An 8 amp supply to power an Arduino that needs perhaps 100 milliamps? In general, why? Just because that’s what you have?
Looks questionable to power a micro. But, we know nothing about the measurement conditions so you’re asking people to make assumptions. Lots of assumptions.
One thing for sure. If you do power an Arduino with a supply with 400 mv of noise on it, the A/D performance is going to be big time el-stink-o. When using analog, 9vdc from a wall wort into an Uno or similar to utilize the built in linear regulator usually provides a cleaner A/D reference.
Disclaimer: I have a strong aversion to those bargain no-name eBay power supplies from China. You get what you pay for. On a good day, they’re only good for about 50% of their current ratings and the other specs, well, you’re looking at the typical performance. Maybe okay for LEDs but I don’t trust them, safety wise. Of course, YMMV.