I only use the USB for programming. The circuit either runs from battery or a power supply. The 9 volt regulator runs the pumps and the nsno. It is a 7809, so in retrospect the nano is very well regulated and in turn should be a great 5volts regulated as well as good 3.3v.
I don't really have a power issue, it is just the pulses from the pumps which in reality does offer theoretical noise in what the senser sees. Anything other than a constant pressure could be consider noise in my opinion. That is why the low pass filter does work. No matter what anyone says it's proven on the circuit and on the scope.
The pulses from the pumps cause the voltage out from the pressor sensor to waver, voltage up and down a bit, but the low pass filter takes most of that out. I was will to post it but nobody was interested.
Sometimes theory and what is on paper isn't always the fix. Sometimes introducing issues like this requires looking outside the box. I am not doubting anyone but the proof is in the operation.
Right now even though it is a little slow the accuracy is within 1 mmhg. That is way better then what the tolerance is on the pressure sensor. It is just a little slow with the averaging code I have and it leaves a remainder.