that the Arduino is powered over USB only when no DC supply is present. This is implemented with a comparator, comparing half the voltage on the Vin pin to the 3.3v rail - so the switch will work correctly as long as power is supplied to Vin.